From: Stefan Bühler <source@stbuehler.de>
Date: Fri, 19 Apr 2019 09:57:45 +0000 (+0200)
Subject: io_uring: fix race condition when sq threads goes sleeping
X-Git-Tag: v5.1-rc7~3^2~2
X-Git-Url: http://git.ipfire.org/?a=commitdiff_plain;h=0d7bae69c574c5f25802f8a71252e7d66933a3ab;p=thirdparty%2Flinux.git

io_uring: fix race condition when sq threads goes sleeping

Reading the SQ tail needs to come after setting IORING_SQ_NEED_WAKEUP in
flags; there is no cheap barrier for ordering a store before a load, a
full memory barrier is required.

Userspace needs a full memory barrier between updating SQ tail and
checking for the IORING_SQ_NEED_WAKEUP too.

Signed-off-by: Stefan Bühler <source@stbuehler.de>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
---

diff --git a/fs/io_uring.c b/fs/io_uring.c
index 41e3a6f6a096a..69910fd9ccca0 100644
--- a/fs/io_uring.c
+++ b/fs/io_uring.c
@@ -1865,7 +1865,8 @@ static int io_sq_thread(void *data)
 
 			/* Tell userspace we may need a wakeup call */
 			ctx->sq_ring->flags |= IORING_SQ_NEED_WAKEUP;
-			smp_wmb();
+			/* make sure to read SQ tail after writing flags */
+			smp_mb();
 
 			if (!io_get_sqring(ctx, &sqes[0])) {
 				if (kthread_should_stop()) {