linux-kernel - Re: [RESEND PATCH v5] locking/pvqspinlock: Relax cmpxchg's to improve performance on some archs

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Date:   Wed, 9 Aug 2017 17:15:33 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Waiman Long <longman@...hat.com>
Cc:     Ingo Molnar <mingo@...hat.com>, linux-kernel@...r.kernel.org,
        Pan Xinhui <xinhui@...ux.vnet.ibm.com>,
        Boqun Feng <boqun.feng@...il.com>,
        Andrea Parri <parri.andrea@...il.com>,
        Will Deacon <will.deacon@....com>,
        Paul McKenney <paulmck@...ux.vnet.ibm.com>
Subject: Re: [RESEND PATCH v5] locking/pvqspinlock: Relax cmpxchg's to
 improve performance on some archs

On Wed, Aug 09, 2017 at 05:06:03PM +0200, Peter Zijlstra wrote:
> Now, ARM64 for instance plays funny games, it does something along the
> lines of:
> 
> cmpxchg(ptr, old, new)
> {
> 	do {
> 		r = LL(ptr);
> 		if (r != old)
> 			return r; /* no barriers */
> 		r = new
> 	} while (SC_release(ptr, r));
> 	smp_mb();
> 	return r;
> }
> 
> Thereby ordering things relative to the store on ptr, but the load can
> very much escape. The thinking is that if success, we must observe the
> latest value of ptr, but even in that case the load is not ordered and
> could happen before.
> 
> However, since we're guaranteed to observe the latest value of ptr (on
> success) it doesn't matter if we reordered the load, there is no newer
> value possible.
> 
> So heaps of tricky, but correct afaict. Will?

And could not PPC do something similar:

cmpxchg(ptr, old, new)
{
	lwsync();
	dp {
		r = LL(ptr);
		if (r != old)
			return;
		r = new;
	} while (SC(ptr, r));
	sync();
	return r;
}

?

the lwsync would make it store-release on SC with similar reasoning as
above.

And lwsync allows 'stores reordered after loads', which allows the prior
smp_store_release() to leak past.

Or is the reason this doesn't work on PPC that its RCpc?