[PATCH] printk: fix pr_debug and pr_devel to elide function calls

[Aaron Conole]

Currently, pr_debug and pr_devel will not elide function call arguments
appearing in calls to 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
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.

fixes commit 5264f2f75d86 ("include/linux/printk.h: use and neaten
no_printk")

Signed-off-by: Aaron Conole <aconole@xxxxxxxxxx>
Reported-by: Dmitry Vyukov <dvyukov@xxxxxxxxxx>
Cc: Joe Perches <joe@xxxxxxxxxxx>

---
 include/linux/printk.h | 42 +++++++++++++++++++++++++++++++++---------
 1 file changed, 33 insertions(+), 9 deletions(-)

diff --git a/include/linux/printk.h b/include/linux/printk.h
index 9729565..87dfdd2 100644
--- a/include/linux/printk.h
+++ b/include/linux/printk.h
@@ -270,8 +270,12 @@ extern asmlinkage void dump_stack(void) __cold;
 #define pr_devel(fmt, ...) \
 	printk(KERN_DEBUG pr_fmt(fmt), ##__VA_ARGS__)
 #else
-#define pr_devel(fmt, ...) \
-	no_printk(KERN_DEBUG pr_fmt(fmt), ##__VA_ARGS__)
+#define pr_devel(fmt, ...)					\
+do {								\
+	if (0) {						\
+		no_printk(KERN_DEBUG pr_fmt(fmt), ##__VA_ARGS__);\
+	}							 \
+} while (0)
 #endif
 
 #include <linux/dynamic_debug.h>
@@ -286,7 +290,11 @@ extern asmlinkage void dump_stack(void) __cold;
 	printk(KERN_DEBUG pr_fmt(fmt), ##__VA_ARGS__)
 #else
 #define pr_debug(fmt, ...) \
-	no_printk(KERN_DEBUG pr_fmt(fmt), ##__VA_ARGS__)
+do {								\
+	if (0) {						\
+		no_printk(KERN_DEBUG pr_fmt(fmt), ##__VA_ARGS__);\
+	}							 \
+} while (0)
 #endif
 
 /*
@@ -341,7 +349,11 @@ extern asmlinkage void dump_stack(void) __cold;
 	printk_once(KERN_DEBUG pr_fmt(fmt), ##__VA_ARGS__)
 #else
 #define pr_devel_once(fmt, ...)					\
-	no_printk(KERN_DEBUG pr_fmt(fmt), ##__VA_ARGS__)
+do {								\
+	if (0) {						\
+		no_printk(KERN_DEBUG pr_fmt(fmt), ##__VA_ARGS__);\
+	}							 \
+} while (0)
 #endif
 
 /* If you are writing a driver, please use dev_dbg instead */
@@ -350,7 +362,11 @@ extern asmlinkage void dump_stack(void) __cold;
 	printk_once(KERN_DEBUG pr_fmt(fmt), ##__VA_ARGS__)
 #else
 #define pr_debug_once(fmt, ...)					\
-	no_printk(KERN_DEBUG pr_fmt(fmt), ##__VA_ARGS__)
+do {								\
+	if (0) {						\
+		no_printk(KERN_DEBUG pr_fmt(fmt), ##__VA_ARGS__);\
+	}							 \
+} while (0)
 #endif
 
 /*
@@ -392,8 +408,12 @@ extern asmlinkage void dump_stack(void) __cold;
 #define pr_devel_ratelimited(fmt, ...)					\
 	printk_ratelimited(KERN_DEBUG pr_fmt(fmt), ##__VA_ARGS__)
 #else
-#define pr_devel_ratelimited(fmt, ...)					\
-	no_printk(KERN_DEBUG pr_fmt(fmt), ##__VA_ARGS__)
+#define pr_devel_ratelimited(fmt, ...)				\
+do {								\
+	if (0) {						\
+		no_printk(KERN_DEBUG pr_fmt(fmt), ##__VA_ARGS__);\
+	}							 \
+} while (0)
 #endif
 
 /* If you are writing a driver, please use dev_dbg instead */
@@ -413,8 +433,12 @@ do {									\
 #define pr_debug_ratelimited(fmt, ...)					\
 	printk_ratelimited(KERN_DEBUG pr_fmt(fmt), ##__VA_ARGS__)
 #else
-#define pr_debug_ratelimited(fmt, ...) \
-	no_printk(KERN_DEBUG pr_fmt(fmt), ##__VA_ARGS__)
+#define pr_debug_ratelimited(fmt, ...)					\
+do {								\
+	if (0) {						\
+		no_printk(KERN_DEBUG pr_fmt(fmt), ##__VA_ARGS__);\
+	}							 \
+} while (0)
 #endif
 
 extern const struct file_operations kmsg_fops;
-- 
2.6.1.133.gf5b6079

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