Re: [PATCH v2] barriers: introduce smp_mb__release_acquire and update documentation

From	Peter Zijlstra <peterz@infradead.org>
Newsgroups	linux.kernel
Subject	Re: [PATCH v2] barriers: introduce smp_mb__release_acquire and update documentation
Date	2015-10-21 10:30 +0200
Message-ID	<qlWWu-5OT-25@gated-at.bofh.it> (permalink)
References	(5 earlier) <qhreB-7rq-87@gated-at.bofh.it> <qhBnz-5g1-1@gated-at.bofh.it> <qhCtk-6Ne-25@gated-at.bofh.it> <ql7hf-5TU-7@gated-at.bofh.it> <qlOFz-20f-1@gated-at.bofh.it>
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On Tue, Oct 20, 2015 at 04:34:51PM -0700, Paul E. McKenney wrote:
> There is also the question of whether the barrier forces ordering
> of unrelated stores, everything initially zero and all accesses
> READ_ONCE() or WRITE_ONCE():
> 
> 	P0		P1		P2		P3
> 	X = 1;		Y = 1;		r1 = X;		r3 = Y;
> 					some_barrier();	some_barrier();
> 					r2 = Y;		r4 = X;
> 
> P2's and P3's ordering could be globally visible without requiring
> P0's and P1's independent stores to be ordered, for example, if you
> used smp_rmb() for some_barrier().  In contrast, if we used smp_mb()
> for barrier, everyone would agree on the order of P0's and P0's stores.

Oh!?

> There are actually a fair number of different combinations of
> aspects of memory ordering.  We will need to choose wisely.  ;-)
> 
> My hope is that the store-ordering gets folded into the globally
> visible transitive level.  Especially given that I have not (yet)
> seen any algorithms used in production that relied on the ordering of
> independent stores.

I would hope not, that's quite insane.
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Thread

Re: [PATCH v2] barriers: introduce smp_mb__release_acquire and  update documentation Boqun Feng <boqun.feng@gmail.com> - 2015-10-19 03:20 +0200
  Re: [PATCH v2] barriers: introduce smp_mb__release_acquire and  update documentation Peter Zijlstra <peterz@infradead.org> - 2015-10-19 12:30 +0200
    Re: [PATCH v2] barriers: introduce smp_mb__release_acquire and  update documentation Boqun Feng <boqun.feng@gmail.com> - 2015-10-20 09:40 +0200
  Re: [PATCH v2] barriers: introduce smp_mb__release_acquire and  update documentation "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2015-10-21 01:40 +0200
    Re: [PATCH v2] barriers: introduce smp_mb__release_acquire and  update documentation Peter Zijlstra <peterz@infradead.org> - 2015-10-21 10:30 +0200
      Re: [PATCH v2] barriers: introduce smp_mb__release_acquire and  update documentation "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2015-10-21 21:30 +0200
        Re: [PATCH v2] barriers: introduce smp_mb__release_acquire and  update documentation Peter Zijlstra <peterz@infradead.org> - 2015-10-21 21:40 +0200
          Re: [PATCH v2] barriers: introduce smp_mb__release_acquire and  update documentation "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2015-10-21 22:00 +0200
    RE: [PATCH v2] barriers: introduce smp_mb__release_acquire and  update documentation David Laight <David.Laight@ACULAB.COM> - 2015-10-21 18:10 +0200
      Re: [PATCH v2] barriers: introduce smp_mb__release_acquire and  update documentation "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2015-10-21 21:40 +0200

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