Re: [RFC PATCH v2] memory-barriers: remove smp_mb__after_unlock_lock()

From	Michael Ellerman <mpe@ellerman.id.au>
Newsgroups	linux.kernel
Subject	Re: [RFC PATCH v2] memory-barriers: remove smp_mb__after_unlock_lock()
Date	2015-08-26 06:10 +0200
Message-ID	<q1Aca-2SV-7@gated-at.bofh.it> (permalink)
References	(6 earlier) <pYElX-7Y-3@gated-at.bofh.it> <pYKB5-Rg-29@gated-at.bofh.it> <pZuDU-15a-1@gated-at.bofh.it> <pZApX-YM-1@gated-at.bofh.it> <q1wLf-69E-9@gated-at.bofh.it>
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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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Re: [RFC PATCH v2] memory-barriers: remove  smp_mb__after_unlock_lock() Michael Ellerman <mpe@ellerman.id.au> - 2015-08-20 11:50 +0200
  Re: [RFC PATCH v2] memory-barriers: remove  smp_mb__after_unlock_lock() Will Deacon <will.deacon@arm.com> - 2015-08-20 18:00 +0200
    Re: [RFC PATCH v2] memory-barriers: remove  smp_mb__after_unlock_lock() "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2015-08-26 02:30 +0200
      Re: [RFC PATCH v2] memory-barriers: remove  smp_mb__after_unlock_lock() Michael Ellerman <mpe@ellerman.id.au> - 2015-08-26 06:10 +0200

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