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

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Message-ID: <20150817061501.GF11078@linux.vnet.ibm.com>
Date:	Sun, 16 Aug 2015 23:15:01 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Michael Ellerman <mpe@...erman.id.au>
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 Mon, Aug 17, 2015 at 02:06:07PM +1000, Michael Ellerman wrote:
> On Wed, 2015-08-12 at 08:43 -0700, Paul E. McKenney wrote:
> > On Wed, Aug 12, 2015 at 02:44:15PM +0100, Will Deacon wrote:
> > > Hello Paul,
> > > 
> > > On Fri, Jul 24, 2015 at 04:30:46PM +0100, Paul E. McKenney wrote:
> > > > On Fri, Jul 24, 2015 at 12:31:01PM +0100, Will Deacon wrote:
> > > > > On Wed, Jul 15, 2015 at 02:12:21PM +0100, Paul E. McKenney wrote:
> > > > > > > > commit 695c05d4b9666c50b40a1c022678b5f6e2e3e771
> > > > > > > > Author: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
> > > > > > > > Date:   Tue Jul 14 18:35:23 2015 -0700
> > > > > > > > 
> > > > > > > >     rcu,locking: Privatize smp_mb__after_unlock_lock()
> > > > > > > >     
> > > > > > > >     RCU is the only thing that uses smp_mb__after_unlock_lock(), and is
> > > > > > > >     likely the only thing that ever will use it, so this commit makes this
> > > > > > > >     macro private to RCU.
> > > > > > > >     
> > > > > > > >     Signed-off-by: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
> > > > > > > >     Cc: Will Deacon <will.deacon@....com>
> > > > > > > >     Cc: Peter Zijlstra <peterz@...radead.org>
> > > > > > > >     Cc: Benjamin Herrenschmidt <benh@...nel.crashing.org>
> > > > > > > >     Cc: "linux-arch@...r.kernel.org" <linux-arch@...r.kernel.org>
> > > > > 
> > > > > Are you planning to queue this somewhere? I think it makes sense regardless
> > > > > of whether we change PowerPc or not and ideally it would be merged around
> > > > > the same time as my relaxed atomics series.
> > > > 
> > > > I have is in -rcu.  By default, I will push it to the 4.4 merge window.
> > > > Please let me know if you need it sooner.
> > > 
> > > The generic relaxed atomics are now queued in -tip, so it would be really
> > > good to see this Documentation update land in 4.3 if at all possible. I
> > > appreciate it's late in the cycle, but it's always worth asking.
> > 
> > Can't hurt to give it a try.  I have set -rcu's rcu/next branch to this
> > commit, and if it passes a few day's worth of testing, I will see what
> > Ingo has to say about a pull request.
> > 
> > This commit also privatizes smp_mb__after_unlock_lock() as well as
> > updating documentation.  Looks like we need to strengthen powerpc's
> > locking primitives, then get rid of smp_mb__after_unlock_lock() entirely.
> > Or did that already happen and I just missed it?
> 
> No it didn't.
> 
> I thought the end result of this thread was that we didn't *need* to change the
> powerpc lock semantics? Or did I read it wrong?
> 
> ie. the docs now say that RELEASE+ACQUIRE is not a full barrier, which is
> consistent with our current implementation.

That change happened about 1.5 years ago, and I thought that the
current discussion was about reversing it, based in part on the
recent powerpc benchmarks of locking primitives with and without the
sync instruction.  But regardless, I clearly cannot remove either the
smp_mb__after_unlock_lock() or the powerpc definition of it to be smp_mb()
if powerpc unlock/lock is not strengthened.

							Thanx, Paul

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