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

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Date:	Fri, 24 Jul 2015 12:31:01 +0100
From:	Will Deacon <will.deacon@....com>
To:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc:	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()

Hi Paul,

On Wed, Jul 15, 2015 at 02:12:21PM +0100, Paul E. McKenney wrote:
> On Wed, Jul 15, 2015 at 11:51:35AM +0100, Will Deacon wrote:
> > On Wed, Jul 15, 2015 at 02:38:20AM +0100, Paul E. McKenney wrote:
> > > On Tue, Jul 14, 2015 at 12:31:44PM -0700, Paul E. McKenney wrote:
> > > > On Tue, Jul 14, 2015 at 03:12:16PM +0100, Will Deacon wrote:
> > > > > On Tue, Jul 14, 2015 at 03:00:14PM +0100, Paul E. McKenney wrote:
> > > > > > On Tue, Jul 14, 2015 at 01:51:46PM +0100, Will Deacon wrote:
> > > > > > > On Tue, Jul 14, 2015 at 01:45:40PM +0100, Paul E. McKenney wrote:
> > > > > > > > On Tue, Jul 14, 2015 at 11:04:29AM +0100, Will Deacon wrote:
> > > > > > > > > Given that RCU is currently the only user of this barrier, how would you
> > > > > > > > > feel about making the barrier local to RCU and not part of the general
> > > > > > > > > memory-barrier API?
> > > > > > > > 
> > > > > > > > In theory, no objection.  Your thought is to leave the definitions where
> > > > > > > > they are, mark them as being used only by RCU, and removing mention from
> > > > > > > > memory-barriers.txt?  Or did you have something else in mind?
> > > > > > > 
> > > > > > > Actually, I was thinking of defining them in an RCU header file with an
> > > > > > > #ifdef CONFIG_POWERPC for the smb_mb() version. Then you could have a big
> > > > > > > comment describing the semantics, or put that in an RCU Documentation file
> > > > > > > instead of memory-barriers.txt.
> > > > > > > 
> > > > > > > That *should* then mean we notice anybody else trying to use the barrier,
> > > > > > > because they'd need to send patches to either add something equivalent
> > > > > > > or move the definition out again.
> > > > > > 
> > > > > > My concern with this approach is that someone putting together a new
> > > > > > architecture might miss this.  That said, this approach certainly would
> > > > > > work for the current architectures.
> > > > > 
> > > > > I don't think they're any more likely to miss it than with the current
> > > > > situation where the generic code defines the macro as a NOP unless you
> > > > > explicitly override it.
> > > > 
> > > > Fair enough...
> > > 
> > > Like this?
> > 
> > Precisely! Thanks for cooking the patch -- this lays all my worries to
> > rest, so:
> > 
> >   Acked-by: Will Deacon <will.deacon@....com>
> 
> Thank you!

[...]

> > > 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.

Will
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