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 <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Christian Brauner <[email protected]>

Author: oleg-nesterov

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;