linux-kernel - Re: [RFC PATCH v2] memory-barriers: remove smp_mb__after_unlock

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Date:	Wed, 26 Aug 2015 14:06:11 +1000
From:	Michael Ellerman <mpe@...erman.id.au>
To:	paulmck@...ux.vnet.ibm.com
Cc:	Will Deacon <will.deacon@....com>,
	Peter Zijlstra <peterz@...radead.org>,
	"linux-arch@...r.kernel.org" <linux-arch@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>
Subject: Re: [RFC PATCH v2] memory-barriers: remove
 smp_mb__after_unlock_lock()

On Tue, 2015-08-25 at 17:27 -0700, Paul E. McKenney wrote:
> On Thu, Aug 20, 2015 at 04:56:04PM +0100, Will Deacon wrote:
> > On Thu, Aug 20, 2015 at 10:45:05AM +0100, Michael Ellerman wrote:
> > > On Tue, 2015-08-18 at 09:37 +0100, Will Deacon wrote:
> > > > 
> > > > Thanks, that sounds great. FWIW, there are multiple ways of implementing
> > > > the patch (i.e. whether you strengthen lock or unlock). I had a crack at
> > > > something here, but it's not tested:
> > > > 
> > > >   http://marc.info/?l=linux-arch&m=143758379023849&w=2
> > > 
> > > I notice you are not changing PPC_RELEASE_BARRIER, but only the spin unlock
> > > code. But from my reading of the docs we need to make sure any UNLOCK+LOCK is a
> > > full barrier, not just spin unlock/lock?
> > > 
> > > So don't we need to worry about some of the other locks as well? At least
> > > rwlock, and mutex fast path?
> > 
> > Hmm, that's a good question. I notice that you don't do any of the SYNC_IO
> > stuff for any locks other than spinlocks but I don't know whether
> > smp_mb__after_unlock_lock is similarly limited in scope.
> > 
> > Paul?
> 
> I would expect the various locks to have similar ordering characteristics.
> 
> Or am I missing something subtle here?

I don't think so.

The docs just talk about ACQUIRE/RELEASE, so I think it needs to apply to all
lock types. Or at least the list mentioned in the docs which is:

 (*) spin locks
 (*) R/W spin locks
 (*) mutexes
 (*) semaphores
 (*) R/W semaphores
 (*) RCU

cheers


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