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

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Message-ID: <20150717124040.GB3717@linux.vnet.ibm.com>
Date:	Fri, 17 Jul 2015 05:40:40 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Will Deacon <will.deacon@....com>,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	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>
Subject: Re: [RFC PATCH v2] memory-barriers: remove
 smp_mb__after_unlock_lock()

On Fri, Jul 17, 2015 at 12:15:35PM +0200, Peter Zijlstra wrote:
> On Fri, Jul 17, 2015 at 10:32:21AM +0100, Will Deacon wrote:
> > @@ -158,9 +140,7 @@ void arch_spin_lock_flags(arch_spinlock_t *lock, unsigned long flags)
> >  
> >  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;
> >  }
> 
> Should we then also make smp_store_release() use sync instead of lwsync
> to keep it consistent?

Unless smp_store_release() needs to interact with MMIO accesses, it
should still be able to be lwsync.  This means that unlock-lock is a full
barrier, but relase-acquire is not necessarily, which should be just fine.

							Thanx, Paul

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