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    SubjectRe: [RESEND PATCH v5] locking/pvqspinlock: Relax cmpxchg's to improve performance on some archs
    On Thu, Aug 10, 2017 at 04:12:13PM +0800, Boqun Feng wrote:

    > > Or is the reason this doesn't work on PPC that its RCpc?

    So that :-)

    > Here is an example why PPC needs a sync() before the cmpxchg():
    >
    > https://marc.info/?l=linux-kernel&m=144485396224519&w=2
    >
    > and Paul Mckenney's detailed explanation about why this could happen:
    >
    > https://marc.info/?l=linux-kernel&m=144485909826241&w=2
    >
    > (Somehow, I feel like he was answering to a similar question question as
    > you ask here ;-))

    Yes, and I had vague memories of having gone over this before, but
    couldn't quickly find things. Thanks!

    > And I think aarch64 doesn't have a problem here because it is "(other)
    > multi-copy atomic". Will?

    Right, its the RCpc vs RCsc thing. The ARM64 release is as you say
    multi-copy atomic, whereas the PPC lwsync is not.


    This still leaves us with the situation that we need an smp_mb() between
    smp_store_release() and a possibly failing cmpxchg() if we want to
    guarantee the cmpxchg()'s load comes after the store-release.

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