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

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Message-ID: <20150714141202.GN16213@arm.com>
Date:	Tue, 14 Jul 2015 15:12:16 +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()

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.

Will
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