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io_uring: fix race condition when sq threads goes sleeping

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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>
This commit is contained in:
Stefan Bühler 2019-04-19 11:57:45 +02:00 committed by Jens Axboe
parent e523a29c4f
commit 0d7bae69c5
1 changed files with 2 additions and 1 deletions
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3
fs/io_uring.c
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@ -1865,7 +1865,8 @@ static int io_sq_thread(void *data)
/* Tell userspace we may need a wakeup call */ /* Tell userspace we may need a wakeup call */
ctx->sq_ring->flags |= IORING_SQ_NEED_WAKEUP; 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 (!io_get_sqring(ctx, &sqes[0])) {
if (kthread_should_stop()) { if (kthread_should_stop()) {