Re: [RFC][PATCH 0/5] arch: atomic rework

From: Will Deacon
Date: Fri Feb 07 2014 - 11:56:40 EST

Hi Paul,

On Fri, Feb 07, 2014 at 04:50:28PM +0000, Paul E. McKenney wrote:
> On Fri, Feb 07, 2014 at 08:44:05AM +0100, Peter Zijlstra wrote:
> > On Thu, Feb 06, 2014 at 08:20:51PM -0800, Paul E. McKenney wrote:
> > > Hopefully some discussion of out-of-thin-air values as well.
> >
> > Yes, absolutely shoot store speculation in the head already. Then drive
> > a wooden stake through its hart.
> >
> > C11/C++11 should not be allowed to claim itself a memory model until that
> > is sorted.
> There actually is a proposal being put forward, but it might not make ARM
> and Power people happy because it involves adding a compare, a branch,
> and an ISB/isync after every relaxed load... Me, I agree with you,
> much preferring the no-store-speculation approach.

Can you elaborate a bit on this please? We don't permit speculative stores
in the ARM architecture, so it seems counter-intuitive that GCC needs to
emit any additional instructions to prevent that from happening.

Stores can, of course, be observed out-of-order but that's a lot more
reasonable :)

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