Messages in this thread Patch in this message | | | From | Nikanth Karthikesan <> | Subject | Re: [PATCH] Detect and warn on atomic_inc/atomic_dec wrapping around | Date | Thu, 30 Apr 2009 19:21:44 +0530 |
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On Thursday 30 April 2009 19:07:57 Ingo Molnar wrote: > * Nikanth Karthikesan <knikanth@novell.com> wrote: > > > Then there could be a single, straightforward value check: > > > > > > static inline void atomic_inc(atomic_t *v) > > > { > > > debug_atomic_check_value(v); > > > raw_atomic_inc(v); > > > } > > > > > > Where debug_atomic_check_value() is just an atomic_read(): > > > > > > static inline void debug_atomic_check_value(atomic_t *v) > > > { > > > WARN_ONCE(in_range(atomic_read(v), UINT_MAX/4, UINT_MAX/4*3), > > > KERN_ERR "atomic counter check failure!"); > > > } > > > > I do not understand, why UINT_MAX/4 to UINT_MAX/4*3? > > Roughly, > > UINT_MAX/4 = INT_MAX/2 > > UINT_MAX/4*3 = INT_MAX/2*3 which we will never reach with an int. > > i mean: > > WARN_ONCE(in_range((u32)atomic_read(v), UINT_MAX/4, UINT_MAX/4*3), > KERN_ERR "atomic counter check failure!"); > > that's a single range check on an u32, selecting 'too large' and > 'too small' s32 values. > > > > It's a constant check. > > > > > > If are overflowing on such a massive rate, it doesnt matter how > > > early or late we check the value. > > > > UINT_MAX/4 early, might be too early. And if it doesn't matter how > > early or late, why try to be over-cautious and produce false > > warnings. ;-) > > UINT_MAX/4 is ~1 billion. If we reach a value of 1 billion we are > leaking. Your check basically is a sharp test for the specific case > of overflowing the boundary - but it makes the code slower (it uses > more complex atomic ops) and uglifies it via #ifdefs as well. > > It doesnt matter whether we wrap over at around +2 billion into -2 > billion, or treat the whole above-1-billion and > below-minus-1-billion range as invalid. (other than we'll catch bugs > sooner via this method, and have faster and cleaner code) >
Ah.. got it. But, range checking is not required as we are just verifying it during increment and decrement, not atomic_add, atomic_sub etc... Should we add debug checks to those operations as well? If we want to test those operations as well, range check would be useful.
Here is a patch, without the overhead of a costly atomic operation which would warn if it goes out of [INT_MIN/2 .. INT_MAX/2].
Thanks Nikanth
Detect and warn on atomic_inc/atomic_dec overflow.
Add a debug option to detect and warn when the 32-bit atomic_t overflows during atomic_inc and atomic_dec.
Signed-off-by: Nikanth Karthikesan <knikanth@suse.de> Acked-by: Ingo Molnar <mingo@elte.hu>
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diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig index df9e885..dd49be3 100644 --- a/arch/x86/Kconfig +++ b/arch/x86/Kconfig @@ -46,6 +46,7 @@ config X86 select HAVE_KERNEL_GZIP select HAVE_KERNEL_BZIP2 select HAVE_KERNEL_LZMA + select HAVE_ARCH_DEBUG_ATOMIC config ARCH_DEFCONFIG string diff --git a/arch/x86/include/asm/atomic_32.h b/arch/x86/include/asm/atomic_32.h index 85b46fb..c6a17bb 100644 --- a/arch/x86/include/asm/atomic_32.h +++ b/arch/x86/include/asm/atomic_32.h @@ -78,24 +78,24 @@ static inline int atomic_sub_and_test(int i, atomic_t *v) } /** - * atomic_inc - increment atomic variable + * raw_atomic_inc - increment atomic variable * @v: pointer of type atomic_t * * Atomically increments @v by 1. */ -static inline void atomic_inc(atomic_t *v) +static