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Date: Tue, 17 Nov 2015 13:01:09 -0800
From: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
To: Will Deacon <will.deacon@arm.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>,
        Peter Zijlstra <peterz@infradead.org>,
        Boqun Feng <boqun.feng@gmail.com>, Oleg Nesterov <oleg@redhat.com>,
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        Michael Ellerman <mpe@ellerman.id.au>,
        Benjamin Herrenschmidt <benh@kernel.crashing.org>,
        Paul Mackerras <paulus@samba.org>
Subject: Re: [PATCH 4/4] locking: Introduce smp_cond_acquire()
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On Tue, Nov 17, 2015 at 11:51:10AM +0000, Will Deacon wrote:
> Hi Linus,
> 
> 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@arm.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.

							Thanx, Paul