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    SubjectRe: [PATCHv5 2/2] memory barrier: adding smp_mb__after_lock
    * 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

    --
    Mathieu Desnoyers
    OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F BA06 3F25 A8FE 3BAE 9A68


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