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

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Message-ID: <20150720133906.GG9908@arm.com>
Date:	Mon, 20 Jul 2015 14:39:06 +0100
From:	Will Deacon <will.deacon@....com>
To:	Benjamin Herrenschmidt <benh@...nel.crashing.org>
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 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?

Will
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