Messages in this thread Patch in this message | | | From | Linus Torvalds <> | Date | Sat, 29 Oct 2011 10:34:30 -0700 | Subject | Re: >Re: [RFC] should VM_BUG_ON(cond) really evaluate cond |
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On Fri, Oct 28, 2011 at 7:55 AM, Linus Torvalds <torvalds@linux-foundation.org> wrote: > > Comments? I think I'm open to tested patches..
Here's a *untested* patch.
In particular, I worry that I'd need to add a "#include <linux/compiler.h>" to some header file, although I suspect it gets included some way regardless.
And when I say "untested", I mean it. I verified that this makes *some* difference to the generated code, but I didn't actually check if it really matters, or if it actually compiles and works in general.
Caveat tester,
Linus arch/x86/include/asm/bitops.h | 5 +++-- include/asm-generic/atomic.h | 2 +- include/linux/compiler.h | 7 +++++++ 3 files changed, 11 insertions(+), 3 deletions(-)
diff --git a/arch/x86/include/asm/bitops.h b/arch/x86/include/asm/bitops.h index 1775d6e5920e..e3982cb42fe5 100644 --- a/arch/x86/include/asm/bitops.h +++ b/arch/x86/include/asm/bitops.h @@ -308,8 +308,9 @@ static inline int test_and_change_bit(int nr, volatile unsigned long *addr) static __always_inline int constant_test_bit(unsigned int nr, const volatile unsigned long *addr) { - return ((1UL << (nr % BITS_PER_LONG)) & - (addr[nr / BITS_PER_LONG])) != 0; + unsigned long *word = (nr / BITS_PER_LONG) + (unsigned long *)addr; + unsigned long bit = 1UL << (nr % BITS_PER_LONG); + return (bit & ACCESS_AT_MOST_ONCE(*word)) != 0; } static inline int variable_test_bit(int nr, volatile const unsigned long *addr) diff --git a/include/asm-generic/atomic.h b/include/asm-generic/atomic.h index e37963c1df4d..c05e21f7a985 100644 --- a/include/asm-generic/atomic.h +++ b/include/asm-generic/atomic.h @@ -39,7 +39,7 @@ * Atomically reads the value of @v. */ #ifndef atomic_read -#define atomic_read(v) (*(volatile int *)&(v)->counter) +#define atomic_read(v) ACCESS_AT_MOST_ONCE((v)->counter) #endif /** diff --git a/include/linux/compiler.h b/include/linux/compiler.h index 320d6c94ff84..d30ffc685241 100644 --- a/include/linux/compiler.h +++ b/include/linux/compiler.h @@ -308,4 +308,11 @@ void ftrace_likely_update(struct ftrace_branch_data *f, int val, int expect); */ #define ACCESS_ONCE(x) (*(volatile typeof(x) *)&(x)) +/* + * Like ACCESS_ONCE, but can be optimized away if nothing uses the value, + * and/or merged with previous non-ONCE accesses. + */ +#define ACCESS_AT_MOST_ONCE(x) \ + ({ typeof(x) __y; asm("":"=r" (__y):"0" (x)); __y; }) + #endif /* __LINUX_COMPILER_H */ | |