[PATCHSET v2 0/8] Add io_uring futex/futexv support

[PATCHSET v2 0/8] Add io_uring futex/futexv support

[Jens Axboe]

Hi,

This patchset adds support for first futex wake and wait, and then
futexv. Patches 1..3 are just prep patches, patch 4 adds the wait
and wake support for io_uring, and then patches 5..7 are again prep
patches to end up with futexv support in patch 8.

For both wait/wake/waitv, we support the bitset variant, as the
"normal" variants can be easily implemented on top of that.

PI and requeue are not supported through io_uring, just the above
mentioned parts. This may change in the future, but in the spirit
of keeping this small (and based on what people have been asking for),
this is what we currently have.

When I did these patches, I forgot that Pavel had previously posted a
futex variant for io_uring. The major thing that had been holding me
back from people asking about futexes and io_uring, is that I wanted
to do this what I consider the right way - no usage of io-wq or thread
offload, an actually async implementation that is efficient to use
and don't rely on a blocking thread for futex wait/waitv. This is what
this patchset attempts to do, while being minimally invasive on the
futex side. I believe the diffstat reflects that.

As far as I can recall, the first request for futex support with
io_uring came from Andres Freund, working on postgres. His aio rework
of postgres was one of the early adopters of io_uring, and futex
support was a natural extension for that. This is relevant from both
a usability point of view, as well as for effiency and performance.
In Andres's words, for the former:

"Futex wait support in io_uring makes it a lot easier to avoid deadlocks
in concurrent programs that have their own buffer pool: Obviously pages in
the application buffer pool have to be locked during IO. If the initiator
of IO A needs to wait for a held lock B, the holder of lock B might wait
for the IO A to complete.  The ability to wait for a lock and IO
completions at the same time provides an efficient way to avoid such
deadlocks."

and in terms of effiency, even without unlocking the full potential yet,
Andres says:

"Futex wake support in io_uring is useful because it allows for more
efficient directed wakeups.  For some "locks" postgres has queues
implemented in userspace, with wakeup logic that cannot easily be
implemented with FUTEX_WAKE_BITSET on a single "futex word" (imagine
waiting for journal flushes to have completed up to a certain point). Thus
a "lock release" sometimes need to wake up many processes in a row.  A
quick-and-dirty conversion to doing these wakeups via io_uring lead to a
3% throughput increase, with 12% fewer context switches, albeit in a
fairly extreme workload."

Some basic io_uring futex support and test cases are available in the
liburing 'futex' branch:

https://git.kernel.dk/cgit/liburing/log/?h=futex

testing all of the variants. I originally wrote this code about a
month ago and Andres has been using it with postgres, and I'm not
aware of any bugs in it. That's not to say it's perfect, obviously,
and I welcome some feedback so we can move this forward and hash out
any potential issues.

 include/linux/io_uring_types.h |   3 +
 include/uapi/linux/io_uring.h  |   4 +
 io_uring/Makefile              |   4 +-
 io_uring/cancel.c              |   5 +
 io_uring/cancel.h              |   4 +
 io_uring/futex.c               | 376 +++++++++++++++++++++++++++++++++
 io_uring/futex.h               |  36 ++++
 io_uring/io_uring.c            |   5 +
 io_uring/opdef.c               |  35 ++-
 kernel/futex/futex.h           |  33 +++
 kernel/futex/requeue.c         |   3 +-
 kernel/futex/syscalls.c        |  27 ++-
 kernel/futex/waitwake.c        |  49 +++--
 13 files changed, 548 insertions(+), 36 deletions(-)

You can also find the code here:

https://git.kernel.dk/cgit/linux/log/?h=io_uring-futex

V2:
- Abstract out __futex_wake_mark() helper. Use it both in the
  futex and io_uring code. This also fixes a missing WARN_ON
  on the io_uring side.
- Have futex_op_to_flags() unconditionally clear flags to
  zero rather than do that in both callers.
- Remove comment on needing to open-code futex_queue(),
  and associated hunk doing that. This was a leftover
  from an earlier version.
- Expand the commit message logs in various patches.

-- 
Jens Axboe

[PATCH 1/8] futex: abstract out futex_op_to_flags() helper

[Jens Axboe]

Rather than needing to duplicate this for the io_uring hook of futexes,
abstract out a helper.

No functional changes intended in this patch.

Signed-off-by: Jens Axboe <axboe@kernel.dk>
---
 kernel/futex/futex.h    | 17 +++++++++++++++++
 kernel/futex/syscalls.c | 13 +++----------
 2 files changed, 20 insertions(+), 10 deletions(-)

diff --git a/kernel/futex/futex.h b/kernel/futex/futex.h
index b5379c0e6d6d..b8f454792304 100644
--- a/kernel/futex/futex.h
+++ b/kernel/futex/futex.h
@@ -291,4 +291,21 @@ extern int futex_unlock_pi(u32 __user *uaddr, unsigned int flags);
 
 extern int futex_lock_pi(u32 __user *uaddr, unsigned int flags, ktime_t *time, int trylock);
 
+static inline bool futex_op_to_flags(int op, int cmd, unsigned int *flags)
+{
+	*flags = 0;
+
+	if (!(op & FUTEX_PRIVATE_FLAG))
+		*flags |= FLAGS_SHARED;
+
+	if (op & FUTEX_CLOCK_REALTIME) {
+		*flags |= FLAGS_CLOCKRT;
+		if (cmd != FUTEX_WAIT_BITSET && cmd != FUTEX_WAIT_REQUEUE_PI &&
+		    cmd != FUTEX_LOCK_PI2)
+			return false;
+	}
+
+	return true;
+}
+
 #endif /* _FUTEX_H */
diff --git a/kernel/futex/syscalls.c b/kernel/futex/syscalls.c
index a8074079b09e..0b63d5bcdc77 100644
--- a/kernel/futex/syscalls.c
+++ b/kernel/futex/syscalls.c
@@ -86,17 +86,10 @@ long do_futex(u32 __user *uaddr, int op, u32 val, ktime_t *timeout,
 		u32 __user *uaddr2, u32 val2, u32 val3)
 {
 	int cmd = op & FUTEX_CMD_MASK;
-	unsigned int flags = 0;
+	unsigned int flags;
 
-	if (!(op & FUTEX_PRIVATE_FLAG))
-		flags |= FLAGS_SHARED;
-
-	if (op & FUTEX_CLOCK_REALTIME) {
-		flags |= FLAGS_CLOCKRT;
-		if (cmd != FUTEX_WAIT_BITSET && cmd != FUTEX_WAIT_REQUEUE_PI &&
-		    cmd != FUTEX_LOCK_PI2)
-			return -ENOSYS;
-	}
+	if (!futex_op_to_flags(op, cmd, &flags))
+		return -ENOSYS;
 
 	switch (cmd) {
 	case FUTEX_WAIT:
-- 
2.40.1

[PATCH 2/8] futex: factor out the futex wake handling

[Jens Axboe]

In preparation for having another waker that isn't futex_wake_mark(),
add a wake handler in futex_q. No extra data is associated with the
handler outside of struct futex_q itself. futex_wake_mark() is defined as
the standard wakeup helper, now set through futex_q_init like other
defaults.

Normal sync futex waiting relies on wake_q holding tasks that should
be woken up. This is what futex_wake_mark() does, it'll unqueue the
futex and add the associated task to the wake queue. For async usage of
futex waiting, rather than having tasks sleeping on the futex, we'll
need to deal with a futex wake differently. For the planned io_uring
case, that means posting a completion event for the task in question.
Having a definable wake handler can help support that use case.

Signed-off-by: Jens Axboe <axboe@kernel.dk>
---
 kernel/futex/futex.h    | 4 ++++
 kernel/futex/requeue.c  | 3 ++-
 kernel/futex/waitwake.c | 6 +++---
 3 files changed, 9 insertions(+), 4 deletions(-)

diff --git a/kernel/futex/futex.h b/kernel/futex/futex.h
index b8f454792304..54b7de21ef00 100644
--- a/kernel/futex/futex.h
+++ b/kernel/futex/futex.h
@@ -69,6 +69,9 @@ struct futex_pi_state {
 	union futex_key key;
 } __randomize_layout;
 
+struct futex_q;
+typedef void (futex_wake_fn)(struct wake_q_head *wake_q, struct futex_q *q);
+
 /**
  * struct futex_q - The hashed futex queue entry, one per waiting task
  * @list:		priority-sorted list of tasks waiting on this futex
@@ -98,6 +101,7 @@ struct futex_q {
 
 	struct task_struct *task;
 	spinlock_t *lock_ptr;
+	futex_wake_fn *wake;
 	union futex_key key;
 	struct futex_pi_state *pi_state;
 	struct rt_mutex_waiter *rt_waiter;
diff --git a/kernel/futex/requeue.c b/kernel/futex/requeue.c
index cba8b1a6a4cc..e892bc6c41d8 100644
--- a/kernel/futex/requeue.c
+++ b/kernel/futex/requeue.c
@@ -58,6 +58,7 @@ enum {
 
 const struct futex_q futex_q_init = {
 	/* list gets initialized in futex_queue()*/
+	.wake		= futex_wake_mark,
 	.key		= FUTEX_KEY_INIT,
 	.bitset		= FUTEX_BITSET_MATCH_ANY,
 	.requeue_state	= ATOMIC_INIT(Q_REQUEUE_PI_NONE),
@@ -591,7 +592,7 @@ int futex_requeue(u32 __user *uaddr1, unsigned int flags, u32 __user *uaddr2,
 		/* Plain futexes just wake or requeue and are done */
 		if (!requeue_pi) {
 			if (++task_count <= nr_wake)
-				futex_wake_mark(&wake_q, this);
+				this->wake(&wake_q, this);
 			else
 				requeue_futex(this, hb1, hb2, &key2);
 			continue;
diff --git a/kernel/futex/waitwake.c b/kernel/futex/waitwake.c
index ba01b9408203..3471af87cb7d 100644
--- a/kernel/futex/waitwake.c
+++ b/kernel/futex/waitwake.c
@@ -174,7 +174,7 @@ int futex_wake(u32 __user *uaddr, unsigned int flags, int nr_wake, u32 bitset)
 			if (!(this->bitset & bitset))
 				continue;
 
-			futex_wake_mark(&wake_q, this);
+			this->wake(&wake_q, this);
 			if (++ret >= nr_wake)
 				break;
 		}
@@ -289,7 +289,7 @@ int futex_wake_op(u32 __user *uaddr1, unsigned int flags, u32 __user *uaddr2,
 				ret = -EINVAL;
 				goto out_unlock;
 			}
-			futex_wake_mark(&wake_q, this);
+			this->wake(&wake_q, this);
 			if (++ret >= nr_wake)
 				break;
 		}
@@ -303,7 +303,7 @@ int futex_wake_op(u32 __user *uaddr1, unsigned int flags, u32 __user *uaddr2,
 					ret = -EINVAL;
 					goto out_unlock;
 				}
-				futex_wake_mark(&wake_q, this);
+				this->wake(&wake_q, this);
 				if (++op_ret >= nr_wake2)
 					break;
 			}
-- 
2.40.1

[PATCH 3/8] futex: abstract out a __futex_wake_mark() helper

[Jens Axboe]

Move the unqueue and lock_ptr clear into a helper that futex_wake_mark()
calls. Add it to the public functions as well, in preparation for using
it outside the core futex code.

