linux-kernel - Re: [PATCH v5 tip/core/locking 6/7] locking: Add an smp_mb__after_unlock

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Message-ID: <20131210201154.GF4208@linux.vnet.ibm.com>
Date:	Tue, 10 Dec 2013 12:11:54 -0800
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Josh Triplett <josh@...htriplett.org>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	linux-kernel@...r.kernel.org, mingo@...nel.org,
	laijs@...fujitsu.com, dipankar@...ibm.com,
	akpm@...ux-foundation.org, mathieu.desnoyers@...icios.com,
	niv@...ibm.com, tglx@...utronix.de, rostedt@...dmis.org,
	dhowells@...hat.com, edumazet@...gle.com, darren@...art.com,
	fweisbec@...il.com, sbw@....edu,
	Linux-Arch <linux-arch@...r.kernel.org>,
	Ingo Molnar <mingo@...hat.com>,
	Oleg Nesterov <oleg@...hat.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH v5 tip/core/locking 6/7] locking: Add an
 smp_mb__after_unlock_lock() for UNLOCK+LOCK barrier

On Tue, Dec 10, 2013 at 09:45:08AM -0800, Josh Triplett wrote:
> On Tue, Dec 10, 2013 at 01:37:26PM +0100, Peter Zijlstra wrote:
> > On Mon, Dec 09, 2013 at 05:28:02PM -0800, Paul E. McKenney wrote:
> > > diff --git a/arch/powerpc/include/asm/barrier.h b/arch/powerpc/include/asm/barrier.h
> > > index f89da808ce31..abf645799991 100644
> > > --- a/arch/powerpc/include/asm/barrier.h
> > > +++ b/arch/powerpc/include/asm/barrier.h
> > > @@ -84,4 +84,6 @@ do {									\
> > >  	___p1;								\
> > >  })
> > >  
> > > +#define smp_mb__after_unlock_lock()	do { } while (0)
> > > +
> > >  #endif /* _ASM_POWERPC_BARRIER_H */
> > 
> > Didn't ben said ppc actually violates the current unlock+lock assumtion
> > and therefore this barrier woulnd't actually be a nop on ppc
> 
> Or, ppc could fix its lock primitives to preserve the unlock+lock
> assumption, and avoid subtle breakage across half the kernel.

Indeed.  However, another motivation for this change was the difficulty
in proving that x86 really provided the equivalent of a full barrier
for the MCS lock handoff case:

	http://www.spinics.net/lists/linux-mm/msg65653.html

							Thanx, Paul

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