linux-kernel - [RFCv7 PATCH 3/4] net/sock.h: use poll_does_not_wait() in sock_poll

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Date:	Thu,  2 Feb 2012 11:26:56 +0100
From:	Hans Verkuil <hverkuil@...all.nl>
To:	linux-media@...r.kernel.org
Cc:	Al Viro <viro@...iv.linux.org.uk>,
	Jonathan Corbet <corbet@....net>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Davide Libenzi <davidel@...ilserver.org>,
	linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
	"David S. Miller" <davem@...emloft.net>,
	Enke Chen <enkechen@...co.com>,
	Hans Verkuil <hans.verkuil@...co.com>
Subject: [RFCv7 PATCH 3/4] net/sock.h: use poll_does_not_wait() in sock_poll_wait()

From: Hans Verkuil <hans.verkuil@...co.com>

In order to determine whether poll_wait() might actually wait the
poll_table pointer was tested in sock_poll_wait(). This is no longer
sufficient, instead poll_does_not_wait() should be called. That function
also tests whether pt->pq_proc is non-NULL.

Without this change smp_mb() could be called unnecessarily in some
circumstances, causing a performance hit.

Signed-off-by: Hans Verkuil <hans.verkuil@...co.com>
---
 include/net/sock.h |    2 +-
 1 files changed, 1 insertions(+), 1 deletions(-)

diff --git a/include/net/sock.h b/include/net/sock.h
index 91c1c8b..da7f2ec 100644
--- a/include/net/sock.h
+++ b/include/net/sock.h
@@ -1824,7 +1824,7 @@ static inline bool wq_has_sleeper(struct socket_wq *wq)
 static inline void sock_poll_wait(struct file *filp,
 		wait_queue_head_t *wait_address, poll_table *p)
 {
-	if (p && wait_address) {
+	if (!poll_does_not_wait(p) && wait_address) {
 		poll_wait(filp, wait_address, p);
 		/*
 		 * We need to be sure we are in sync with the
-- 
1.7.8.3

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