Suggested-by: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
---
 kernel/futex/futex.h    |  1 +
 kernel/futex/waitwake.c | 33 ++++++++++++++++++++++-----------
 2 files changed, 23 insertions(+), 11 deletions(-)

diff --git a/kernel/futex/futex.h b/kernel/futex/futex.h
index 54b7de21ef00..1d531244fb71 100644
--- a/kernel/futex/futex.h
+++ b/kernel/futex/futex.h
@@ -148,6 +148,7 @@ extern int futex_wait_setup(u32 __user *uaddr, u32 val, unsigned int flags,
 			    struct futex_q *q, struct futex_hash_bucket **hb);
 extern void futex_wait_queue(struct futex_hash_bucket *hb, struct futex_q *q,
 				   struct hrtimer_sleeper *timeout);
+extern bool __futex_wake_mark(struct futex_q *q);
 extern void futex_wake_mark(struct wake_q_head *wake_q, struct futex_q *q);
 
 extern int fault_in_user_writeable(u32 __user *uaddr);
diff --git a/kernel/futex/waitwake.c b/kernel/futex/waitwake.c
index 3471af87cb7d..f8fb6550061d 100644
--- a/kernel/futex/waitwake.c
+++ b/kernel/futex/waitwake.c
@@ -106,20 +106,11 @@
  * double_lock_hb() and double_unlock_hb(), respectively.
  */
 
-/*
- * The hash bucket lock must be held when this is called.
- * Afterwards, the futex_q must not be accessed. Callers
- * must ensure to later call wake_up_q() for the actual
- * wakeups to occur.
- */
-void futex_wake_mark(struct wake_q_head *wake_q, struct futex_q *q)
+bool __futex_wake_mark(struct futex_q *q)
 {
-	struct task_struct *p = q->task;
-
 	if (WARN(q->pi_state || q->rt_waiter, "refusing to wake PI futex\n"))
-		return;
+		return false;
 
-	get_task_struct(p);
 	__futex_unqueue(q);
 	/*
 	 * The waiting task can free the futex_q as soon as q->lock_ptr = NULL
@@ -130,6 +121,26 @@ void futex_wake_mark(struct wake_q_head *wake_q, struct futex_q *q)
 	 */
 	smp_store_release(&q->lock_ptr, NULL);
 
+	return true;
+}
+
+/*
+ * The hash bucket lock must be held when this is called.
+ * Afterwards, the futex_q must not be accessed. Callers
+ * must ensure to later call wake_up_q() for the actual
+ * wakeups to occur.
+ */
+void futex_wake_mark(struct wake_q_head *wake_q, struct futex_q *q)
+{
+	struct task_struct *p = q->task;
+
+	get_task_struct(p);
+
+	if (!__futex_wake_mark(q)) {
+		put_task_struct(p);
+		return;
+	}
+
 	/*
 	 * Queue the task for later wakeup for after we've released
 	 * the hb->lock.
-- 
2.40.1

[PATCH 4/8] io_uring: add support for futex wake and wait

[Jens Axboe]

Add support for FUTEX_WAKE/WAIT primitives.

IORING_OP_FUTEX_WAKE is mix of FUTEX_WAKE and FUTEX_WAKE_BITSET, as
it does support passing in a bitset.

Similary, IORING_OP_FUTEX_WAIT is a mix of FUTEX_WAIT and
FUTEX_WAIT_BITSET.

FUTEX_WAKE is straight forward, as those can always be done directly from
the io_uring submission without needing async handling. For FUTEX_WAIT,
things are a bit more complicated. If the futex isn't ready, then we
rely on a callback via futex_queue->wake() when someone wakes up the
futex. From that calback, we queue up task_work with the original task,
which will post a CQE and wake it, if necessary.

Cancelations are supported, both from the application point-of-view,
but also to be able to cancel pending waits if the ring exits before
all events have occurred.

This is just the barebones wait/wake support. PI or REQUEUE support is
not added at this point, unclear if we might look into that later.

Likewise, explicit timeouts are not supported either. It is expected
that users that need timeouts would do so via the usual io_uring
mechanism to do that using linked timeouts.

Signed-off-by: Jens Axboe <axboe@kernel.dk>
---
 include/linux/io_uring_types.h |   3 +
 include/uapi/linux/io_uring.h  |   3 +
 io_uring/Makefile              |   4 +-
 io_uring/cancel.c              |   5 +
 io_uring/cancel.h              |   4 +
 io_uring/futex.c               | 232 +++++++++++++++++++++++++++++++++
 io_uring/futex.h               |  34 +++++
 io_uring/io_uring.c            |   5 +
 io_uring/opdef.c               |  24 +++-
 9 files changed, 312 insertions(+), 2 deletions(-)
 create mode 100644 io_uring/futex.c
 create mode 100644 io_uring/futex.h

diff --git a/include/linux/io_uring_types.h b/include/linux/io_uring_types.h
index f04ce513fadb..a7f03d8d879f 100644
--- a/include/linux/io_uring_types.h
+++ b/include/linux/io_uring_types.h
@@ -273,6 +273,9 @@ struct io_ring_ctx {
 	struct io_wq_work_list	locked_free_list;
 	unsigned int		locked_free_nr;
 
+	struct hlist_head	futex_list;
+	struct io_alloc_cache	futex_cache;
+
 	const struct cred	*sq_creds;	/* cred used for __io_sq_thread() */
 	struct io_sq_data	*sq_data;	/* if using sq thread polling */
 
diff --git a/include/uapi/linux/io_uring.h b/include/uapi/linux/io_uring.h
index 36f9c73082de..3bd2d765f593 100644
--- a/include/uapi/linux/io_uring.h
+++ b/include/uapi/linux/io_uring.h
@@ -65,6 +65,7 @@ struct io_uring_sqe {
 		__u32		xattr_flags;
 		__u32		msg_ring_flags;
 		__u32		uring_cmd_flags;
+		__u32		futex_flags;
 	};
 	__u64	user_data;	/* data to be passed back at completion time */
 	/* pack this to avoid bogus arm OABI complaints */
@@ -235,6 +236,8 @@ enum io_uring_op {
 	IORING_OP_URING_CMD,
 	IORING_OP_SEND_ZC,
 	IORING_OP_SENDMSG_ZC,
+	IORING_OP_FUTEX_WAIT,
+	IORING_OP_FUTEX_WAKE,
 
 	/* this goes last, obviously */
 	IORING_OP_LAST,
diff --git a/io_uring/Makefile b/io_uring/Makefile
index 8cc8e5387a75..2e4779bc550c 100644
--- a/io_uring/Makefile
+++ b/io_uring/Makefile
@@ -7,5 +7,7 @@ obj-$(CONFIG_IO_URING)		+= io_uring.o xattr.o nop.o fs.o splice.o \
 					openclose.o uring_cmd.o epoll.o \
 					statx.o net.o msg_ring.o timeout.o \
 					sqpoll.o fdinfo.o tctx.o poll.o \
-					cancel.o kbuf.o rsrc.o rw.o opdef.o notif.o
+					cancel.o kbuf.o rsrc.o rw.o opdef.o \
+					notif.o
 obj-$(CONFIG_IO_WQ)		+= io-wq.o
+obj-$(CONFIG_FUTEX)		+= futex.o
diff --git a/io_uring/cancel.c b/io_uring/cancel.c
index 7b23607cf4af..3dba8ccb1cd8 100644
--- a/io_uring/cancel.c
+++ b/io_uring/cancel.c
@@ -15,6 +15,7 @@
 #include "tctx.h"
 #include "poll.h"
 #include "timeout.h"
+#include "futex.h"
 #include "cancel.h"
 
 struct io_cancel {
@@ -119,6 +120,10 @@ int io_try_cancel(struct io_uring_task *tctx, struct io_cancel_data *cd,
 	if (ret != -ENOENT)
 		return ret;
 
+	ret = io_futex_cancel(ctx, cd, issue_flags);
+	if (ret != -ENOENT)
+		return ret;
+
 	spin_lock(&ctx->completion_lock);
 	if (!(cd->flags & IORING_ASYNC_CANCEL_FD))
 		ret = io_timeout_cancel(ctx, cd);
diff --git a/io_uring/cancel.h b/io_uring/cancel.h
index fc98622e6166..c0a8e7c520b6 100644
--- a/io_uring/cancel.h
+++ b/io_uring/cancel.h
@@ -1,4 +1,6 @@
 // SPDX-License-Identifier: GPL-2.0
+#ifndef IORING_CANCEL_H
+#define IORING_CANCEL_H
 
 #include <linux/io_uring_types.h>
 
@@ -22,3 +24,5 @@ void init_hash_table(struct io_hash_table *table, unsigned size);
 
