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

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Message-ID: <1437427094.3775.22.camel@kernel.crashing.org>
Date:	Tue, 21 Jul 2015 07:18:14 +1000
From:	Benjamin Herrenschmidt <benh@...nel.crashing.org>
To:	Will Deacon <will.deacon@....com>
Cc:	"paulmck@...ux.vnet.ibm.com" <paulmck@...ux.vnet.ibm.com>,
	Michael Ellerman <mpe@...erman.id.au>,
	"linux-arch@...r.kernel.org" <linux-arch@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Peter Zijlstra <peterz@...radead.org>
Subject: Re: [RFC PATCH v2] memory-barriers: remove
 smp_mb__after_unlock_lock()

On Mon, 2015-07-20 at 14:39 +0100, Will Deacon wrote:
> On Fri, Jul 17, 2015 at 11:14:14PM +0100, Benjamin Herrenschmidt wrote:
> > On Fri, 2015-07-17 at 10:32 +0100, Will Deacon wrote:
> > >  static inline void arch_spin_unlock(arch_spinlock_t *lock)
> > >  {
> > > -       SYNC_IO;
> > > -       __asm__ __volatile__("# arch_spin_unlock\n\t"
> > > -                               PPC_RELEASE_BARRIER: : :"memory");
> > > +       smp_mb();
> > >         lock->slock = 0;
> > >  }
> > 
> > That probably needs to be mb() in case somebody has the expectation that
> > it does a barrier vs. DMA on UP.
> 
> Hmm, but on !SMP doesn't arch_spin_unlock effectively expand to barrier()
> in the core code?

include/linux/spinlock_up.h:# define arch_spin_unlock(lock)	do { barrier(); (void)(lock); } while (0)

Indeed...

Ben.


> Will
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