Date: Fri, 9 Jun 2017 20:49:58 +0200
From: Peter Zijlstra <peterz@infradead.org>
To: Vineet Gupta <Vineet.Gupta1@synopsys.com>
Cc: Will Deacon <will.deacon@arm.com>,
        Paul McKenney <paulmck@linux.vnet.ibm.com>,
        Boqun Feng <boqun.feng@gmail.com>, linux-kernel@vger.kernel.org,
        Ingo Molnar <mingo@kernel.org>, Thomas Gleixner <tglx@linutronix.de>,
        rkuo@codeaurora.org, james.hogan@imgtec.com, jejb@parisc-linux.org,
        davem@davemloft.net, cmetcalf@mellanox.com,
        arcml <linux-snps-arc@lists.infradead.org>
Subject: Re: [RFC][PATCH] atomic: Fix atomic_set_release() for 'funny'
 architectures
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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 ;-)