 int io_sync_cancel(struct io_ring_ctx *ctx, void __user *arg);
 bool io_cancel_req_match(struct io_kiocb *req, struct io_cancel_data *cd);
+
+#endif
diff --git a/io_uring/futex.c b/io_uring/futex.c
new file mode 100644
index 000000000000..fa92ee795c83
--- /dev/null
+++ b/io_uring/futex.c
@@ -0,0 +1,232 @@
+// SPDX-License-Identifier: GPL-2.0
+#include <linux/kernel.h>
+#include <linux/errno.h>
+#include <linux/fs.h>
+#include <linux/file.h>
+#include <linux/io_uring.h>
+
+#include <uapi/linux/io_uring.h>
+
+#include "../kernel/futex/futex.h"
+#include "io_uring.h"
+#include "rsrc.h"
+#include "futex.h"
+
+struct io_futex {
+	struct file	*file;
+	u32 __user	*uaddr;
+	int		futex_op;
+	unsigned int	futex_val;
+	unsigned int	futex_flags;
+	unsigned int	futex_mask;
+};
+
+struct io_futex_data {
+	union {
+		struct futex_q		q;
+		struct io_cache_entry	cache;
+	};
+	struct io_kiocb	*req;
+};
+
+void io_futex_cache_init(struct io_ring_ctx *ctx)
+{
+	io_alloc_cache_init(&ctx->futex_cache, IO_NODE_ALLOC_CACHE_MAX,
+				sizeof(struct io_futex_data));
+}
+
+static void io_futex_cache_entry_free(struct io_cache_entry *entry)
+{
+	kfree(container_of(entry, struct io_futex_data, cache));
+}
+
+void io_futex_cache_free(struct io_ring_ctx *ctx)
+{
+	io_alloc_cache_free(&ctx->futex_cache, io_futex_cache_entry_free);
+}
+
+static void io_futex_complete(struct io_kiocb *req, struct io_tw_state *ts)
+{
+	struct io_futex_data *ifd = req->async_data;
+	struct io_ring_ctx *ctx = req->ctx;
+
+	io_tw_lock(ctx, ts);
+	if (!io_alloc_cache_put(&ctx->futex_cache, &ifd->cache))
+		kfree(ifd);
+	req->async_data = NULL;
+	hlist_del_init(&req->hash_node);
+	io_req_task_complete(req, ts);
+}
+
+static bool __io_futex_cancel(struct io_ring_ctx *ctx, struct io_kiocb *req)
+{
+	struct io_futex_data *ifd = req->async_data;
+
+	/* futex wake already done or in progress */
+	if (!futex_unqueue(&ifd->q))
+		return false;
+
+	hlist_del_init(&req->hash_node);
+	io_req_set_res(req, -ECANCELED, 0);
+	req->io_task_work.func = io_futex_complete;
+	io_req_task_work_add(req);
+	return true;
+}
+
+int io_futex_cancel(struct io_ring_ctx *ctx, struct io_cancel_data *cd,
+		    unsigned int issue_flags)
+{
+	struct hlist_node *tmp;
+	struct io_kiocb *req;
+	int nr = 0;
+
+	if (cd->flags & (IORING_ASYNC_CANCEL_FD|IORING_ASYNC_CANCEL_FD_FIXED))
+		return -ENOENT;
+
+	io_ring_submit_lock(ctx, issue_flags);
+	hlist_for_each_entry_safe(req, tmp, &ctx->futex_list, hash_node) {
+		if (req->cqe.user_data != cd->data &&
+		    !(cd->flags & IORING_ASYNC_CANCEL_ANY))
+			continue;
+		if (__io_futex_cancel(ctx, req))
+			nr++;
+		if (!(cd->flags & IORING_ASYNC_CANCEL_ALL))
+			break;
+	}
+	io_ring_submit_unlock(ctx, issue_flags);
+
+	if (nr)
+		return nr;
+
+	return -ENOENT;
+}
+
+bool io_futex_remove_all(struct io_ring_ctx *ctx, struct task_struct *task,
+			 bool cancel_all)
+{
+	struct hlist_node *tmp;
+	struct io_kiocb *req;
+	bool found = false;
+
+	lockdep_assert_held(&ctx->uring_lock);
+
+	hlist_for_each_entry_safe(req, tmp, &ctx->futex_list, hash_node) {
+		if (!io_match_task_safe(req, task, cancel_all))
+			continue;
+		__io_futex_cancel(ctx, req);
+		found = true;
+	}
+
+	return found;
+}
+
+int io_futex_prep(struct io_kiocb *req, const struct io_uring_sqe *sqe)
+{
+	struct io_futex *iof = io_kiocb_to_cmd(req, struct io_futex);
+
+	if (unlikely(sqe->addr2 || sqe->buf_index || sqe->addr3))
+		return -EINVAL;
+
+	iof->futex_op = READ_ONCE(sqe->fd);
+	iof->uaddr = u64_to_user_ptr(READ_ONCE(sqe->addr));
+	iof->futex_val = READ_ONCE(sqe->len);
+	iof->futex_mask = READ_ONCE(sqe->file_index);
+	iof->futex_flags = READ_ONCE(sqe->futex_flags);
+	if (iof->futex_flags & FUTEX_CMD_MASK)
+		return -EINVAL;
+
+	return 0;
+}
+
+static void io_futex_wake_fn(struct wake_q_head *wake_q, struct futex_q *q)
+{
+	struct io_futex_data *ifd = container_of(q, struct io_futex_data, q);
+	struct io_kiocb *req = ifd->req;
+
+	if (unlikely(!__futex_wake_mark(q)))
+		return;
+
+	io_req_set_res(req, 0, 0);
+	req->io_task_work.func = io_futex_complete;
+	io_req_task_work_add(req);
+}
+
+static struct io_futex_data *io_alloc_ifd(struct io_ring_ctx *ctx)
+{
+	struct io_cache_entry *entry;
+
+	entry = io_alloc_cache_get(&ctx->futex_cache);
+	if (entry)
+		return container_of(entry, struct io_futex_data, cache);
+
+	return kmalloc(sizeof(struct io_futex_data), GFP_NOWAIT);
+}
+
+int io_futex_wait(struct io_kiocb *req, unsigned int issue_flags)
+{
+	struct io_futex *iof = io_kiocb_to_cmd(req, struct io_futex);
+	struct io_ring_ctx *ctx = req->ctx;
+	struct io_futex_data *ifd = NULL;
+	struct futex_hash_bucket *hb;
+	unsigned int flags;
+	int ret;
+
+	if (!iof->futex_mask) {
+		ret = -EINVAL;
+		goto done;
+	}
+	if (!futex_op_to_flags(FUTEX_WAIT, iof->futex_flags, &flags)) {
+		ret = -ENOSYS;
+		goto done;
+	}
+
+	io_ring_submit_lock(ctx, issue_flags);
+	ifd = io_alloc_ifd(ctx);
+	if (!ifd) {
+		ret = -ENOMEM;
+		goto done_unlock;
+	}
+
+	req->async_data = ifd;
+	ifd->q = futex_q_init;
+	ifd->q.bitset = iof->futex_mask;
+	ifd->q.wake = io_futex_wake_fn;
+	ifd->req = req;
+
+	ret = futex_wait_setup(iof->uaddr, iof->futex_val, flags, &ifd->q, &hb);
+	if (!ret) {
+		hlist_add_head(&req->hash_node, &ctx->futex_list);
+		io_ring_submit_unlock(ctx, issue_flags);
+
+		futex_queue(&ifd->q, hb);
+		return IOU_ISSUE_SKIP_COMPLETE;
+	}
+
+done_unlock:
+	io_ring_submit_unlock(ctx, issue_flags);
+done:
+	if (ret < 0)
+		req_set_fail(req);
+	io_req_set_res(req, ret, 0);
+	kfree(ifd);
+	return IOU_OK;
+}
+
+int io_futex_wake(struct io_kiocb *req, unsigned int issue_flags)
+{
+	struct io_futex *iof = io_kiocb_to_cmd(req, struct io_futex);
+	unsigned int flags = 0;
+	int ret;
+
+	if (!futex_op_to_flags(FUTEX_WAKE, iof->futex_flags, &flags)) {
+		ret = -ENOSYS;
+		goto done;
+	}
+
+	ret = futex_wake(iof->uaddr, flags, iof->futex_val, iof->futex_mask);
+done:
+	if (ret < 0)
+		req_set_fail(req);
+	io_req_set_res(req, ret, 0);
+	return IOU_OK;
+}
diff --git a/io_uring/futex.h b/io_uring/futex.h
new file mode 100644
index 000000000000..ddc9e0d73c52
--- /dev/null
+++ b/io_uring/futex.h
@@ -0,0 +1,34 @@
+// SPDX-License-Identifier: GPL-2.0
+
+#include "cancel.h"
+
+int io_futex_prep(struct io_kiocb *req, const struct io_uring_sqe *sqe);
+int io_futex_wait(struct io_kiocb *req, unsigned int issue_flags);
+int io_futex_wake(struct io_kiocb *req, unsigned int issue_flags);
+
+#if defined(CONFIG_FUTEX)
+int io_futex_cancel(struct io_ring_ctx *ctx, struct io_cancel_data *cd,
+		    unsigned int issue_flags);
+bool io_futex_remove_all(struct io_ring_ctx *ctx, struct task_struct *task,
+			 bool cancel_all);
+void io_futex_cache_init(struct io_ring_ctx *ctx);
+void io_futex_cache_free(struct io_ring_ctx *ctx);
+#else
+static inline int io_futex_cancel(struct io_ring_ctx *ctx,
+				  struct io_cancel_data *cd,
+				  unsigned int issue_flags)
+{
+	return 0;
+}
+static inline bool io_futex_remove_all(struct io_ring_ctx *ctx,
+				       struct task_struct *task, bool cancel_all)
+{
+	return false;
+}
+static inline void io_futex_cache_init(struct io_ring_ctx *ctx)
+{
+}
+static inline void io_futex_cache_free(struct io_ring_ctx *ctx)
+{
+}
+#endif
diff --git a/io_uring/io_uring.c b/io_uring/io_uring.c
index e8096d502a7c..67ff148bc394 100644
--- a/io_uring/io_uring.c
+++ b/io_uring/io_uring.c
@@ -92,6 +92,7 @@
 #include "cancel.h"
 #include "net.h"
 #include "notif.h"
+#include "futex.h"
 
 #include "timeout.h"
 #include "poll.h"
@@ -314,6 +315,7 @@ static __cold struct io_ring_ctx *io_ring_ctx_alloc(struct io_uring_params *p)
 			    sizeof(struct async_poll));
 	io_alloc_cache_init(&ctx->netmsg_cache, IO_ALLOC_CACHE_MAX,
 			    sizeof(struct io_async_msghdr));
+	io_futex_cache_init(ctx);
 	init_completion(&ctx->ref_comp);
 	xa_init_flags(&ctx->personalities, XA_FLAGS_ALLOC1);
 	mutex_init(&ctx->uring_lock);
@@ -333,6 +335,7 @@ static __cold struct io_ring_ctx *io_ring_ctx_alloc(struct io_uring_params *p)
 	INIT_LIST_HEAD(&ctx->tctx_list);
 	ctx->submit_state.free_list.next = NULL;
 	INIT_WQ_LIST(&ctx->locked_free_list);
+	INIT_HLIST_HEAD(&ctx->futex_list);
 	INIT_DELAYED_WORK(&ctx->fallback_work, io_fallback_req_func);
 	INIT_WQ_LIST(&ctx->submit_state.compl_reqs);
 	return ctx;
@@ -2842,6 +2845,7 @@ static __cold void io_ring_ctx_free(struct io_ring_ctx *ctx)
 	io_eventfd_unregister(ctx);
 	io_alloc_cache_free(&ctx->apoll_cache, io_apoll_cache_free);
 	io_alloc_cache_free(&ctx->netmsg_cache, io_netmsg_cache_free);
+	io_futex_cache_free(ctx);
 	io_destroy_buffers(ctx);
 	mutex_unlock(&ctx->uring_lock);
 	if (ctx->sq_creds)
@@ -3254,6 +3258,7 @@ static __cold bool io_uring_try_cancel_requests(struct io_ring_ctx *ctx,
 	ret |= io_cancel_defer_files(ctx, task, cancel_all);
 	mutex_lock(&ctx->uring_lock);
 	ret |= io_poll_remove_all(ctx, task, cancel_all);
+	ret |= io_futex_remove_all(ctx, task, cancel_all);
 	mutex_unlock(&ctx->uring_lock);
 	ret |= io_kill_timeouts(ctx, task, cancel_all);
 	if (task)
diff --git a/io_uring/opdef.c b/io_uring/opdef.c
index 3b9c6489b8b6..c9f23c21a031 100644
--- a/io_uring/opdef.c
+++ b/io_uring/opdef.c
@@ -33,6 +33,7 @@
 #include "poll.h"
 #include "cancel.h"
 #include "rw.h"
+#include "futex.h"
 
 static int io_no_issue(struct io_kiocb *req, unsigned int issue_flags)
 {
@@ -426,11 +427,26 @@ const struct io_issue_def io_issue_defs[] = {
 		.issue			= io_sendmsg_zc,
 #else
 		.prep			= io_eopnotsupp_prep,
+#endif
+	},
+	[IORING_OP_FUTEX_WAIT] = {
+#if defined(CONFIG_FUTEX)
+		.prep			= io_futex_prep,
+		.issue			= io_futex_wait,
+#else
+		.prep			= io_eopnotsupp_prep,
+#endif
+	},
+	[IORING_OP_FUTEX_WAKE] = {
+#if defined(CONFIG_FUTEX)
+		.prep			= io_futex_prep,
+		.issue			= io_futex_wake,
+#else
+		.prep			= io_eopnotsupp_prep,
 #endif
 	},
 };
 
