linux-kernel - [PATCH 3.16.y-ckt 172/180] printk: help pr_debug and pr

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Message-Id: <1454538786-12215-173-git-send-email-luis.henriques@canonical.com>
Date:	Wed,  3 Feb 2016 22:32:58 +0000
From:	Luis Henriques <luis.henriques@...onical.com>
To:	linux-kernel@...r.kernel.org, stable@...r.kernel.org,
	kernel-team@...ts.ubuntu.com
Cc:	Aaron Conole <aconole@...hat.com>, Joe Perches <joe@...ches.com>,
	Jason Baron <jbaron@...mai.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Luis Henriques <luis.henriques@...onical.com>
Subject: [PATCH 3.16.y-ckt 172/180] printk: help pr_debug and pr_devel to optimize out arguments

3.16.7-ckt24 -stable review patch.  If anyone has any objections, please let me know.

---8<------------------------------------------------------------

From: Aaron Conole <aconole@...hat.com>

commit fe22cd9b7c980b8b948ec85f034a8668c57ec867 upstream.

Currently, pr_debug and pr_devel will not elide function call arguments
appearing in calls to the no_printk for these macros.  This is because
all side effects must be honored before proceeding to the 0-value
assignment in no_printk.

The behavior is contrary to documentation found in the CodingStyle and
the header file where these functions are declared.

This patch corrects that behavior by shunting out the call to no_printk
completely.  The format string is still checked by gcc for correctness,
but no code seems to be emitted in common cases.

[akpm@...ux-foundation.org: remove braces, per Joe]
Fixes: 5264f2f75d86 ("include/linux/printk.h: use and neaten no_printk")
Signed-off-by: Aaron Conole <aconole@...hat.com>
Reported-by: Dmitry Vyukov <dvyukov@...gle.com>
Cc: Joe Perches <joe@...ches.com>
Cc: Jason Baron <jbaron@...mai.com>
Signed-off-by: Andrew Morton <akpm@...ux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@...ux-foundation.org>
Signed-off-by: Luis Henriques <luis.henriques@...onical.com>
---
 include/linux/printk.h | 12 ++++++------
 1 file changed, 6 insertions(+), 6 deletions(-)

diff --git a/include/linux/printk.h b/include/linux/printk.h
index 319ff7e53efb..4b0636bcf268 100644
--- a/include/linux/printk.h
+++ b/include/linux/printk.h
@@ -104,13 +104,13 @@ struct va_format {
 
 /*
  * Dummy printk for disabled debugging statements to use whilst maintaining
- * gcc's format and side-effect checking.
+ * gcc's format checking.
  */
-static inline __printf(1, 2)
-int no_printk(const char *fmt, ...)
-{
-	return 0;
-}
+#define no_printk(fmt, ...)			\
+do {						\
+	if (0)					\
+		printk(fmt, ##__VA_ARGS__);	\
+} while (0)
 
 #ifdef CONFIG_EARLY_PRINTK
 extern asmlinkage __printf(1, 2)