[PATCH 2/2] io_uring: close lookup gap for dependent next work

[Jens Axboe <axboe@kernel.dk>]

When we find new work to process within the work handler, we queue the
linked timeout before we have issued the new work. This can be
problematic for very short timeouts, as we have a window where the new
work isn't visible.

Allow the work handler to store a callback function for this in the work
item, and flag it with IO_WQ_WORK_CB if the caller has done so. If that
is set, then io-wq will call the callback when it has setup the new work
item.

Reported-by: Pavel Begunkov <asml.silence@gmail.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
---
 fs/io-wq.c    |  3 +++
 fs/io-wq.h    | 12 +++++++++++-
 fs/io_uring.c | 14 ++++++++++++--
 3 files changed, 26 insertions(+), 3 deletions(-)

diff --git a/fs/io-wq.c b/fs/io-wq.c
index b4bc377dda61..2666384aaf44 100644
--- a/fs/io-wq.c
+++ b/fs/io-wq.c
@@ -427,6 +427,9 @@ static void io_worker_handle_work(struct io_worker *worker)
 		worker->cur_work = work;
 		spin_unlock_irq(&worker->lock);
 
+		if (work->flags & IO_WQ_WORK_CB)
+			work->cb.fn(work->cb.data);
+
 		if ((work->flags & IO_WQ_WORK_NEEDS_FILES) &&
 		    current->files != work->files) {
 			task_lock(current);
diff --git a/fs/io-wq.h b/fs/io-wq.h
index 4b29f922f80c..892989f3e41e 100644
--- a/fs/io-wq.h
+++ b/fs/io-wq.h
@@ -11,6 +11,7 @@ enum {
 	IO_WQ_WORK_NEEDS_FILES	= 16,
 	IO_WQ_WORK_UNBOUND	= 32,
 	IO_WQ_WORK_INTERNAL	= 64,
+	IO_WQ_WORK_CB		= 128,
 
 	IO_WQ_HASH_SHIFT	= 24,	/* upper 8 bits are used for hash key */
 };
@@ -21,8 +22,17 @@ enum io_wq_cancel {
 	IO_WQ_CANCEL_NOTFOUND,	/* work not found */
 };
 
+struct io_wq_work;
+struct io_wq_work_cb {
+	void (*fn)(void *data);
+	void *data;
+};
+
 struct io_wq_work {
-	struct list_head list;
+	union {
+		struct list_head list;
+		struct io_wq_work_cb cb;
+	};
 	void (*func)(struct io_wq_work **);
 	unsigned flags;
 	struct files_struct *files;
diff --git a/fs/io_uring.c b/fs/io_uring.c
index 132a890368bf..6175e2e195c0 100644
--- a/fs/io_uring.c
+++ b/fs/io_uring.c
@@ -2664,6 +2664,13 @@ static int __io_submit_sqe(struct io_kiocb *req, struct io_kiocb **nxt,
 	return 0;
 }
 
+static void io_link_work_cb(void *data)
+{
+	struct io_kiocb *link = data;
+
+	io_queue_linked_timeout(link);
+}
+
 static void io_wq_submit_work(struct io_wq_work **workptr)
 {
 	struct io_wq_work *work = *workptr;
@@ -2710,8 +2717,11 @@ static void io_wq_submit_work(struct io_wq_work **workptr)
 
 		io_prep_async_work(nxt, &link);
 		*workptr = &nxt->work;
-		if (link)
-			io_queue_linked_timeout(link);
+		if (link) {
+			nxt->work.flags |= IO_WQ_WORK_CB;
+			nxt->work.cb.fn = io_link_work_cb;
+			nxt->work.cb.data = link;
+		}
 	}
 }
 
-- 
2.24.0