Messages in this thread | | | Date | Fri, 3 Jul 2009 10:04:01 -0400 | From | Mathieu Desnoyers <> | Subject | Re: [PATCHv5 2/2] memory barrier: adding smp_mb__after_lock |
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* Ingo Molnar (mingo@elte.hu) wrote: > > * Jiri Olsa <jolsa@redhat.com> wrote: > > > +++ b/arch/x86/include/asm/spinlock.h > > @@ -302,4 +302,7 @@ static inline void __raw_write_unlock(raw_rwlock_t *rw) > > #define _raw_read_relax(lock) cpu_relax() > > #define _raw_write_relax(lock) cpu_relax() > > > > +/* The {read|write|spin}_lock() on x86 are full memory barriers. */ > > +#define smp_mb__after_lock() do { } while (0) >
Hm. Looking at http://lkml.org/lkml/2009/6/23/192, a very basic question comes to my mind :
Why don't we create a read_lock without acquire semantic instead (e.g. read_lock_nomb(), or something with a better name like __read_lock()) ? On architectures where memory barriers are needed to provide the acquire semantic, it would be faster to do :
__read_lock(); smp_mb();
than :
read_lock(); <- e.g. lwsync + isync or something like that smp_mb(); <- full sync.
Second point : __add_wait_queue/waitqueue_active/wake_up_interruptible would probably benefit from adding comments about their combined use with other checks and how nice memory barriers are.
Mathieu
> Two small stylistic comments, please make this an inline function: > > static inline void smp_mb__after_lock(void) { } > #define smp_mb__after_lock > > (untested) > > > +/* The lock does not imply full memory barrier. */ > > +#ifndef smp_mb__after_lock > > +#define smp_mb__after_lock() smp_mb() > > +#endif > > ditto. > > Ingo
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