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io_uring: clean up local tw add-wait sync

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Kill a smp_mb__after_atomic() right before wake_up, it's useless, and
add a comment explaining implicit barriers from cmpxchg and
synchronsation around ->cq_wait_nr with the waiter.

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://lore.kernel.org/r/3007f3c2d53c72b61de56919ef56b53158b8276f.1705438669.git.asml.silence@gmail.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>
This commit is contained in:
Pavel Begunkov 2024-01-17 00:57:27 +00:00 committed by Jens Axboe
parent dc12d1799c
commit d381099f98
1 changed files with 8 additions and 2 deletions
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10
io_uring/io_uring.c
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@ -1332,6 +1332,14 @@ static inline void io_req_local_work_add(struct io_kiocb *req, unsigned flags)
} while (!try_cmpxchg(&ctx->work_llist.first, &first,
&req->io_task_work.node));
/*
* cmpxchg implies a full barrier, which pairs with the barrier
* in set_current_state() on the io_cqring_wait() side. It's used
* to ensure that either we see updated ->cq_wait_nr, or waiters
* going to sleep will observe the work added to the list, which
* is similar to the wait/wawke task state sync.
*/
if (!first) {
if (ctx->flags & IORING_SETUP_TASKRUN_FLAG)
atomic_or(IORING_SQ_TASKRUN, &ctx->rings->sq_flags);
@ -1346,8 +1354,6 @@ static inline void io_req_local_work_add(struct io_kiocb *req, unsigned flags)
/* either not enough or the previous add has already woken it up */
if (nr_wait > nr_tw || nr_tw_prev >= nr_wait)
return;
/* pairs with set_current_state() in io_cqring_wait() */
smp_mb__after_atomic();
wake_up_state(ctx->submitter_task, TASK_INTERRUPTIBLE);
}