-
 const struct io_cold_def io_cold_defs[] = {
 	[IORING_OP_NOP] = {
 		.name			= "NOP",
@@ -648,6 +664,12 @@ const struct io_cold_def io_cold_defs[] = {
 		.fail			= io_sendrecv_fail,
 #endif
 	},
+	[IORING_OP_FUTEX_WAIT] = {
+		.name			= "FUTEX_WAIT",
+	},
+	[IORING_OP_FUTEX_WAKE] = {
+		.name			= "FUTEX_WAKE",
+	},
 };
 
 const char *io_uring_get_opcode(u8 opcode)
-- 
2.40.1

Re: [PATCH 4/8] io_uring: add support for futex wake and wait

[Peter Zijlstra]

On Wed, Jul 12, 2023 at 10:20:13AM -0600, Jens Axboe wrote:

> +int io_futex_prep(struct io_kiocb *req, const struct io_uring_sqe *sqe)
> +{
> +	struct io_futex *iof = io_kiocb_to_cmd(req, struct io_futex);
> +
> +	if (unlikely(sqe->addr2 || sqe->buf_index || sqe->addr3))
> +		return -EINVAL;
> +
> +	iof->futex_op = READ_ONCE(sqe->fd);
> +	iof->uaddr = u64_to_user_ptr(READ_ONCE(sqe->addr));
> +	iof->futex_val = READ_ONCE(sqe->len);
> +	iof->futex_mask = READ_ONCE(sqe->file_index);
> +	iof->futex_flags = READ_ONCE(sqe->futex_flags);
> +	if (iof->futex_flags & FUTEX_CMD_MASK)
> +		return -EINVAL;
> +
> +	return 0;
> +}

I'm a little confused on the purpose of iof->futex_op, it doesn't appear
to be used. Instead iof->futex_flags is used as the ~FUTEX_CMD_MASK part
of ops.

The latter actually makes sense since you encode the actual op in the
IOURING_OP_ space.

> +int io_futex_wait(struct io_kiocb *req, unsigned int issue_flags)
> +{
> +	struct io_futex *iof = io_kiocb_to_cmd(req, struct io_futex);
> +	struct io_ring_ctx *ctx = req->ctx;
> +	struct io_futex_data *ifd = NULL;
> +	struct futex_hash_bucket *hb;
> +	unsigned int flags;
> +	int ret;
> +
> +	if (!iof->futex_mask) {
> +		ret = -EINVAL;
> +		goto done;
> +	}
> +	if (!futex_op_to_flags(FUTEX_WAIT, iof->futex_flags, &flags)) {

A little confusing since you then implement FUTEX_WAIT_BITSET, but using
FUTEX_WAIT ensures this goes -ENOSYS when setting FUTEX_CLOCK_REALTIME,
since you handle timeouts through the iouring thing.

Perhaps a comment?

> +		ret = -ENOSYS;
> +		goto done;
> +	}
> +
> +	io_ring_submit_lock(ctx, issue_flags);
> +	ifd = io_alloc_ifd(ctx);
> +	if (!ifd) {
> +		ret = -ENOMEM;
> +		goto done_unlock;
> +	}
> +
> +	req->async_data = ifd;
> +	ifd->q = futex_q_init;
> +	ifd->q.bitset = iof->futex_mask;
> +	ifd->q.wake = io_futex_wake_fn;
> +	ifd->req = req;
> +
> +	ret = futex_wait_setup(iof->uaddr, iof->futex_val, flags, &ifd->q, &hb);
> +	if (!ret) {
> +		hlist_add_head(&req->hash_node, &ctx->futex_list);
> +		io_ring_submit_unlock(ctx, issue_flags);
> +
> +		futex_queue(&ifd->q, hb);
> +		return IOU_ISSUE_SKIP_COMPLETE;
> +	}
> +
> +done_unlock:
> +	io_ring_submit_unlock(ctx, issue_flags);
> +done:
> +	if (ret < 0)
> +		req_set_fail(req);
> +	io_req_set_res(req, ret, 0);
> +	kfree(ifd);
> +	return IOU_OK;
> +}

Other than that, I think these things are indeed transparant wrt the
existing futex interface. If we add a flag this shouldn't care.

Re: [PATCH 4/8] io_uring: add support for futex wake and wait

[Jens Axboe]

On 7/13/23 5:15?AM, Peter Zijlstra wrote:
> On Wed, Jul 12, 2023 at 10:20:13AM -0600, Jens Axboe wrote:
> 
>> +int io_futex_prep(struct io_kiocb *req, const struct io_uring_sqe *sqe)
>> +{
>> +	struct io_futex *iof = io_kiocb_to_cmd(req, struct io_futex);
>> +
>> +	if (unlikely(sqe->addr2 || sqe->buf_index || sqe->addr3))
>> +		return -EINVAL;
>> +
>> +	iof->futex_op = READ_ONCE(sqe->fd);
>> +	iof->uaddr = u64_to_user_ptr(READ_ONCE(sqe->addr));
>> +	iof->futex_val = READ_ONCE(sqe->len);
>> +	iof->futex_mask = READ_ONCE(sqe->file_index);
>> +	iof->futex_flags = READ_ONCE(sqe->futex_flags);
>> +	if (iof->futex_flags & FUTEX_CMD_MASK)
>> +		return -EINVAL;
>> +
>> +	return 0;
>> +}
> 
> I'm a little confused on the purpose of iof->futex_op, it doesn't appear
> to be used. Instead iof->futex_flags is used as the ~FUTEX_CMD_MASK part
> of ops.
> 
> The latter actually makes sense since you encode the actual op in the
> IOURING_OP_ space.

Yep, I think this is also a leftover from when I had it multiplexed a
bit more. The liburing side got fixed for that, but neglected this bit.
Good catch. I'll fold the below in.

> 
>> +int io_futex_wait(struct io_kiocb *req, unsigned int issue_flags)
>> +{
>> +	struct io_futex *iof = io_kiocb_to_cmd(req, struct io_futex);
>> +	struct io_ring_ctx *ctx = req->ctx;
>> +	struct io_futex_data *ifd = NULL;
>> +	struct futex_hash_bucket *hb;
>> +	unsigned int flags;
>> +	int ret;
>> +
>> +	if (!iof->futex_mask) {
>> +		ret = -EINVAL;
>> +		goto done;
>> +	}
>> +	if (!futex_op_to_flags(FUTEX_WAIT, iof->futex_flags, &flags)) {
> 
> A little confusing since you then implement FUTEX_WAIT_BITSET, but using
> FUTEX_WAIT ensures this goes -ENOSYS when setting FUTEX_CLOCK_REALTIME,
> since you handle timeouts through the iouring thing.
> 
> Perhaps a comment?

OK, will add a comment on that.

>> +		ret = -ENOSYS;
>> +		goto done;
>> +	}
>> +
>> +	io_ring_submit_lock(ctx, issue_flags);
>> +	ifd = io_alloc_ifd(ctx);
>> +	if (!ifd) {
>> +		ret = -ENOMEM;
>> +		goto done_unlock;
>> +	}
>> +
>> +	req->async_data = ifd;
>> +	ifd->q = futex_q_init;
>> +	ifd->q.bitset = iof->futex_mask;
>> +	ifd->q.wake = io_futex_wake_fn;
>> +	ifd->req = req;
>> +
>> +	ret = futex_wait_setup(iof->uaddr, iof->futex_val, flags, &ifd->q, &hb);
>> +	if (!ret) {
>> +		hlist_add_head(&req->hash_node, &ctx->futex_list);
>> +		io_ring_submit_unlock(ctx, issue_flags);
>> +
>> +		futex_queue(&ifd->q, hb);
>> +		return IOU_ISSUE_SKIP_COMPLETE;
>> +	}
>> +
>> +done_unlock:
>> +	io_ring_submit_unlock(ctx, issue_flags);
>> +done:
>> +	if (ret < 0)
>> +		req_set_fail(req);
>> +	io_req_set_res(req, ret, 0);
>> +	kfree(ifd);
>> +	return IOU_OK;
>> +}
> 
> Other than that, I think these things are indeed transparant wrt the
> existing futex interface. If we add a flag this shouldn't care.

Not sure I follow, what kind of flag do you want/need?


diff --git a/io_uring/futex.c b/io_uring/futex.c
index df65b8f3593f..bced11c87896 100644
--- a/io_uring/futex.c
+++ b/io_uring/futex.c
@@ -18,7 +18,6 @@ struct io_futex {
 		u32 __user			*uaddr;
 		struct futex_waitv __user	*uwaitv;
 	};
-	int		futex_op;
 	unsigned int	futex_val;
 	unsigned int	futex_flags;
 	unsigned int	futex_mask;
@@ -173,10 +172,9 @@ int io_futex_prep(struct io_kiocb *req, const struct io_uring_sqe *sqe)
 {
 	struct io_futex *iof = io_kiocb_to_cmd(req, struct io_futex);
 
-	if (unlikely(sqe->buf_index || sqe->addr3))
+	if (unlikely(sqe->fd || sqe->buf_index || sqe->addr3))
 		return -EINVAL;
 
-	iof->futex_op = READ_ONCE(sqe->fd);
 	iof->uaddr = u64_to_user_ptr(READ_ONCE(sqe->addr));
 	iof->futex_val = READ_ONCE(sqe->len);
 	iof->futex_mask = READ_ONCE(sqe->file_index);

-- 
Jens Axboe

Re: [PATCH 4/8] io_uring: add support for futex wake and wait

[Peter Zijlstra]

On Thu, Jul 13, 2023 at 01:15:13PM +0200, Peter Zijlstra wrote:
> On Wed, Jul 12, 2023 at 10:20:13AM -0600, Jens Axboe wrote:
> 
> > +int io_futex_prep(struct io_kiocb *req, const struct io_uring_sqe *sqe)
> > +{
> > +	struct io_futex *iof = io_kiocb_to_cmd(req, struct io_futex);
> > +
> > +	if (unlikely(sqe->addr2 || sqe->buf_index || sqe->addr3))
> > +		return -EINVAL;
> > +
> > +	iof->futex_op = READ_ONCE(sqe->fd);
> > +	iof->uaddr = u64_to_user_ptr(READ_ONCE(sqe->addr));
> > +	iof->futex_val = READ_ONCE(sqe->len);
> > +	iof->futex_mask = READ_ONCE(sqe->file_index);
> > +	iof->futex_flags = READ_ONCE(sqe->futex_flags);
> > +	if (iof->futex_flags & FUTEX_CMD_MASK)
> > +		return -EINVAL;
> > +
> > +	return 0;
> > +}
> 
> I'm a little confused on the purpose of iof->futex_op, it doesn't appear
> to be used. Instead iof->futex_flags is used as the ~FUTEX_CMD_MASK part
> of ops.
> 
> The latter actually makes sense since you encode the actual op in the
> IOURING_OP_ space.

