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@xxxxxxxxx>
---
io_uring/io_uring.c | 10 ++++++++--
1 file 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);
}
--
2.43.0
