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From: Will Deacon <will.deacon@arm.com>
To: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
Cc: Peter Zijlstra <peterz@infradead.org>,
	"torvalds@linux-foundation.org" <torvalds@linux-foundation.org>,
	"oleg@redhat.com" <oleg@redhat.com>,
	"benh@kernel.crashing.org" <benh@kernel.crashing.org>,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
	"linux-arch@vger.kernel.org" <linux-arch@vger.kernel.org>
Subject: Re: Behaviour of smp_mb__{before,after}_spin* and acquire/release
Date: Wed, 21 Jan 2015 13:56:59 +0000	[thread overview]
Message-ID: <20150121135659.GG4549@arm.com> (raw)
In-Reply-To: <20150120213545.GT9719@linux.vnet.ibm.com>

On Tue, Jan 20, 2015 at 09:35:45PM +0000, Paul E. McKenney wrote:
> On Tue, Jan 20, 2015 at 10:34:43AM +0100, Peter Zijlstra wrote:
> > On Tue, Jan 13, 2015 at 04:33:54PM +0000, Will Deacon wrote:
> > > Hi Paul,
> > > 
> > > I started dusting off a series I've been working to implement a relaxed
> > > atomic API in Linux (i.e. things like atomic_read(v, ACQUIRE)) but I'm
> > > having trouble making sense of the ordering semantics we have in mainline
> > > today:
> > 
> > >   2. Does smp_mb__after_unlock_lock order smp_store_release against
> > >      smp_load_acquire? Again, Documentation/memory-barriers.txt puts
> > >      these operations into the RELEASE and ACQUIRE classes respectively,
> > >      but since smp_mb__after_unlock_lock is a NOP everywhere other than
> > >      PowerPC, I don't think this is enforced by the current code. 
> > 
> > Yeah, wasn't Paul going to talk to Ben about that? PPC is the only arch
> > that has the weak ACQUIRE/RELEASE for its spinlocks.
> 
> I thought that you guys were going to propose something and we would see
> what the reaction was.  ;-)
> 
> > >      Most
> > >      architectures follow the pattern used by asm-generic/barrier.h:
> > > 
> > >        release: smp_mb(); STORE
> > >        acquire: LOAD; smp_mb();
> > > 
> > >      which doesn't provide any release -> acquire ordering afaict.
> > 
> > Only when combined on the same address, if the LOAD observes the result
> > of the STORE we can guarantee the rest of the ordering. And if you
> > build a locking primitive with them (or circular lists or whatnot) you
> > have that extra condition.
> > 
> > But yes, I see your argument that this implementation is weak like the
> > PPC.
> 
> A more complete example would be as follows:
> 
>        STOREs followed by release: smp_mb(); STORE A
>        acquire: LOAD A; smp_mb(); preceding LOADs
> 
> If the LOAD A gets the value from the STORE A, then the LOADs following
> the acquire are guaranteed to see the STOREs preceding the release.
> 
> And yes, this really truly does work fine with weaker ordering.

I agree, but if we consider the case where the acquire and the release are
operating on *different* addresses, then the current Documentation would
tell us to use smp_mb__after_unlock_lock to ensure ordering, which won't
work on anything other than Power.

Will

  reply	other threads:[~2015-01-21 13:57 UTC|newest]

Thread overview: 11+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2015-01-13 16:33 Will Deacon
2015-01-13 18:45 ` Oleg Nesterov
2015-01-14 11:31   ` Will Deacon
2015-01-20  3:40     ` Paul E. McKenney
2015-01-20 10:43       ` Will Deacon
2015-01-20  9:34 ` Peter Zijlstra
2015-01-20 10:38   ` Will Deacon
2015-01-20 21:35   ` Paul E. McKenney
2015-01-21 13:56     ` Will Deacon [this message]
2015-01-23 14:08     ` Will Deacon
2015-01-23 21:21       ` Paul E. McKenney

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