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

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Message-ID: <20170810091317.hv6smfz5fxgolu2n@hirez.programming.kicks-ass.net>
Date:   Thu, 10 Aug 2017 11:13:17 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Boqun Feng <boqun.feng@...il.com>
Cc:     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>,
        Paul McKenney <paulmck@...ux.vnet.ibm.com>
Subject: Re: [RESEND PATCH v5] locking/pvqspinlock: Relax cmpxchg's to
 improve performance on some archs

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.