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Subject[RESEND PATCH] bug: Recursion avoidance for WARN_ONCE() and friends
Date
Currently WARN_ONCE() and similar macros set __warned *after* calling
the underlying macro. This risks infinite recursion if WARN_ONCE() is
used to implement sanity tests in any code that can be called by printk.

This can be fixed by restructuring the macros to set __warned before
calling further macros.

Signed-off-by: Daniel Thompson <daniel.thompson@linaro.org>
---

Notes:
I discovered this problem when I temporarily added sanity tests to the
irqflags macros during some of my development work but I suspect the
scope is a little wider. I admit I was tempted to throw this change
away after I had finished debugging but for two things prompted me to
post it.

1. It did cost me a few minutes head scratching and I'd like to spare
others the pain.

2. I realized the new code is potentially (and very fractionally) more
efficient: the register containing address of __warned can be reused
and a cache hit is a near certainty for the write.

Don't get too excited about the efficiency gains though they are
extremely modest. Measures as code size benefit and using v4.0-rc4 the
results are:

Kernel GCC version Code size reduction

arm multi_v7_defconfig Linaro 4.8-2014.01 224 bytes
arm64 defconfig Linaro 4.9-2014.09 32 bytes
i386_defconfig Redhat 4.9.2-6 62 bytes
x86_64_defconfig Redhat 4.9.2-6 380 bytes


include/asm-generic/bug.h | 21 ++++++++++++---------
1 file changed, 12 insertions(+), 9 deletions(-)

diff --git a/include/asm-generic/bug.h b/include/asm-generic/bug.h
index 630dd2372238..f8c8a819c563 100644
--- a/include/asm-generic/bug.h
+++ b/include/asm-generic/bug.h
@@ -110,9 +110,10 @@ extern void warn_slowpath_null(const char *file, const int line);
static bool __section(.data.unlikely) __warned; \
int __ret_warn_once = !!(condition); \
\
- if (unlikely(__ret_warn_once)) \
- if (WARN_ON(!__warned)) \
- __warned = true; \
+ if (unlikely(__ret_warn_once) && !__warned) { \
+ __warned = true; \
+ WARN_ON(true); \
+ } \
unlikely(__ret_warn_once); \
})

@@ -120,9 +121,10 @@ extern void warn_slowpath_null(const char *file, const int line);
static bool __section(.data.unlikely) __warned; \
int __ret_warn_once = !!(condition); \
\
- if (unlikely(__ret_warn_once)) \
- if (WARN(!__warned, format)) \
- __warned = true; \
+ if (unlikely(__ret_warn_once) && !__warned) { \
+ __warned = true; \
+ WARN(true, format); \
+ } \
unlikely(__ret_warn_once); \
})

@@ -130,9 +132,10 @@ extern void warn_slowpath_null(const char *file, const int line);
static bool __section(.data.unlikely) __warned; \
int __ret_warn_once = !!(condition); \
\
- if (unlikely(__ret_warn_once)) \
- if (WARN_TAINT(!__warned, taint, format)) \
- __warned = true; \
+ if (unlikely(__ret_warn_once) && !__warned) { \
+ __warned = true; \
+ WARN_TAINT(true, taint, format); \
+ } \
unlikely(__ret_warn_once); \
})

--
2.1.0


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