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io_uring: fix memory ordering when SQPOLL thread goes to sleep

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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>
Link: https://lore.kernel.org/r/20220321090059.46313-1-almogkh@gmail.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>
This commit is contained in:
Almog Khaikin 2022-03-21 11:00:59 +02:00 committed by Jens Axboe
parent f63cf5192f
commit 649bb75d19
1 changed files with 7 additions and 0 deletions
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7
fs/io_uring.c
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@ -8042,6 +8042,13 @@ static int io_sq_thread(void *data)
needs_sched = false; needs_sched = false;
break; 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)) { if (io_sqring_entries(ctx)) {
needs_sched = false; needs_sched = false;
break; break;