Futex is slowly getting back to me; I'm thinking these io_uring
interfaces should perhaps use the futex2 flags instead of the futex_op
encoded muck.

I'll try and send out a few patches tomorrow to clarify things a little
-- the futex2 work seems to have stalled somewhere halfway :/

Re: [PATCH 4/8] io_uring: add support for futex wake and wait

[Jens Axboe]

On 7/13/23 11:24?AM, Peter Zijlstra wrote:
> On Thu, Jul 13, 2023 at 01:15:13PM +0200, Peter Zijlstra wrote:
>> On Wed, Jul 12, 2023 at 10:20:13AM -0600, Jens Axboe wrote:
>>
>>> +int io_futex_prep(struct io_kiocb *req, const struct io_uring_sqe *sqe)
>>> +{
>>> +	struct io_futex *iof = io_kiocb_to_cmd(req, struct io_futex);
>>> +
>>> +	if (unlikely(sqe->addr2 || sqe->buf_index || sqe->addr3))
>>> +		return -EINVAL;
>>> +
>>> +	iof->futex_op = READ_ONCE(sqe->fd);
>>> +	iof->uaddr = u64_to_user_ptr(READ_ONCE(sqe->addr));
>>> +	iof->futex_val = READ_ONCE(sqe->len);
>>> +	iof->futex_mask = READ_ONCE(sqe->file_index);
>>> +	iof->futex_flags = READ_ONCE(sqe->futex_flags);
>>> +	if (iof->futex_flags & FUTEX_CMD_MASK)
>>> +		return -EINVAL;
>>> +
>>> +	return 0;
>>> +}
>>
>> I'm a little confused on the purpose of iof->futex_op, it doesn't appear
>> to be used. Instead iof->futex_flags is used as the ~FUTEX_CMD_MASK part
>> of ops.
>>
>> The latter actually makes sense since you encode the actual op in the
>> IOURING_OP_ space.
> 
> Futex is slowly getting back to me; I'm thinking these io_uring
> interfaces should perhaps use the futex2 flags instead of the futex_op
> encoded muck.
> 
> I'll try and send out a few patches tomorrow to clarify things a little
> -- the futex2 work seems to have stalled somewhere halfway :/

Saw your series - I'll take a look. In terms of staging when we get
there, would it be possible to split your flags series into the bare
minimum and trivial, and then have that as a dependency for this series
and the rest of your series?

-- 
Jens Axboe

Re: [PATCH 4/8] io_uring: add support for futex wake and wait

[Peter Zijlstra]

On Fri, Jul 14, 2023 at 08:52:40AM -0600, Jens Axboe wrote:
> Saw your series - I'll take a look. In terms of staging when we get
> there, would it be possible to split your flags series into the bare
> minimum and trivial, and then have that as a dependency for this series
> and the rest of your series?

I think you only really need the first three patches, and I hope those
are the least controversial of the lot.

After those, I can implement the extra flags independently of the
io_uring thing and all interfaces should just have it work.

So yes :-)

Re: [PATCH 4/8] io_uring: add support for futex wake and wait

[Jens Axboe]

On 7/14/23 9:08?AM, Peter Zijlstra wrote:
> On Fri, Jul 14, 2023 at 08:52:40AM -0600, Jens Axboe wrote:
>> Saw your series - I'll take a look. In terms of staging when we get
>> there, would it be possible to split your flags series into the bare
>> minimum and trivial, and then have that as a dependency for this series
>> and the rest of your series?
> 
> I think you only really need the first three patches, and I hope those
> are the least controversial of the lot.

That's what it looks like to me, and yes those look trivial and risk
free :-)

I'll just put those at the base of my series for now, and once they are
stamped, would be great if you could then stuff them in a stable branch
that I could pull in.

> After those, I can implement the extra flags independently of the
> io_uring thing and all interfaces should just have it work.
> 
> So yes :-)

That's the plan then!

-- 
Jens Axboe

[PATCH 5/8] futex: add wake_data to struct futex_q

[Jens Axboe]

With handling multiple futex_q for waitv, we cannot easily go from the
futex_q to data related to that request or queue. Add a wake_data
argument that belongs to the wake handler assigned.

Signed-off-by: Jens Axboe <axboe@kernel.dk>
---
 kernel/futex/futex.h | 1 +
 1 file changed, 1 insertion(+)

diff --git a/kernel/futex/futex.h b/kernel/futex/futex.h
index 1d531244fb71..24180a95bdc8 100644
--- a/kernel/futex/futex.h
+++ b/kernel/futex/futex.h
@@ -102,6 +102,7 @@ struct futex_q {
 	struct task_struct *task;
 	spinlock_t *lock_ptr;
 	futex_wake_fn *wake;
+	void *wake_data;
 	union futex_key key;
 	struct futex_pi_state *pi_state;
 	struct rt_mutex_waiter *rt_waiter;
-- 
2.40.1

[PATCH 6/8] futex: make futex_parse_waitv() available as a helper

[Jens Axboe]

To make it more generically useful, augment it with allowing the caller
to pass in the wake handler and wake data. Convert the futex_waitv()
syscall, passing in the default handlers.

Signed-off-by: Jens Axboe <axboe@kernel.dk>
---
 kernel/futex/futex.h    |  5 +++++
 kernel/futex/syscalls.c | 14 ++++++++++----
 2 files changed, 15 insertions(+), 4 deletions(-)

diff --git a/kernel/futex/futex.h b/kernel/futex/futex.h
index 24180a95bdc8..f6598d8451fb 100644
--- a/kernel/futex/futex.h
+++ b/kernel/futex/futex.h
@@ -285,6 +285,11 @@ struct futex_vector {
 	struct futex_q q;
 };
 
+extern int futex_parse_waitv(struct futex_vector *futexv,
+			     struct futex_waitv __user *uwaitv,
+			     unsigned int nr_futexes, futex_wake_fn *wake,
+			     void *wake_data);
+
 extern int futex_wait_multiple(struct futex_vector *vs, unsigned int count,
 			       struct hrtimer_sleeper *to);
 
diff --git a/kernel/futex/syscalls.c b/kernel/futex/syscalls.c
index 0b63d5bcdc77..fc3cdd0fefab 100644
--- a/kernel/futex/syscalls.c
+++ b/kernel/futex/syscalls.c
@@ -184,12 +184,15 @@ SYSCALL_DEFINE6(futex, u32 __user *, uaddr, int, op, u32, val,
  * @futexv:	Kernel side list of waiters to be filled
  * @uwaitv:     Userspace list to be parsed
  * @nr_futexes: Length of futexv
+ * @wake:	Wake to call when futex is woken
+ * @wake_data:	Data for the wake handler
  *
  * Return: Error code on failure, 0 on success
  */
-static int futex_parse_waitv(struct futex_vector *futexv,
-			     struct futex_waitv __user *uwaitv,
-			     unsigned int nr_futexes)
+int futex_parse_waitv(struct futex_vector *futexv,
+		      struct futex_waitv __user *uwaitv,
+		      unsigned int nr_futexes, futex_wake_fn *wake,
+		      void *wake_data)
 {
 	struct futex_waitv aux;
 	unsigned int i;
@@ -208,6 +211,8 @@ static int futex_parse_waitv(struct futex_vector *futexv,
 		futexv[i].w.val = aux.val;
 		futexv[i].w.uaddr = aux.uaddr;
 		futexv[i].q = futex_q_init;
+		futexv[i].q.wake = wake;
+		futexv[i].q.wake_data = wake_data;
 	}
 
 	return 0;
@@ -284,7 +289,8 @@ SYSCALL_DEFINE5(futex_waitv, struct futex_waitv __user *, waiters,
 		goto destroy_timer;
 	}
 
-	ret = futex_parse_waitv(futexv, waiters, nr_futexes);
+	ret = futex_parse_waitv(futexv, waiters, nr_futexes, futex_wake_mark,
+				NULL);
 	if (!ret)
 		ret = futex_wait_multiple(futexv, nr_futexes, timeout ? &to : NULL);
 
-- 
2.40.1

[PATCH 7/8] futex: make the vectored futex operations available

[Jens Axboe]

Rename unqueue_multiple() as futex_unqueue_multiple(), and make both
that and futex_wait_multiple_setup() available for external users. This
is in preparation for wiring up vectored waits in io_uring.

Signed-off-by: Jens Axboe <axboe@kernel.dk>
---
 kernel/futex/futex.h    |  5 +++++
 kernel/futex/waitwake.c | 10 +++++-----
 2 files changed, 10 insertions(+), 5 deletions(-)

diff --git a/kernel/futex/futex.h b/kernel/futex/futex.h
index f6598d8451fb..4d73d2978e50 100644
--- a/kernel/futex/futex.h
+++ b/kernel/futex/futex.h
@@ -290,6 +290,11 @@ extern int futex_parse_waitv(struct futex_vector *futexv,
 			     unsigned int nr_futexes, futex_wake_fn *wake,
 			     void *wake_data);
 
+extern int futex_wait_multiple_setup(struct futex_vector *vs, int count,
+				     int *woken);
+
+extern int futex_unqueue_multiple(struct futex_vector *v, int count);
+
 extern int futex_wait_multiple(struct futex_vector *vs, unsigned int count,
 			       struct hrtimer_sleeper *to);
 
diff --git a/kernel/futex/waitwake.c b/kernel/futex/waitwake.c
index f8fb6550061d..0383da9f737f 100644
--- a/kernel/futex/waitwake.c
+++ b/kernel/futex/waitwake.c
@@ -369,7 +369,7 @@ void futex_wait_queue(struct futex_hash_bucket *hb, struct futex_q *q,
 }
 
 /**
- * unqueue_multiple - Remove various futexes from their hash bucket
+ * futex_unqueue_multiple - Remove various futexes from their hash bucket
  * @v:	   The list of futexes to unqueue
  * @count: Number of futexes in the list
  *
@@ -379,7 +379,7 @@ void futex_wait_queue(struct futex_hash_bucket *hb, struct futex_q *q,
  *  - >=0 - Index of the last futex that was awoken;
  *  - -1  - No futex was awoken
  */
-static int unqueue_multiple(struct futex_vector *v, int count)
+int futex_unqueue_multiple(struct futex_vector *v, int count)
 {
 	int ret = -1, i;
 
