[RFC 4/4] io_uring: implement futex wait

[Pavel Begunkov <asml.silence@gmail.com>]

Add futex wait requests, those always go through io-wq for simplicity.

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
---
 fs/io_uring.c                 | 38 ++++++++++++++++++++++++++++-------
 include/uapi/linux/io_uring.h |  2 +-
 2 files changed, 32 insertions(+), 8 deletions(-)

diff --git a/fs/io_uring.c b/fs/io_uring.c
index 99f4f8d9f685..9c3d075a6647 100644
--- a/fs/io_uring.c
+++ b/fs/io_uring.c
@@ -670,8 +670,15 @@ struct io_futex {
 	struct file			*file;
 	unsigned int			futex_op;
 
-	unsigned int			nr_wake;
-	unsigned int			wake_op_arg;
+	union {
+		/* wake */
+		struct {
+			unsigned int	nr_wake;
+			unsigned int	wake_op_arg;
+		};
+		/* wait */
+		u32 			val;
+	};
 	unsigned int			flags;
 	void __user			*uaddr;
 };
@@ -5894,10 +5901,23 @@ static int io_futex_prep(struct io_kiocb *req, const struct io_uring_sqe *sqe)
 		return -EINVAL;
 
 	v = READ_ONCE(sqe->off);
-	f->nr_wake = (u32)v;
-	f->wake_op_arg = (u32)(v >> 32);
-	f->futex_op = READ_ONCE(sqe->futex_op);
 	f->uaddr = u64_to_user_ptr(READ_ONCE(sqe->addr));
+	f->futex_op = READ_ONCE(sqe->futex_op);
+
+	switch (f->futex_op) {
+	case IORING_FUTEX_WAKE_OP:
+		f->nr_wake = (u32)v;
+		f->wake_op_arg = (u32)(v >> 32);
+		break;
+	case IORING_FUTEX_WAIT:
+		f->val = (u32)v;
+		if (unlikely(v >> 32))
+			return -EINVAL;
+		break;
+	default:
+		return -EINVAL;
+	}
+
 	return 0;
 }
 
@@ -5916,8 +5936,12 @@ static int io_futex(struct io_kiocb *req, unsigned int issue_flags)
 		if (nonblock && ret == -EAGAIN)
 			return -EAGAIN;
 		break;
-	default:
-		ret = -EINVAL;
+	case IORING_FUTEX_WAIT:
+		if (nonblock)
+			return -EAGAIN;
+		ret = futex_wait(f->uaddr, f->flags, f->val, NULL,
+				 FUTEX_BITSET_MATCH_ANY);
+		break;
 	}
 
 	if (ret < 0)
diff --git a/include/uapi/linux/io_uring.h b/include/uapi/linux/io_uring.h
index 6fa5a6e59934..7ab11ee2ce12 100644
--- a/include/uapi/linux/io_uring.h
+++ b/include/uapi/linux/io_uring.h
@@ -186,7 +186,7 @@ enum {
 
 enum {
 	IORING_FUTEX_WAKE_OP = 0,
-
+	IORING_FUTEX_WAIT,
 	IORING_FUTEX_LAST,
 };
 
-- 
2.31.1