Commit-ID: ccd0d44fad38dc1bb4b26dcc7a30e9f2c3b36870
Gitweb: http://git.kernel.org/tip/ccd0d44fad38dc1bb4b26dcc7a30e9f2c3b36870
Author: Steven Rostedt <srostedt@xxxxxxxxxx>
AuthorDate: Fri, 25 Mar 2011 16:21:06 -0400
Committer: Steven Rostedt <rostedt@xxxxxxxxxxx>
CommitDate: Mon, 28 Mar 2011 10:10:42 -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>
---
include/asm-generic/bug.h | 7 +++++++
1 files changed, 7 insertions(+), 0 deletions(-)
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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