@@ -407,7 +407,7 @@ static int unqueue_multiple(struct futex_vector *v, int count)
  *  -  0 - Success
  *  - <0 - -EFAULT, -EWOULDBLOCK or -EINVAL
  */
-static int futex_wait_multiple_setup(struct futex_vector *vs, int count, int *woken)
+int futex_wait_multiple_setup(struct futex_vector *vs, int count, int *woken)
 {
 	struct futex_hash_bucket *hb;
 	bool retry = false;
@@ -469,7 +469,7 @@ static int futex_wait_multiple_setup(struct futex_vector *vs, int count, int *wo
 		 * was woken, we don't return error and return this index to
 		 * userspace
 		 */
-		*woken = unqueue_multiple(vs, i);
+		*woken = futex_unqueue_multiple(vs, i);
 		if (*woken >= 0)
 			return 1;
 
@@ -554,7 +554,7 @@ int futex_wait_multiple(struct futex_vector *vs, unsigned int count,
 
 		__set_current_state(TASK_RUNNING);
 
-		ret = unqueue_multiple(vs, count);
+		ret = futex_unqueue_multiple(vs, count);
 		if (ret >= 0)
 			return ret;
 
-- 
2.40.1

[PATCH 8/8] io_uring: add support for vectored futex waits

[Jens Axboe]

This adds support for IORING_OP_FUTEX_WAITV, which allows registering a
notification for a number of futexes at once. If one of the futexes are
woken, then the request will complete with the index of the futex that got
woken as the result. This is identical to what the normal vectored futex
waitv operation does.

Use like IORING_OP_FUTEX_WAIT, except sqe->addr must now contain the a
pointer to a struct futex_waitv array, and sqe->off must now contain the
number of elements in that array.

Waiting on N futexes could be done with IORING_OP_FUTEX_WAIT as well,
but that punts a lot of the work to the application:

1) Application would need to submit N IORING_OP_FUTEX_WAIT requests,
   rather than just a single IORING_OP_FUTEX_WAITV.

2) When one futex is woken, application would need to cancel the
   remaining N-1 requests that didn't trigger.

While this is of course doable, having a single vectored futex wait
makes for much simpler application code.

Signed-off-by: Jens Axboe <axboe@kernel.dk>
---
 include/uapi/linux/io_uring.h |   1 +
 io_uring/futex.c              | 164 +++++++++++++++++++++++++++++++---
 io_uring/futex.h              |   2 +
 io_uring/opdef.c              |  11 +++
 4 files changed, 168 insertions(+), 10 deletions(-)

diff --git a/include/uapi/linux/io_uring.h b/include/uapi/linux/io_uring.h
index 3bd2d765f593..420f38675769 100644
--- a/include/uapi/linux/io_uring.h
+++ b/include/uapi/linux/io_uring.h
@@ -238,6 +238,7 @@ enum io_uring_op {
 	IORING_OP_SENDMSG_ZC,
 	IORING_OP_FUTEX_WAIT,
 	IORING_OP_FUTEX_WAKE,
+	IORING_OP_FUTEX_WAITV,
 
 	/* this goes last, obviously */
 	IORING_OP_LAST,
diff --git a/io_uring/futex.c b/io_uring/futex.c
index fa92ee795c83..df65b8f3593f 100644
--- a/io_uring/futex.c
+++ b/io_uring/futex.c
@@ -14,11 +14,16 @@
 
 struct io_futex {
 	struct file	*file;
-	u32 __user	*uaddr;
+	union {
+		u32 __user			*uaddr;
+		struct futex_waitv __user	*uwaitv;
+	};
 	int		futex_op;
 	unsigned int	futex_val;
 	unsigned int	futex_flags;
 	unsigned int	futex_mask;
+	unsigned int	futex_nr;
+	unsigned long	futexv_owned;
 };
 
 struct io_futex_data {
@@ -45,6 +50,13 @@ void io_futex_cache_free(struct io_ring_ctx *ctx)
 	io_alloc_cache_free(&ctx->futex_cache, io_futex_cache_entry_free);
 }
 
+static void __io_futex_complete(struct io_kiocb *req, struct io_tw_state *ts)
+{
+	req->async_data = NULL;
+	hlist_del_init(&req->hash_node);
+	io_req_task_complete(req, ts);
+}
+
 static void io_futex_complete(struct io_kiocb *req, struct io_tw_state *ts)
 {
 	struct io_futex_data *ifd = req->async_data;
@@ -53,22 +65,59 @@ static void io_futex_complete(struct io_kiocb *req, struct io_tw_state *ts)
 	io_tw_lock(ctx, ts);
 	if (!io_alloc_cache_put(&ctx->futex_cache, &ifd->cache))
 		kfree(ifd);
-	req->async_data = NULL;
-	hlist_del_init(&req->hash_node);
-	io_req_task_complete(req, ts);
+	__io_futex_complete(req, ts);
 }
 
-static bool __io_futex_cancel(struct io_ring_ctx *ctx, struct io_kiocb *req)
+static void io_futexv_complete(struct io_kiocb *req, struct io_tw_state *ts)
 {
-	struct io_futex_data *ifd = req->async_data;
+	struct io_futex *iof = io_kiocb_to_cmd(req, struct io_futex);
+	struct futex_vector *futexv = req->async_data;
+	struct io_ring_ctx *ctx = req->ctx;
+	int res = 0;
 
-	/* futex wake already done or in progress */
-	if (!futex_unqueue(&ifd->q))
+	io_tw_lock(ctx, ts);
+
+	res = futex_unqueue_multiple(futexv, iof->futex_nr);
+	if (res != -1)
+		io_req_set_res(req, res, 0);
+
+	kfree(req->async_data);
+	req->flags &= ~REQ_F_ASYNC_DATA;
+	__io_futex_complete(req, ts);
+}
+
+static bool io_futexv_claimed(struct io_futex *iof)
+{
+	return test_bit(0, &iof->futexv_owned);
+}
+
+static bool io_futexv_claim(struct io_futex *iof)
+{
+	if (test_bit(0, &iof->futexv_owned) ||
+	    test_and_set_bit(0, &iof->futexv_owned))
 		return false;
+	return true;
+}
+
+static bool __io_futex_cancel(struct io_ring_ctx *ctx, struct io_kiocb *req)
+{
+	/* futex wake already done or in progress */
+	if (req->opcode == IORING_OP_FUTEX_WAIT) {
+		struct io_futex_data *ifd = req->async_data;
+
+		if (!futex_unqueue(&ifd->q))
+			return false;
+		req->io_task_work.func = io_futex_complete;
+	} else {
+		struct io_futex *iof = io_kiocb_to_cmd(req, struct io_futex);
+
+		if (!io_futexv_claim(iof))
+			return false;
+		req->io_task_work.func = io_futexv_complete;
+	}
 
 	hlist_del_init(&req->hash_node);
 	io_req_set_res(req, -ECANCELED, 0);
-	req->io_task_work.func = io_futex_complete;
 	io_req_task_work_add(req);
 	return true;
 }
@@ -124,7 +173,7 @@ int io_futex_prep(struct io_kiocb *req, const struct io_uring_sqe *sqe)
 {
 	struct io_futex *iof = io_kiocb_to_cmd(req, struct io_futex);
 
-	if (unlikely(sqe->addr2 || sqe->buf_index || sqe->addr3))
+	if (unlikely(sqe->buf_index || sqe->addr3))
 		return -EINVAL;
 
 	iof->futex_op = READ_ONCE(sqe->fd);
@@ -135,6 +184,52 @@ int io_futex_prep(struct io_kiocb *req, const struct io_uring_sqe *sqe)
 	if (iof->futex_flags & FUTEX_CMD_MASK)
 		return -EINVAL;
 
+	iof->futexv_owned = 0;
+	return 0;
+}
+
+static void io_futex_wakev_fn(struct wake_q_head *wake_q, struct futex_q *q)
+{
+	struct io_kiocb *req = q->wake_data;
+	struct io_futex *iof = io_kiocb_to_cmd(req, struct io_futex);
+
+	if (!io_futexv_claim(iof))
+		return;
+	if (unlikely(!__futex_wake_mark(q)))
+		return;
+
+	io_req_set_res(req, 0, 0);
+	req->io_task_work.func = io_futexv_complete;
+	io_req_task_work_add(req);
+}
+
+int io_futexv_prep(struct io_kiocb *req, const struct io_uring_sqe *sqe)
+{
+	struct io_futex *iof = io_kiocb_to_cmd(req, struct io_futex);
+	struct futex_vector *futexv;
+	int ret;
+
+	ret = io_futex_prep(req, sqe);
+	if (ret)
+		return ret;
+
+	iof->futex_nr = READ_ONCE(sqe->off);
+	if (!iof->futex_nr || iof->futex_nr > FUTEX_WAITV_MAX)
+		return -EINVAL;
+
+	futexv = kcalloc(iof->futex_nr, sizeof(*futexv), GFP_KERNEL);
+	if (!futexv)
+		return -ENOMEM;
+
+	ret = futex_parse_waitv(futexv, iof->uwaitv, iof->futex_nr,
+				io_futex_wakev_fn, req);
+	if (ret) {
+		kfree(futexv);
+		return ret;
+	}
+
+	req->flags |= REQ_F_ASYNC_DATA;
+	req->async_data = futexv;
 	return 0;
 }
 
@@ -162,6 +257,55 @@ static struct io_futex_data *io_alloc_ifd(struct io_ring_ctx *ctx)
 	return kmalloc(sizeof(struct io_futex_data), GFP_NOWAIT);
 }
 
