Commit ccd0d44f authored Mar 25, 2011 by Steven Rostedt Committed by Steven Rostedt Mar 28, 2011

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>

parent 18bcd0c8

include/asm-generic/bug.h

+7 −0

Original line number	Diff line number	Diff line
		@@ -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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