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authorSteven Rostedt <srostedt@redhat.com>2011-03-25 16:21:06 -0400
committerNicolas Pitre <nicolas.pitre@linaro.org>2011-06-14 16:40:27 -0400
commit5078fc4d3e14f09eb52fa5e975729d1a873fafb0 (patch)
tree7825bdf3d87ee93d48ebc58949bc08ccc73b3fe1
parentc7983304c310103f270ac507d199f8614a61fdfd (diff)
downloadlinux-linaro-android-5078fc4d3e14f09eb52fa5e975729d1a873fafb0.tar.gz
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> (cherry picked from commit ccd0d44fad38dc1bb4b26dcc7a30e9f2c3b36870)
-rw-r--r--include/asm-generic/bug.h7
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