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authorPavel Begunkov <asml.silence@gmail.com>2024-01-17 00:57:27 +0000
committerJens Axboe <axboe@kernel.dk>2024-01-17 09:45:24 -0700
commitd381099f980b5f6c3c7e150baf13b0aaefc66c29 (patch)
treec53607ec46b10a246c94519271b04f557fb919bf /io_uring
parentdc12d1799ce710fd90abbe0ced71e7e1ae0894fc (diff)
io_uring: clean up local tw add-wait sync
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>
Diffstat (limited to 'io_uring')
-rw-r--r--io_uring/io_uring.c10
1 files changed, 8 insertions, 2 deletions
diff --git a/io_uring/io_uring.c b/io_uring/io_uring.c
index d40c767a6216..3ab7e6a46149 100644
--- a/io_uring/io_uring.c
+++ b/io_uring/io_uring.c
@@ -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);
}