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

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Message-ID: <20150714140014.GD3717@linux.vnet.ibm.com>
Date:	Tue, 14 Jul 2015 07:00:14 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Will Deacon <will.deacon@....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 01:51:46PM +0100, Will Deacon wrote:
> Hi Paul,
> 
> 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.

> > > My main reason for proposing its removal is because I don't want to see
> > > it being used (incorrectly) all over the place to order the new RELEASE
> > > and ACQUIRE operations I posted separately, at which point we have to try
> > > fixing up all the callers or retrofitting some semantics. It doesn't help
> > > that memory-barriers.txt lumps things like LOCK and ACQUIRE together,
> > > whereas this barrier is currently only intended to be used in conjunction
> > > with the former.
> > 
> > Heh!  That lumping was considered to be a feature at the time.  ;-)
> 
> Oh, I'm sure it was added with good intentions!

And we all know which road is paved with good intentions!  ;-)

							Thanx, Paul

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