[PATCH v2] io_uring: fix memory ordering when SQPOLL thread goes to sleep

[PATCH v2] io_uring: fix memory ordering when SQPOLL thread goes to sleep

[Almog Khaikin]

Without a full memory barrier between the store to the flags and the
load of the SQ tail the two operations can be reordered and this can
lead to a situation where the SQPOLL thread goes to sleep while the
application writes to the SQ tail and doesn't see the wakeup flag.
This memory barrier pairs with a full memory barrier in the application
between its store to the SQ tail and its load of the flags.

Signed-off-by: Almog Khaikin <almogkh@gmail.com>
---

v2: Fix trailing whitespace and rebase to latest tag

 fs/io_uring.c | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/fs/io_uring.c b/fs/io_uring.c
index 5fa736344b67..695ee5fddc84 100644
--- a/fs/io_uring.c
+++ b/fs/io_uring.c
@@ -8035,6 +8035,13 @@ static int io_sq_thread(void *data)
 					needs_sched = false;
 					break;
 				}
+
+				/*
+				 * Ensure the store of the wakeup flag is not
+				 * reordered with the load of the SQ tail
+				 */
+				smp_mb();
+
 				if (io_sqring_entries(ctx)) {
 					needs_sched = false;
 					break;

base-commit: 5e929367468c8f97cd1ffb0417316cecfebef94b
-- 
2.35.1

Re: [PATCH v2] io_uring: fix memory ordering when SQPOLL thread goes to sleep

[Jens Axboe]

On Mon, 21 Mar 2022 11:00:59 +0200, Almog Khaikin wrote:
> Without a full memory barrier between the store to the flags and the
> load of the SQ tail the two operations can be reordered and this can
> lead to a situation where the SQPOLL thread goes to sleep while the
> application writes to the SQ tail and doesn't see the wakeup flag.
> This memory barrier pairs with a full memory barrier in the application
> between its store to the SQ tail and its load of the flags.
> 
> [...]

Applied, thanks!

[1/1] io_uring: fix memory ordering when SQPOLL thread goes to sleep
      commit: 649bb75d19c93f5459f450191953dff4825fda3e

Best regards,
-- 
Jens Axboe