linux-kernel - [PATCH 3/5] io_uring_poll: kill the no longer necessary barrier after poll

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Message-ID: <20250107162730.GA18940@redhat.com>
Date: Tue, 7 Jan 2025 17:27:30 +0100
From: Oleg Nesterov <oleg@...hat.com>
To: Christian Brauner <brauner@...nel.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Manfred Spraul <manfred@...orfullife.com>
Cc: "David S. Miller" <davem@...emloft.net>,
	Eric Dumazet <edumazet@...gle.com>, Jens Axboe <axboe@...nel.dk>,
	Pavel Begunkov <asml.silence@...il.com>,
	WangYuli <wangyuli@...ontech.com>, linux-kernel@...r.kernel.org,
	io-uring@...r.kernel.org, netdev@...r.kernel.org
Subject: [PATCH 3/5] io_uring_poll: kill the no longer necessary barrier
 after poll_wait()

Now that poll_wait() provides a full barrier we can remove smp_rmb() from
io_uring_poll().

In fact I don't think smp_rmb() was correct, it can't serialize LOADs and
STOREs.

Signed-off-by: Oleg Nesterov <oleg@...hat.com>
---
 io_uring/io_uring.c | 9 ++++-----
 1 file changed, 4 insertions(+), 5 deletions(-)

diff --git a/io_uring/io_uring.c b/io_uring/io_uring.c
index 06ff41484e29..a64a82b93b86 100644
--- a/io_uring/io_uring.c
+++ b/io_uring/io_uring.c
@@ -2809,13 +2809,12 @@ static __poll_t io_uring_poll(struct file *file, poll_table *wait)
 
 	if (unlikely(!ctx->poll_activated))
 		io_activate_pollwq(ctx);
-
-	poll_wait(file, &ctx->poll_wq, wait);
 	/*
-	 * synchronizes with barrier from wq_has_sleeper call in
-	 * io_commit_cqring
+	 * provides mb() which pairs with barrier from wq_has_sleeper
+	 * call in io_commit_cqring
 	 */
-	smp_rmb();
+	poll_wait(file, &ctx->poll_wq, wait);
+
 	if (!io_sqring_full(ctx))
 		mask |= EPOLLOUT | EPOLLWRNORM;
 
-- 
2.25.1.362.g51ebf55