linux-kernel - [PATCH 1/4] ring_buffer: Fix warning while ignoring cmpxchg return value

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Date:	Sat, 18 Jul 2009 03:16:58 -0400
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	Ingo Molnar <mingo@...e.hu>, Thomas Gleixner <tglx@...utronix.de>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	Lai Jiangshan <laijs@...fujitsu.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Xiao Guangrong <xiaoguangrong@...fujitsu.com>,
	Frederic Weisbecker <fweisbec@...il.com>
Subject: [PATCH 1/4] ring_buffer: Fix warning while ignoring cmpxchg return value

From: Lai Jiangshan <laijs@...fujitsu.com>

kernel/trace/ring_buffer.c: In function 'rb_tail_page_update':
kernel/trace/ring_buffer.c:849: warning: value computed is not used
kernel/trace/ring_buffer.c:850: warning: value computed is not used

Add "(void)"s to fix this warning, because we don't need here to handle
the fail case of cmpxchg, it's fine if an interrupt already did the
job.

Changed from V1:
  Add a comment(which is written by Steven) for it.

Signed-off-by: Lai Jiangshan <laijs@...fujitsu.com>
Acked-by: Steven Rostedt <rostedt@...dmis.org>
Signed-off-by: Frederic Weisbecker <fweisbec@...il.com>
---
 kernel/trace/ring_buffer.c |    9 +++++++--
 1 files changed, 7 insertions(+), 2 deletions(-)

diff --git a/kernel/trace/ring_buffer.c b/kernel/trace/ring_buffer.c
index e648ba4..51633d7 100644
--- a/kernel/trace/ring_buffer.c
+++ b/kernel/trace/ring_buffer.c
@@ -845,9 +845,14 @@ static int rb_tail_page_update(struct ring_buffer_per_cpu *cpu_buffer,
 		 * This will only succeed if an interrupt did
 		 * not come in and change it. In which case, we
 		 * do not want to modify it.
+		 *
+		 * We add (void) to let the compiler know that we do not care
+		 * about the return value of these functions. We use the
+		 * cmpxchg to only update if an interrupt did not already
+		 * do it for us. If the cmpxchg fails, we don't care.
 		 */
-		local_cmpxchg(&next_page->write, old_write, val);
-		local_cmpxchg(&next_page->entries, old_entries, eval);
+		(void)local_cmpxchg(&next_page->write, old_write, val);
+		(void)local_cmpxchg(&next_page->entries, old_entries, eval);
 
 		/*
 		 * No need to worry about races with clearing out the commit.
-- 
1.6.2.3

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