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Date:   Mon, 16 Jan 2017 07:56:47 +0100
From:   Ingo Molnar <mingo@...nel.org>
To:     "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc:     linux-kernel@...r.kernel.org, jiangshanlai@...il.com,
        dipankar@...ibm.com, akpm@...ux-foundation.org,
        mathieu.desnoyers@...icios.com, josh@...htriplett.org,
        tglx@...utronix.de, peterz@...radead.org, rostedt@...dmis.org,
        dhowells@...hat.com, edumazet@...gle.com, dvhart@...ux.intel.com,
        fweisbec@...il.com, oleg@...hat.com, bobby.prani@...il.com,
        will.deacon@....com, boqun.feng@...ux.vnet.ibm.com,
        linuxppc-dev@...ts.ozlabs.org,
        Peter Zijlstra <a.p.zijlstra@...llo.nl>
Subject: Re: [PATCH tip/core/rcu 2/3] srcu: Force full grace-period ordering


* Paul E. McKenney <paulmck@...ux.vnet.ibm.com> wrote:

> On Sun, Jan 15, 2017 at 10:40:58AM +0100, Ingo Molnar wrote:
> > 
> > * Paul E. McKenney <paulmck@...ux.vnet.ibm.com> wrote:
> > 
> > > > [sounds of rummaging around in the Git tree]
> > > > 
> > > > I found this commit of yours from ancient history (more than a year ago!):
> > > > 
> > > >   commit 12d560f4ea87030667438a169912380be00cea4b
> > > >   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>
> > > > 
> > > > So I concur and I'm fine with your patch - or with the status quo code as well.
> > > 
> > > I already have the patch queued, so how about I keep it if I get an ack
> > > from the powerpc guys and drop it otherwise?
> > 
> > Yeah, sounds good! Your patch made me look up 'RelAcq' so it has documentation 
> > value as well ;-)
> 
> ;-) ;-) ;-)
> 
> Looking forward, my guess would be that if some other code needs
> smp_mb__after_unlock_lock() or if some other architecture needs
> non-smb_mb() special handling, I should consider making it work the
> same as smp_mb__after_atomic() and friends.  Does that seem like a
> reasonable thought?

Yeah, absolutely - it's just that the pattern triggered the 'this looks a bit too 
specialized' response in me, but after seeing the details (again ...) I agree that 
this time is different!

Thanks,

	Ingo

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