inline void raw_atomic_inc(atomic_t *v) { asm volatile(LOCK_PREFIX "incl %0" : "+m" (v->counter)); } /** - * atomic_dec - decrement atomic variable + * raw_atomic_dec - decrement atomic variable * @v: pointer of type atomic_t * * Atomically decrements @v by 1. */ -static inline void atomic_dec(atomic_t *v) +static inline void raw_atomic_dec(atomic_t *v) { asm volatile(LOCK_PREFIX "decl %0" : "+m" (v->counter)); diff --git a/arch/x86/include/asm/atomic_64.h b/arch/x86/include/asm/atomic_64.h index 8c21731..1183b85 100644 --- a/arch/x86/include/asm/atomic_64.h +++ b/arch/x86/include/asm/atomic_64.h @@ -77,12 +77,12 @@ static inline int atomic_sub_and_test(int i, atomic_t *v) } /** - * atomic_inc - increment atomic variable + * raw_atomic_inc - increment atomic variable * @v: pointer of type atomic_t * * Atomically increments @v by 1. */ -static inline void atomic_inc(atomic_t *v) +static inline void raw_atomic_inc(atomic_t *v) { asm volatile(LOCK_PREFIX "incl %0" : "=m" (v->counter) @@ -90,12 +90,12 @@ static inline void atomic_inc(atomic_t *v) } /** - * atomic_dec - decrement atomic variable + * raw_atomic_dec - decrement atomic variable * @v: pointer of type atomic_t * * Atomically decrements @v by 1. */ -static inline void atomic_dec(atomic_t *v) +static inline void raw_atomic_dec(atomic_t *v) { asm volatile(LOCK_PREFIX "decl %0" : "=m" (v->counter) diff --git a/include/asm-generic/atomic.h b/include/asm-generic/atomic.h index 7abdaa9..007931a 100644 --- a/include/asm-generic/atomic.h +++ b/include/asm-generic/atomic.h @@ -4,15 +4,51 @@ * Copyright (C) 2005 Silicon Graphics, Inc. * Christoph Lameter * - * Allows to provide arch independent atomic definitions without the need to - * edit all arch specific atomic.h files. */ +#include <linux/kernel.h> #include <asm/types.h> +#include <asm/bug.h> + + +/** + * atomic_inc - increment atomic variable + * @v: pointer of type atomic_t + * + * Atomically increments @v by 1. + */ +static inline void atomic_inc(atomic_t *v) +{ +#ifdef CONFIG_ENABLE_WARN_ATOMIC_INC_WRAP + WARN_ONCE((atomic_read(v) > (INT_MAX / 2)), + KERN_ERR "atomic counter check failure!"); +#endif + raw_atomic_inc(v); +} + +/** + * atomic_dec - decrement atomic variable + * @v: pointer of type atomic_t + * + * Atomically decrements @v by 1. + */ +static inline void atomic_dec(atomic_t *v) +{ +#ifdef CONFIG_ENABLE_WARN_ATOMIC_INC_WRAP + WARN_ONCE((atomic_read(v) < (INT_MIN / 2)), + KERN_ERR "atomic counter check failure!"); +#endif + raw_atomic_dec(v); + +} + /* * Suppport for atomic_long_t * + * Allows to provide arch independent atomic definitions without the need to + * edit all arch specific atomic.h files. + * * Casts for parameters are avoided for existing atomic functions in order to * avoid issues with cast-as-lval under gcc 4.x and other limitations that the * macros of a platform may have. diff --git a/lib/Kconfig.debug b/lib/Kconfig.debug index 812c282..773c1a4 100644 --- a/lib/Kconfig.debug +++ b/lib/Kconfig.debug @@ -17,6 +17,17 @@ config ENABLE_WARN_DEPRECATED Disable this to suppress the "warning: 'foo' is deprecated (declared at kernel/power/somefile.c:1234)" messages. +config HAVE_ARCH_DEBUG_ATOMIC + bool + +config ENABLE_WARN_ATOMIC_INC_WRAP + bool "Enable warning on atomic_inc()/atomic_dec() wrap" + depends on HAVE_ARCH_DEBUG_ATOMIC + default y + help + Enable printing a warning when atomic_inc() or atomic_dec() + operation wraps around the 32-bit value. + config ENABLE_MUST_CHECK bool "Enable __must_check logic" default y
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