io_uring: remove extra barrier for non-sqpoll iopoll

smp_mb() in io_cqring_ev_posted_iopoll() is only there because of
waitqueue_active(). However, non-SQPOLL IOPOLL ring doesn't wake the CQ
and so the barrier there is useless. Kill it, it's usually pretty
expensive.

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://lore.kernel.org/r/d72e8ef6f7a3f6a72e18fad8409f7d47afc8da7d.1647481208.git.asml.silence@gmail.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>

diff --git a/fs/io_uring.c b/fs/io_uring.c
index f039706..692dbe7 100644 (file)
--- a/fs/io_uring.c
+++ b/fs/io_uring.c
@@ -1877,11 +1877,8 @@ static void io_cqring_ev_posted(struct io_ring_ctx *ctx)
 
 static void io_cqring_ev_posted_iopoll(struct io_ring_ctx *ctx)
 {
-       /* see waitqueue_active() comment */
-       smp_mb();
-
        if (ctx->flags & IORING_SETUP_SQPOLL) {
-               if (waitqueue_active(&ctx->cq_wait))
+               if (wq_has_sleeper(&ctx->cq_wait))
                        wake_up_all(&ctx->cq_wait);
        }
        io_eventfd_signal(ctx);