From ccd0d44fad38dc1bb4b26dcc7a30e9f2c3b36870 Mon Sep 17 00:00:00 2001 From: Steven Rostedt Date: Fri, 25 Mar 2011 16:21:06 -0400 Subject: [PATCH] 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 Cc: Alexey Dobriyan Signed-off-by: Steven Rostedt --- include/asm-generic/bug.h | 7 +++++++ 1 file changed, 7 insertions(+) diff --git a/include/asm-generic/bug.h b/include/asm-generic/bug.h index f2d2faf4d9ae..e5a3f5880001 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