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SubjectRe: [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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