linux-kernel - Re: [PATCH 4/4] locking: Introduce smp_cond

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Date:	Wed, 18 Nov 2015 11:25:14 +0000
From:	Will Deacon <will.deacon@....com>
To:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Boqun Feng <boqun.feng@...il.com>,
	Oleg Nesterov <oleg@...hat.com>,
	Ingo Molnar <mingo@...nel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Jonathan Corbet <corbet@....net>,
	Michal Hocko <mhocko@...nel.org>,
	David Howells <dhowells@...hat.com>,
	Michael Ellerman <mpe@...erman.id.au>,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	Paul Mackerras <paulus@...ba.org>
Subject: Re: [PATCH 4/4] locking: Introduce smp_cond_acquire()

On Tue, Nov 17, 2015 at 01:01:09PM -0800, Paul E. McKenney wrote:
> On Tue, Nov 17, 2015 at 11:51:10AM +0000, Will Deacon wrote:
> > On Mon, Nov 16, 2015 at 01:58:49PM -0800, Linus Torvalds wrote:
> > > On Mon, Nov 16, 2015 at 8:24 AM, Will Deacon <will.deacon@....com> wrote:
> > > >
> > > > ... or we upgrade spin_unlock_wait to a LOCK operation, which might be
> > > > slightly cheaper than spin_lock()+spin_unlock().
> > > 
> > > So traditionally the real concern has been the cacheline ping-pong
> > > part of spin_unlock_wait(). I think adding a memory barrier (that
> > > doesn't force any exclusive states, just ordering) to it is fine, but
> > > I don't think we want to necessarily have it have to get the cacheline
> > > into exclusive state.
> > 
> > The problem is, I don't think the memory-barrier buys you anything in
> > the context of Boqun's example. In fact, he already had smp_mb() either
> > side of the spin_unlock_wait() and its still broken on arm64 and ppc.
> > 
> > Paul is proposing adding a memory barrier after spin_lock() in the racing
> > thread, but I personally think people will forget to add that.
> 
> A mechanical check would certainly make me feel better about it, so that
> any lock that was passed to spin_unlock_wait() was required to have all
> acquisitions followed by smp_mb__after_unlock_lock() or some such.
> But I haven't yet given up on finding a better solution.

Right-o. I'll hack together the arm64 spin_unlock_wait fix, but hold off
merging it for a few weeks in case we get struck by a sudden flash of
inspiration.

Will
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