+int io_futex_waitv(struct io_kiocb *req, unsigned int issue_flags)
+{
+	struct io_futex *iof = io_kiocb_to_cmd(req, struct io_futex);
+	struct futex_vector *futexv = req->async_data;
+	struct io_ring_ctx *ctx = req->ctx;
+	int ret, woken = -1;
+
+	io_ring_submit_lock(ctx, issue_flags);
+
+	ret = futex_wait_multiple_setup(futexv, iof->futex_nr, &woken);
+
+	/*
+	 * The above call leaves us potentially non-running. This is fine
+	 * for the sync syscall as it'll be blocking unless we already got
+	 * one of the futexes woken, but it obviously won't work for an async
+	 * invocation. Mark is runnable again.
+	 */
+	__set_current_state(TASK_RUNNING);
+
+	/*
+	 * We got woken while setting up, let that side do the completion
+	 */
+	if (io_futexv_claimed(iof)) {
+skip:
+		io_ring_submit_unlock(ctx, issue_flags);
+		return IOU_ISSUE_SKIP_COMPLETE;
+	}
+
+	/*
+	 * 0 return means that we successfully setup the waiters, and that
+	 * nobody triggered a wakeup while we were doing so. < 0 or 1 return
+	 * is either an error or we got a wakeup while setting up.
+	 */
+	if (!ret) {
+		hlist_add_head(&req->hash_node, &ctx->futex_list);
+		goto skip;
+	}
+
+	io_ring_submit_unlock(ctx, issue_flags);
+	if (ret < 0)
+		req_set_fail(req);
+	else if (woken != -1)
+		ret = woken;
+	io_req_set_res(req, ret, 0);
+	kfree(futexv);
+	req->flags &= ~REQ_F_ASYNC_DATA;
+	return IOU_OK;
+}
+
 int io_futex_wait(struct io_kiocb *req, unsigned int issue_flags)
 {
 	struct io_futex *iof = io_kiocb_to_cmd(req, struct io_futex);
diff --git a/io_uring/futex.h b/io_uring/futex.h
index ddc9e0d73c52..7828e27e4184 100644
--- a/io_uring/futex.h
+++ b/io_uring/futex.h
@@ -3,7 +3,9 @@
 #include "cancel.h"
 
 int io_futex_prep(struct io_kiocb *req, const struct io_uring_sqe *sqe);
+int io_futexv_prep(struct io_kiocb *req, const struct io_uring_sqe *sqe);
 int io_futex_wait(struct io_kiocb *req, unsigned int issue_flags);
+int io_futex_waitv(struct io_kiocb *req, unsigned int issue_flags);
 int io_futex_wake(struct io_kiocb *req, unsigned int issue_flags);
 
 #if defined(CONFIG_FUTEX)
diff --git a/io_uring/opdef.c b/io_uring/opdef.c
index c9f23c21a031..2034acfe10d0 100644
--- a/io_uring/opdef.c
+++ b/io_uring/opdef.c
@@ -443,6 +443,14 @@ const struct io_issue_def io_issue_defs[] = {
 		.issue			= io_futex_wake,
 #else
 		.prep			= io_eopnotsupp_prep,
+#endif
+	},
+	[IORING_OP_FUTEX_WAITV] = {
+#if defined(CONFIG_FUTEX)
+		.prep			= io_futexv_prep,
+		.issue			= io_futex_waitv,
+#else
+		.prep			= io_eopnotsupp_prep,
 #endif
 	},
 };
@@ -670,6 +678,9 @@ const struct io_cold_def io_cold_defs[] = {
 	[IORING_OP_FUTEX_WAKE] = {
 		.name			= "FUTEX_WAKE",
 	},
+	[IORING_OP_FUTEX_WAITV] = {
+		.name			= "FUTEX_WAITV",
+	},
 };
 
 const char *io_uring_get_opcode(u8 opcode)
-- 
2.40.1

Re: [PATCH 8/8] io_uring: add support for vectored futex waits

[Peter Zijlstra]

On Wed, Jul 12, 2023 at 10:20:17AM -0600, Jens Axboe wrote:
>  int io_futex_prep(struct io_kiocb *req, const struct io_uring_sqe *sqe);
> +int io_futexv_prep(struct io_kiocb *req, const struct io_uring_sqe *sqe);
>  int io_futex_wait(struct io_kiocb *req, unsigned int issue_flags);
> +int io_futex_waitv(struct io_kiocb *req, unsigned int issue_flags);
>  int io_futex_wake(struct io_kiocb *req, unsigned int issue_flags);

That's an inconsistent naming convention.. I'll stare at the rest later.

Re: [PATCH 8/8] io_uring: add support for vectored futex waits

[Jens Axboe]

On 7/13/23 5:54 AM, Peter Zijlstra wrote:
> On Wed, Jul 12, 2023 at 10:20:17AM -0600, Jens Axboe wrote:
>>  int io_futex_prep(struct io_kiocb *req, const struct io_uring_sqe *sqe);
>> +int io_futexv_prep(struct io_kiocb *req, const struct io_uring_sqe *sqe);
>>  int io_futex_wait(struct io_kiocb *req, unsigned int issue_flags);
>> +int io_futex_waitv(struct io_kiocb *req, unsigned int issue_flags);
>>  int io_futex_wake(struct io_kiocb *req, unsigned int issue_flags);
> 
> That's an inconsistent naming convention.. I'll stare at the rest later.

I had to stare at that a bit.. Yes it is, I'll unify that.

-- 
Jens Axboe

Re: [PATCHSET v2 0/8] Add io_uring futex/futexv support

[Jens Axboe]

Neglected to mention, that I have run this through the ltp futex tests,
and there are no changes. All tests pass.

-- 
Jens Axboe

[PATCHSET v5] Add io_uring futex/futexv support

[Jens Axboe]

Hi,

This patchset adds support for first futex wake and wait, and then
futexv.

For both wait/wake/waitv, we support the bitset variant, as the
"normal" variants can be easily implemented on top of that.

PI and requeue are not supported through io_uring, just the above
mentioned parts. This may change in the future, but in the spirit
of keeping this small (and based on what people have been asking for),
this is what we currently have.

When I did these patches, I forgot that Pavel had previously posted a
futex variant for io_uring. The major thing that had been holding me
back from people asking about futexes and io_uring, is that I wanted
to do this what I consider the right way - no usage of io-wq or thread
offload, an actually async implementation that is efficient to use
and don't rely on a blocking thread for futex wait/waitv. This is what
this patchset attempts to do, while being minimally invasive on the
futex side. I believe the diffstat reflects that.

As far as I can recall, the first request for futex support with
io_uring came from Andres Freund, working on postgres. His aio rework
of postgres was one of the early adopters of io_uring, and futex
support was a natural extension for that. This is relevant from both
a usability point of view, as well as for effiency and performance.
In Andres's words, for the former:

"Futex wait support in io_uring makes it a lot easier to avoid deadlocks
in concurrent programs that have their own buffer pool: Obviously pages in
the application buffer pool have to be locked during IO. If the initiator
of IO A needs to wait for a held lock B, the holder of lock B might wait
for the IO A to complete.  The ability to wait for a lock and IO
completions at the same time provides an efficient way to avoid such
deadlocks."

and in terms of effiency, even without unlocking the full potential yet,
Andres says:

"Futex wake support in io_uring is useful because it allows for more
efficient directed wakeups.  For some "locks" postgres has queues
implemented in userspace, with wakeup logic that cannot easily be
implemented with FUTEX_WAKE_BITSET on a single "futex word" (imagine
waiting for journal flushes to have completed up to a certain point). Thus
a "lock release" sometimes need to wake up many processes in a row.  A
quick-and-dirty conversion to doing these wakeups via io_uring lead to a
3% throughput increase, with 12% fewer context switches, albeit in a
fairly extreme workload."

Some basic io_uring futex support and test cases are available in the
liburing 'futex' branch:

https://git.kernel.dk/cgit/liburing/log/?h=futex

testing all of the variants. I originally wrote this code about a
month ago and Andres has been using it with postgres, and I'm not
aware of any bugs in it. That's not to say it's perfect, obviously,
and I welcome some feedback so we can move this forward and hash out
any potential issues.

In terms of testing, there's a functionality and beat-up test case
in liburing, and I've run all the ltp futex test cases as well to
ensure we didn't inadvertently break anything. It's also been in
linux-next for a long time and haven't heard any complaints.

 include/linux/io_uring_types.h |   5 +
 include/uapi/linux/io_uring.h  |   4 +
 io_uring/Makefile              |   1 +
 io_uring/cancel.c              |   5 +
 io_uring/cancel.h              |   4 +
 io_uring/futex.c               | 376 +++++++++++++++++++++++++++++++++
 io_uring/futex.h               |  36 ++++
 io_uring/io_uring.c            |   7 +
 io_uring/opdef.c               |  34 +++
 kernel/futex/futex.h           |  20 ++
 kernel/futex/requeue.c         |   3 +-
 kernel/futex/syscalls.c        |  18 +-
 kernel/futex/waitwake.c        |  49 +++--
 13 files changed, 535 insertions(+), 27 deletions(-)

You can also find the code here:

https://git.kernel.dk/cgit/linux/log/?h=io_uring-futex

V5:
- Rebase on PeterZ's futex2 changes. Pulled in the tip locking/core
  branch.
- Shuffle order of some io_uring patchsets, which changed the value
  of the futex opcodes.

V4:
- Refactor the prep setup so it's fully independent between the vectoed
  and non-vectored futex handling.
- Ensure we -EINVAL any futex/futexv wait/waitv that specifies unused
  fields.
- Fix a comment typo
- Update the patches from Peter.
- Fix two kerneldoc warnings
- Add a prep patch moving FUTEX2_MASK to futex.h

-- 
Jens Axboe

[PATCH 2/8] futex: factor out the futex wake handling

[Jens Axboe]

In preparation for having another waker that isn't futex_wake_mark(),
add a wake handler in futex_q. No extra data is associated with the
handler outside of struct futex_q itself. futex_wake_mark() is defined as
the standard wakeup helper, now set through futex_q_init like other
defaults.

Normal sync futex waiting relies on wake_q holding tasks that should
be woken up. This is what futex_wake_mark() does, it'll unqueue the
futex and add the associated task to the wake queue. For async usage of
futex waiting, rather than having tasks sleeping on the futex, we'll
need to deal with a futex wake differently. For the planned io_uring
case, that means posting a completion event for the task in question.
Having a definable wake handler can help support that use case.

Signed-off-by: Jens Axboe <axboe@kernel.dk>
---
 kernel/futex/futex.h    | 5 +++++
 kernel/futex/requeue.c  | 3 ++-
 kernel/futex/waitwake.c | 6 +++---
 3 files changed, 10 insertions(+), 4 deletions(-)

diff --git a/kernel/futex/futex.h b/kernel/futex/futex.h
index a173a9d501e1..547f509b2c87 100644
--- a/kernel/futex/futex.h
+++ b/kernel/futex/futex.h
@@ -139,11 +139,15 @@ struct futex_pi_state {
 	union futex_key key;
 } __randomize_layout;
 
