Re: [PATCH] barriers: introduce smp_mb__release_acquire and update documentation

From	Peter Zijlstra <peterz@infradead.org>
Newsgroups	linux.kernel
Subject	Re: [PATCH] barriers: introduce smp_mb__release_acquire and update documentation
Date	2015-09-16 12:50 +0200
Message-ID	<q9irM-226-13@gated-at.bofh.it> (permalink)
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On Wed, Sep 16, 2015 at 11:29:08AM +0100, Will Deacon wrote:
> > Indeed, that is a hole in the definition, that I think we should close.

> I'm struggling to understand the hole, but here's my intuition. If an
> ACQUIRE on CPUx reads from a RELEASE by CPUy, then I'd expect CPUx to
> observe all memory accessed performed by CPUy prior to the RELEASE
> before it observes the RELEASE itself, regardless of this new barrier.
> I think this matches what we currently have in memory-barriers.txt (i.e.
> acquire/release are neither transitive or multi-copy atomic).

Ah agreed. I seem to have gotten my brain in a tangle.

Basically where a program order release+acquire relies on an address
dependency, a cross cpu release+acquire relies on causality. If we
observe the release, we must also observe everything prior to it etc.
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Thread

[PATCH] barriers: introduce smp_mb__release_acquire and update documentation Will Deacon <will.deacon@arm.com> - 2015-09-15 18:20 +0200
  Re: [PATCH] barriers: introduce smp_mb__release_acquire and update  documentation "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2015-09-15 19:50 +0200
    Re: [PATCH] barriers: introduce smp_mb__release_acquire and update  documentation Peter Zijlstra <peterz@infradead.org> - 2015-09-16 11:30 +0200
      Re: [PATCH] barriers: introduce smp_mb__release_acquire and update  documentation Will Deacon <will.deacon@arm.com> - 2015-09-16 12:30 +0200
        Re: [PATCH] barriers: introduce smp_mb__release_acquire and update  documentation Peter Zijlstra <peterz@infradead.org> - 2015-09-16 12:50 +0200
          Re: [PATCH] barriers: introduce smp_mb__release_acquire and update  documentation Will Deacon <will.deacon@arm.com> - 2015-09-16 13:10 +0200
            Re: [PATCH] barriers: introduce smp_mb__release_acquire and update  documentation Boqun Feng <boqun.feng@gmail.com> - 2015-09-17 05:00 +0200
              Re: [PATCH] barriers: introduce smp_mb__release_acquire and update  documentation Boqun Feng <boqun.feng@gmail.com> - 2015-09-17 10:00 +0200
              Re: [PATCH] barriers: introduce smp_mb__release_acquire and update  documentation Will Deacon <will.deacon@arm.com> - 2015-09-17 20:10 +0200
                Re: [PATCH] barriers: introduce smp_mb__release_acquire and update  documentation Boqun Feng <boqun.feng@gmail.com> - 2015-09-21 15:50 +0200
                Re: [PATCH] barriers: introduce smp_mb__release_acquire and update  documentation Boqun Feng <boqun.feng@gmail.com> - 2015-09-21 16:20 +0200
                Re: [PATCH] barriers: introduce smp_mb__release_acquire and update  documentation Will Deacon <will.deacon@arm.com> - 2015-09-22 00:30 +0200
                Re: [PATCH] barriers: introduce smp_mb__release_acquire and update  documentation Boqun Feng <boqun.feng@gmail.com> - 2015-09-22 01:50 +0200
  Re: [PATCH] barriers: introduce smp_mb__release_acquire and update  documentation Boqun Feng <boqun.feng@gmail.com> - 2015-09-16 13:50 +0200
    Re: [PATCH] barriers: introduce smp_mb__release_acquire and update  documentation Will Deacon <will.deacon@arm.com> - 2015-09-16 18:40 +0200
      Re: [PATCH] barriers: introduce smp_mb__release_acquire and update  documentation Boqun Feng <boqun.feng@gmail.com> - 2015-09-17 04:00 +0200

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