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    SubjectRe: [PATCHv5 2/2] memory barrier: adding smp_mb__after_lock
    On Fri, Jul 03, 2009 at 11:47:00AM -0400, Mathieu Desnoyers wrote:
    > * Eric Dumazet (eric.dumazet@gmail.com) wrote:
    > > Herbert Xu a écrit :
    > > > Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca> wrote:
    > > >> 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.
    > > >
    > > > Hmm, why do we even care when read_lock should just die?
    > > >
    > > > Cheers,
    > >
    > > +1 :)
    > >
    > > Do you mean using a spinlock instead or what ?
    > >
    >
    > I think he meant RCU.
    >
    > > Also, how many arches are able to have a true __read_lock()
    > > (or __spin_lock() if that matters), without acquire semantic ?
    >
    > At least PowerPC, MIPS, recent ARM, alpha.

    Are you guys sure you are in agreement about what you all mean by
    "acquire semantics"?

    Clearly, any correct __read_lock() implementation must enforce ordering
    with respect to the most recent __write_unlock(), but this does not
    necesarily imply all possible definitions of "acquire semantics".

    Thanx, Paul
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