Re: [PATCH tip/core/rcu 2/3] srcu: Force full grace-period ordering

[Ingo Molnar <mingo@kernel.org>]

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

> On Sun, Jan 15, 2017 at 10:40:58AM +0100, Ingo Molnar wrote:
> > 
> > * Paul E. McKenney <paulmck@linux.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@linux.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@linux.vnet.ibm.com>
> > > >     Cc: Will Deacon <will.deacon@arm.com>
> > > >     Cc: Peter Zijlstra <peterz@infradead.org>
> > > >     Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
> > > >     Cc: "linux-arch@vger.kernel.org" <linux-arch@vger.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