+struct futex_q;
+typedef void (futex_wake_fn)(struct wake_q_head *wake_q, struct futex_q *q);
+
 /**
  * struct futex_q - The hashed futex queue entry, one per waiting task
  * @list:		priority-sorted list of tasks waiting on this futex
  * @task:		the task waiting on the futex
  * @lock_ptr:		the hash bucket lock
+ * @wake:		the wake handler for this queue
  * @key:		the key the futex is hashed on
  * @pi_state:		optional priority inheritance state
  * @rt_waiter:		rt_waiter storage for use with requeue_pi
@@ -168,6 +172,7 @@ struct futex_q {
 
 	struct task_struct *task;
 	spinlock_t *lock_ptr;
+	futex_wake_fn *wake;
 	union futex_key key;
 	struct futex_pi_state *pi_state;
 	struct rt_mutex_waiter *rt_waiter;
diff --git a/kernel/futex/requeue.c b/kernel/futex/requeue.c
index a0a79954f506..9dc789399a1a 100644
--- a/kernel/futex/requeue.c
+++ b/kernel/futex/requeue.c
@@ -58,6 +58,7 @@ enum {
 
 const struct futex_q futex_q_init = {
 	/* list gets initialized in futex_queue()*/
+	.wake		= futex_wake_mark,
 	.key		= FUTEX_KEY_INIT,
 	.bitset		= FUTEX_BITSET_MATCH_ANY,
 	.requeue_state	= ATOMIC_INIT(Q_REQUEUE_PI_NONE),
@@ -593,7 +594,7 @@ int futex_requeue(u32 __user *uaddr1, unsigned int flags1,
 		/* Plain futexes just wake or requeue and are done */
 		if (!requeue_pi) {
 			if (++task_count <= nr_wake)
-				futex_wake_mark(&wake_q, this);
+				this->wake(&wake_q, this);
 			else
 				requeue_futex(this, hb1, hb2, &key2);
 			continue;
diff --git a/kernel/futex/waitwake.c b/kernel/futex/waitwake.c
index 37860f794bf7..35c6a637a4bb 100644
--- a/kernel/futex/waitwake.c
+++ b/kernel/futex/waitwake.c
@@ -177,7 +177,7 @@ int futex_wake(u32 __user *uaddr, unsigned int flags, int nr_wake, u32 bitset)
 			if (!(this->bitset & bitset))
 				continue;
 
-			futex_wake_mark(&wake_q, this);
+			this->wake(&wake_q, this);
 			if (++ret >= nr_wake)
 				break;
 		}
@@ -292,7 +292,7 @@ int futex_wake_op(u32 __user *uaddr1, unsigned int flags, u32 __user *uaddr2,
 				ret = -EINVAL;
 				goto out_unlock;
 			}
-			futex_wake_mark(&wake_q, this);
+			this->wake(&wake_q, this);
 			if (++ret >= nr_wake)
 				break;
 		}
@@ -306,7 +306,7 @@ int futex_wake_op(u32 __user *uaddr1, unsigned int flags, u32 __user *uaddr2,
 					ret = -EINVAL;
 					goto out_unlock;
 				}
-				futex_wake_mark(&wake_q, this);
+				this->wake(&wake_q, this);
 				if (++op_ret >= nr_wake2)
 					break;
 			}
-- 
2.40.1

[PATCHSET v6] Add io_uring futex/futexv support

[Jens Axboe]

Hi,

This patchset adds support for first futex wake and wait, and then
futexv.

For both wait/wake/waitv, we support the bitset variant, as the
"normal" variants can be easily implemented on top of that.

PI and requeue are not supported through io_uring, just the above
mentioned parts. This may change in the future, but in the spirit
of keeping this small (and based on what people have been asking for),
this is what we currently have.

When I did these patches, I forgot that Pavel had previously posted a
futex variant for io_uring. The major thing that had been holding me
back from people asking about futexes and io_uring, is that I wanted
to do this what I consider the right way - no usage of io-wq or thread
offload, an actually async implementation that is efficient to use
and don't rely on a blocking thread for futex wait/waitv. This is what
this patchset attempts to do, while being minimally invasive on the
futex side. I believe the diffstat reflects that.

As far as I can recall, the first request for futex support with
io_uring came from Andres Freund, working on postgres. His aio rework
of postgres was one of the early adopters of io_uring, and futex
support was a natural extension for that. This is relevant from both
a usability point of view, as well as for effiency and performance.
In Andres's words, for the former:

"Futex wait support in io_uring makes it a lot easier to avoid deadlocks
in concurrent programs that have their own buffer pool: Obviously pages in
the application buffer pool have to be locked during IO. If the initiator
of IO A needs to wait for a held lock B, the holder of lock B might wait
for the IO A to complete.  The ability to wait for a lock and IO
completions at the same time provides an efficient way to avoid such
deadlocks."

and in terms of effiency, even without unlocking the full potential yet,
Andres says:

"Futex wake support in io_uring is useful because it allows for more
efficient directed wakeups.  For some "locks" postgres has queues
implemented in userspace, with wakeup logic that cannot easily be
implemented with FUTEX_WAKE_BITSET on a single "futex word" (imagine
waiting for journal flushes to have completed up to a certain point). Thus
a "lock release" sometimes need to wake up many processes in a row.  A
quick-and-dirty conversion to doing these wakeups via io_uring lead to a
3% throughput increase, with 12% fewer context switches, albeit in a
fairly extreme workload."

Some basic io_uring futex support and test cases are available in the
liburing 'futex' branch:

https://git.kernel.dk/cgit/liburing/log/?h=futex

testing all of the variants. I originally wrote this code about a
month ago and Andres has been using it with postgres, and I'm not
aware of any bugs in it. That's not to say it's perfect, obviously,
and I welcome some feedback so we can move this forward and hash out
any potential issues.

In terms of testing, there's a functionality and beat-up test case
in liburing, and I've run all the ltp futex test cases as well to
ensure we didn't inadvertently break anything. It's also been in
linux-next for a long time and haven't heard any complaints.

 include/linux/io_uring_types.h |   5 +
 include/uapi/linux/io_uring.h  |   4 +
 io_uring/Makefile              |   1 +
 io_uring/cancel.c              |   5 +
 io_uring/cancel.h              |   4 +
 io_uring/futex.c               | 386 +++++++++++++++++++++++++++++++++
 io_uring/futex.h               |  36 +++
 io_uring/io_uring.c            |   7 +
 io_uring/opdef.c               |  34 +++
 kernel/futex/futex.h           |  20 ++
 kernel/futex/requeue.c         |   3 +-
 kernel/futex/syscalls.c        |  18 +-
 kernel/futex/waitwake.c        |  49 +++--
 13 files changed, 545 insertions(+), 27 deletions(-)

You can also find the code here:

https://git.kernel.dk/cgit/linux/log/?h=io_uring-futex

V6:
- Expand the two main commit messages describing command layout
- Fix double conversion of FUTEX2_* flags (Peter)
- Use FLAGS_STRICT for IORING_OP_FUTEX_WAKE, so we return 0 wakes
  when the caller asked for 0 (Peter)
- Fix issue with IORING_OP_FUTEX_WAITV and futex_wait_multiple_setup()
  doing futex_unqueue_multiple() if we had a wakeup while setting up.
- Cleanup IORING_OP_FUTEX_WAITV issue path
- Don't use sqe->futex_flags for the FUTEX2_* flags, reserve it for
  future internal use.
- Add more liburing test cases.
- Rebase on current tree (for-6.7/io_uring + tip locking/core)

-- 
Jens Axboe

[PATCH 2/8] futex: factor out the futex wake handling

[Jens Axboe]

In preparation for having another waker that isn't futex_wake_mark(),
add a wake handler in futex_q. No extra data is associated with the
handler outside of struct futex_q itself. futex_wake_mark() is defined as
the standard wakeup helper, now set through futex_q_init like other
defaults.

Normal sync futex waiting relies on wake_q holding tasks that should
be woken up. This is what futex_wake_mark() does, it'll unqueue the
futex and add the associated task to the wake queue. For async usage of
futex waiting, rather than having tasks sleeping on the futex, we'll
need to deal with a futex wake differently. For the planned io_uring
case, that means posting a completion event for the task in question.
Having a definable wake handler can help support that use case.

Signed-off-by: Jens Axboe <axboe@kernel.dk>
---
 kernel/futex/futex.h    | 5 +++++
 kernel/futex/requeue.c  | 3 ++-
 kernel/futex/waitwake.c | 6 +++---
 3 files changed, 10 insertions(+), 4 deletions(-)

diff --git a/kernel/futex/futex.h b/kernel/futex/futex.h
index a173a9d501e1..547f509b2c87 100644
--- a/kernel/futex/futex.h
+++ b/kernel/futex/futex.h
@@ -139,11 +139,15 @@ struct futex_pi_state {
 	union futex_key key;
 } __randomize_layout;
 
+struct futex_q;
+typedef void (futex_wake_fn)(struct wake_q_head *wake_q, struct futex_q *q);
+
 /**
  * struct futex_q - The hashed futex queue entry, one per waiting task
  * @list:		priority-sorted list of tasks waiting on this futex
  * @task:		the task waiting on the futex
  * @lock_ptr:		the hash bucket lock
+ * @wake:		the wake handler for this queue
  * @key:		the key the futex is hashed on
  * @pi_state:		optional priority inheritance state
  * @rt_waiter:		rt_waiter storage for use with requeue_pi
@@ -168,6 +172,7 @@ struct futex_q {
 
 	struct task_struct *task;
 	spinlock_t *lock_ptr;
+	futex_wake_fn *wake;
 	union futex_key key;
 	struct futex_pi_state *pi_state;
 	struct rt_mutex_waiter *rt_waiter;
diff --git a/kernel/futex/requeue.c b/kernel/futex/requeue.c
index a0a79954f506..9dc789399a1a 100644
--- a/kernel/futex/requeue.c
+++ b/kernel/futex/requeue.c
@@ -58,6 +58,7 @@ enum {
 
 const struct futex_q futex_q_init = {
 	/* list gets initialized in futex_queue()*/
+	.wake		= futex_wake_mark,
 	.key		= FUTEX_KEY_INIT,
 	.bitset		= FUTEX_BITSET_MATCH_ANY,
 	.requeue_state	= ATOMIC_INIT(Q_REQUEUE_PI_NONE),
@@ -593,7 +594,7 @@ int futex_requeue(u32 __user *uaddr1, unsigned int flags1,
 		/* Plain futexes just wake or requeue and are done */
 		if (!requeue_pi) {
 			if (++task_count <= nr_wake)
-				futex_wake_mark(&wake_q, this);
+				this->wake(&wake_q, this);
 			else
 				requeue_futex(this, hb1, hb2, &key2);
 			continue;
diff --git a/kernel/futex/waitwake.c b/kernel/futex/waitwake.c
index 37860f794bf7..35c6a637a4bb 100644
--- a/kernel/futex/waitwake.c
+++ b/kernel/futex/waitwake.c
@@ -177,7 +177,7 @@ int futex_wake(u32 __user *uaddr, unsigned int flags, int nr_wake, u32 bitset)
 			if (!(this->bitset & bitset))
 				continue;
 
-			futex_wake_mark(&wake_q, this);
+			this->wake(&wake_q, this);
 			if (++ret >= nr_wake)
 				break;
 		}
@@ -292,7 +292,7 @@ int futex_wake_op(u32 __user *uaddr1, unsigned int flags, u32 __user *uaddr2,
 				ret = -EINVAL;
 				goto out_unlock;
 			}
-			futex_wake_mark(&wake_q, this);
+			this->wake(&wake_q, this);
 			if (++ret >= nr_wake)
 				break;
 		}
@@ -306,7 +306,7 @@ int futex_wake_op(u32 __user *uaddr1, unsigned int flags, u32 __user *uaddr2,
 					ret = -EINVAL;
 					goto out_unlock;
 				}
-				futex_wake_mark(&wake_q, this);
+				this->wake(&wake_q, this);
 				if (++op_ret >= nr_wake2)
 					break;
 			}
-- 
2.40.1