Re: [RFC v2 4/7] powerpc: atomic: Implement xchg_* and atomic{,64}_xchg_* variants

From	Peter Zijlstra <peterz@infradead.org>
Newsgroups	linux.kernel
Subject	Re: [RFC v2 4/7] powerpc: atomic: Implement xchg_* and atomic{,64}_xchg_* variants
Date	2015-10-01 14:30 +0200
Message-ID	<qeL9N-3Vj-25@gated-at.bofh.it> (permalink)
References	<q9nhM-vs-7@gated-at.bofh.it> <q9nhN-vs-19@gated-at.bofh.it>
Organization	linux.* mail to news gateway

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On Wed, Sep 16, 2015 at 11:49:32PM +0800, Boqun Feng wrote:
> Implement xchg_relaxed and define atomic{,64}_xchg_* as xchg_relaxed,
> based on these _relaxed variants, release/acquire variants can be built.
> 
> Note that xchg_relaxed and atomic_{,64}_xchg_relaxed are not compiler
> barriers.

Hmm, and I note your previous patch creating the regular _relaxed
thingies also removes the memory clobber.

And looking at the ARM _relaxed patch from Will, I see their _relaxed
ops are also not a compiler barrier.

I must say I'm somewhat surprised by this level of relaxation, I had
expected to only loose SMP barriers, not the program order ones.

Is there a good argument for this?
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Re: [RFC v2 4/7] powerpc: atomic: Implement xchg_* and  atomic{,64}_xchg_* variants Peter Zijlstra <peterz@infradead.org> - 2015-10-01 14:30 +0200
  Re: [RFC v2 4/7] powerpc: atomic: Implement xchg_* and  atomic{,64}_xchg_* variants "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2015-10-01 17:10 +0200
    Re: [RFC v2 4/7] powerpc: atomic: Implement xchg_* and  atomic{,64}_xchg_* variants Peter Zijlstra <peterz@infradead.org> - 2015-10-01 19:20 +0200
      Re: [RFC v2 4/7] powerpc: atomic: Implement xchg_* and  atomic{,64}_xchg_* variants "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2015-10-01 20:10 +0200
        Re: [RFC v2 4/7] powerpc: atomic: Implement xchg_* and  atomic{,64}_xchg_* variants Peter Zijlstra <peterz@infradead.org> - 2015-10-01 20:30 +0200
          Re: [RFC v2 4/7] powerpc: atomic: Implement xchg_* and  atomic{,64}_xchg_* variants "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2015-10-01 21:50 +0200
        Re: [RFC v2 4/7] powerpc: atomic: Implement xchg_* and  atomic{,64}_xchg_* variants Will Deacon <will.deacon@arm.com> - 2015-10-05 16:50 +0200

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