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

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Date:   Thu, 10 Aug 2017 13:49:34 -0700
From:   "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Boqun Feng <boqun.feng@...il.com>,
        Waiman Long <longman@...hat.com>,
        Ingo Molnar <mingo@...hat.com>, linux-kernel@...r.kernel.org,
        Pan Xinhui <xinhui@...ux.vnet.ibm.com>,
        Andrea Parri <parri.andrea@...il.com>,
        Will Deacon <will.deacon@....com>
Subject: Re: [RESEND PATCH v5] locking/pvqspinlock: Relax cmpxchg's to
 improve performance on some archs

On Thu, Aug 10, 2017 at 11:13:17AM +0200, Peter Zijlstra wrote:
> On Thu, Aug 10, 2017 at 04:12:13PM +0800, Boqun Feng wrote:
> 
> > > Or is the reason this doesn't work on PPC that its RCpc?
> 
> So that :-)
> 
> > Here is an example why PPC needs a sync() before the cmpxchg():
> > 
> > 	https://marc.info/?l=linux-kernel&m=144485396224519&w=2
> > 
> > and Paul Mckenney's detailed explanation about why this could happen:
> > 
> > 	https://marc.info/?l=linux-kernel&m=144485909826241&w=2
> > 
> > (Somehow, I feel like he was answering to a similar question question as
> > you ask here ;-))
> 
> Yes, and I had vague memories of having gone over this before, but
> couldn't quickly find things. Thanks!
> 
> > And I think aarch64 doesn't have a problem here because it is "(other)
> > multi-copy atomic". Will?
> 
> Right, its the RCpc vs RCsc thing. The ARM64 release is as you say
> multi-copy atomic, whereas the PPC lwsync is not.
> 
> This still leaves us with the situation that we need an smp_mb() between
> smp_store_release() and a possibly failing cmpxchg() if we want to
> guarantee the cmpxchg()'s load comes after the store-release.

For whatever it is worth, this is why C11 allows specifying one
memory-order strength for the success case and another for the failure
case.  But it is not immediately clear that we need another level
of combinatorial API explosion...

							Thanx, Paul