linux-kernel - Re: [RFC PATCH-tip v2 1/6] locking/osq: Make lock/unlock proper acquire/release barrier

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Message-ID: <20160620075900.GB29165@arm.com>
Date:	Mon, 20 Jun 2016 08:59:00 +0100
From:	Will Deacon <will.deacon@....com>
To:	Boqun Feng <boqun.feng@...il.com>
Cc:	Waiman Long <waiman.long@....com>,
	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...hat.com>, linux-kernel@...r.kernel.org,
	x86@...nel.org, linux-alpha@...r.kernel.org,
	linux-ia64@...r.kernel.org, linux-s390@...r.kernel.org,
	linux-arch@...r.kernel.org, Davidlohr Bueso <dave@...olabs.net>,
	Jason Low <jason.low2@...com>,
	Dave Chinner <david@...morbit.com>,
	Scott J Norton <scott.norton@....com>,
	Douglas Hatch <doug.hatch@....com>
Subject: Re: [RFC PATCH-tip v2 1/6] locking/osq: Make lock/unlock proper
 acquire/release barrier

On Sat, Jun 18, 2016 at 04:46:20PM +0800, Boqun Feng wrote:
> On Fri, Jun 17, 2016 at 02:17:27PM -0400, Waiman Long wrote:
> > keep the xchg() function as it is or use smp_store_release(&next->locked,
> > 1). So which one is a better alternative for ARM or PPC?
> > 
> 
> For PPC, I think xchg_release() + smp_store_release() is better than the 
> current code, because the former has two lwsync while the latter has two
> sync, and sync is quite expensive than lwsync on PPC.
> 
> I need to leave the ARM part to Will ;-)

I doubt there's much in it, but xchg() has DMB + release, so xchg_release +
smp_store_release is probably slightly better for us too.

Will