[PATCH -tip V2] ring_buffer: fix warning

[Lai Jiangshan <laijs@cn.fujitsu.com>]

Sorry for missing Reviewed-by and Acked-by, so I resend it.

Subject: [PATCH -tip V2] ring_buffer: fix warning

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.

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

Signed-off-by: Lai Jiangshan <laijs@cn.fujitsu.com>
Reviewed-by: Frederic Weisbecker <fweisbec@gmail.com>
Acked-by: Steven Rostedt <rostedt@goodmis.org>
---
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.