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

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On Thu, Oct 01, 2015 at 02:24:40PM +0200, Peter Zijlstra wrote:
> 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?

Yes, when we say "relaxed", we really mean relaxed.  ;-)

Both the CPU and the compiler are allowed to reorder around relaxed
operations.

							Thanx, Paul

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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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