[PATCH][GIT PULL] WARN_ON_SMP(): Add comment to explain ({0;})

From: Steven Rostedt
Date: Fri Mar 25 2011 - 16:35:24 EST



Ingo,

Please pull the latest tip/futex/devel tree, which can be found at:

git://git.kernel.org/pub/scm/linux/kernel/git/rostedt/linux-2.6-rt.git
tip/futex/devel


Steven Rostedt (1):
WARN_ON_SMP(): Add comment to explain ({0;})

----
include/asm-generic/bug.h | 7 +++++++
1 files changed, 7 insertions(+), 0 deletions(-)
---------------------------
commit 55c6c5c72dbbd6ffb7d97afb80bf0073988899c0
Author: Steven Rostedt <srostedt@xxxxxxxxxx>
Date: Fri Mar 25 16:21:06 2011 -0400

WARN_ON_SMP(): Add comment to explain ({0;})

The define to use ({0;}) for the !CONFIG_SMP case of WARN_ON_SMP()
can be confusing. As the WARN_ON_SMP() needs to be a nop when
CONFIG_SMP is not set, including all its parameters must not be
evaluated, and that it must work as both a stand alone statement
and inside an if condition, we define it to a funky ({0;}).

A simple "0" will not work as it causes gcc to give the warning that
the statement has no effect.

As this strange definition has raised a few eyebrows from some
major kernel developers, it is wise to document why we create such
a work of art.

Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Cc: Alexey Dobriyan <adobriyan@xxxxxxxxx>
Signed-off-by: Steven Rostedt <rostedt@xxxxxxxxxxx>

diff --git a/include/asm-generic/bug.h b/include/asm-generic/bug.h
index f2d2faf..e5a3f58 100644
--- a/include/asm-generic/bug.h
+++ b/include/asm-generic/bug.h
@@ -194,6 +194,13 @@ extern void warn_slowpath_null(const char *file, const int line);
#ifdef CONFIG_SMP
# define WARN_ON_SMP(x) WARN_ON(x)
#else
+/*
+ * Use of ({0;}) because WARN_ON_SMP(x) may be used either as
+ * a stand alone line statement or as a condition in an if ()
+ * statement.
+ * A simple "0" would cause gcc to give a "statement has no effect"
+ * warning.
+ */
# define WARN_ON_SMP(x) ({0;})
#endif



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