* [PATCH][GIT PULL] WARN_ON_SMP(): Add comment to explain ({0;})
@ 2011-03-25 20:35 Steven Rostedt
0 siblings, 0 replies; only message in thread
From: Steven Rostedt @ 2011-03-25 20:35 UTC (permalink / raw)
To: LKML; +Cc: Ingo Molnar, Linus Torvalds, Alexey Dobriyan, Thomas Gleixner
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@redhat.com>
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@linux-foundation.org>
Cc: Alexey Dobriyan <adobriyan@gmail.com>
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>
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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2011-03-25 20:35 [PATCH][GIT PULL] WARN_ON_SMP(): Add comment to explain ({0;}) Steven Rostedt
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