[PATCH] kernel.h: Add for_each_if()

[Daniel Vetter]

To avoid compilers complainig about ambigious else blocks when putting
an if condition into a for_each macro one needs to invert the
condition and add a dummy else. We have a nice little convenience
macro for that in drm headers, let's move it out. Subsequent patches
will roll it out to other places.

The issue the compilers complain about are nested if with an else
block and no {} to disambiguate which if the else belongs to. The C
standard is clear, but in practice people forget:

if (foo)
	if (bar)
		/* something */
	else
		/* something else

The same can happen in a for_each macro when it also contains an if
condition at the end, except the compiler message is now really
confusing since there's only 1 if:

for_each_something()
	if (bar)
		/* something */
	else
		/* something else

The for_each_if() macro, by inverting the condition and adding an
else, avoids the compiler warning.

Motivated by a discussion with Andy and Yisheng, who want to add
another for_each_macro which would benefit from for_each_if() instead
of hand-rolling it.

Cc: Gustavo Padovan <gustavo at padovan.org>
Cc: Maarten Lankhorst <maarten.lankhorst at linux.intel.com>
Cc: Sean Paul <seanpaul at chromium.org>
Cc: David Airlie <airlied at linux.ie>
Cc: Andrew Morton <akpm at linux-foundation.org>
Cc: Kees Cook <keescook at chromium.org>
Cc: Ingo Molnar <mingo at kernel.org>
Cc: Greg Kroah-Hartman <gregkh at linuxfoundation.org>
Cc: NeilBrown <neilb at suse.com>
Cc: Wei Wang <wvw at google.com>
Cc: Stefan Agner <stefan at agner.ch>
Cc: Andrei Vagin <avagin at openvz.org>
Cc: Randy Dunlap <rdunlap at infradead.org>
Cc: Andy Shevchenko <andriy.shevchenko at linux.intel.com>
Cc: Yisheng Xie <ysxie at foxmail.com>
Cc: Peter Zijlstra <peterz at infradead.org>
Reviewed-by: Andy Shevchenko <andriy.shevchenko at linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter at intel.com>
--
v2: Explain a bit better what this is good for, after the discussion
with Peter Z.
---
 include/drm/drmP.h     | 3 ---
 include/linux/kernel.h | 3 +++
 2 files changed, 3 insertions(+), 3 deletions(-)

diff --git a/include/drm/drmP.h b/include/drm/drmP.h
index f7a19c2a7a80..05350424a4d3 100644
--- a/include/drm/drmP.h
+++ b/include/drm/drmP.h
@@ -110,7 +110,4 @@ static inline bool drm_can_sleep(void)
 	return true;
 }
 
-/* helper for handling conditionals in various for_each macros */
-#define for_each_if(condition) if (!(condition)) {} else
-
 #endif
diff --git a/include/linux/kernel.h b/include/linux/kernel.h
index 941dc0a5a877..4cb95ab9a5bc 100644
--- a/include/linux/kernel.h
+++ b/include/linux/kernel.h
@@ -71,6 +71,9 @@
  */
 #define ARRAY_SIZE(arr) (sizeof(arr) / sizeof((arr)[0]) + __must_be_array(arr))
 
+/* helper for handling conditionals in various for_each macros */
+#define for_each_if(condition) if (!(condition)) {} else
+
 #define u64_to_user_ptr(x) (		\
 {					\
 	typecheck(u64, x);		\
-- 
2.18.0