linux-kernel - Re: [RFC][PATCH] atomic: Fix atomic_set

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Message-ID: <20170609184958.zkgcaxk7wezods5d@hirez.programming.kicks-ass.net>
Date:   Fri, 9 Jun 2017 20:49:58 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Vineet Gupta <Vineet.Gupta1@...opsys.com>
Cc:     Will Deacon <will.deacon@....com>,
        Paul McKenney <paulmck@...ux.vnet.ibm.com>,
        Boqun Feng <boqun.feng@...il.com>,
        linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>, rkuo@...eaurora.org,
        james.hogan@...tec.com, jejb@...isc-linux.org, davem@...emloft.net,
        cmetcalf@...lanox.com, arcml <linux-snps-arc@...ts.infradead.org>
Subject: Re: [RFC][PATCH] atomic: Fix atomic_set_release() for 'funny'
 architectures

On Fri, Jun 09, 2017 at 10:28:50AM -0700, Vineet Gupta wrote:
> On 06/09/2017 04:13 AM, Peter Zijlstra wrote:
> > On Fri, Jun 09, 2017 at 01:05:06PM +0200, Peter Zijlstra wrote:
> > 
> > > The spinlock based atomics should be SC, that is, none of them appear to
> > > place extra barriers in atomic_cmpxchg() or any of the other SC atomic
> > > primitives and therefore seem to rely on their spinlock implementation
> > > being SC (I did not fully validate all that).
> > 
> > So I did see that ARC and PARISC have 'superfluous' smp_mb() calls
> > around their spinlock implementation.
> > 
> > That is, for spinlock semantics you only need one _after_ lock and one
> > _before_ unlock. But the atomic stuff relies on being SC and thus would
> > need one before and after both lock and unlock.
> 
> Right we discussed this a while back: https://lkml.org/lkml/2015/6/11/276
> 
> At the time when I tried removing these extra barriers, hackbench regressed.
> I'm about to get a new quad core 1GHz chip (vs. the FPGA before) and will
> re-experiment. Likely we don't need it otherwise I will add a comment of
> this "feature"
> 
> > But ARC could probably optimize (if they still care about that hardware)
> > by pulling out those barriers and putting it in the atomic
> > implementation.
> 
> A bit confused here. Reading the lkml posting for this thread, you posted 2
> patches, and they had to do with atomic_set() for EZChip platform which is
> really special (no ll/sc). The extra smp_mb() is related to ll/sc variants.
> Just tryign to make sure that we are talking 2 different things here :-)

Could be I just got all my variants in a twist... wouldn't be the first
time ;-)