linux-kernel - Re: [RFC][PATCH 1/3] locking: Introduce smp_acquire__after_ctrl

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Date:	Tue, 7 Jun 2016 11:44:41 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Will Deacon <will.deacon@....com>
Cc:	Hannes Frederic Sowa <hannes@...essinduktion.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Vineet Gupta <Vineet.Gupta1@...opsys.com>,
	Waiman Long <waiman.long@....com>,
	linux-kernel@...r.kernel.org, torvalds@...ux-foundation.org,
	manfred@...orfullife.com, dave@...olabs.net, boqun.feng@...il.com,
	tj@...nel.org, pablo@...filter.org, kaber@...sh.net,
	davem@...emloft.net, oleg@...hat.com,
	netfilter-devel@...r.kernel.org, sasha.levin@...cle.com,
	hofrat@...dl.org
Subject: Re: [RFC][PATCH 1/3] locking: Introduce smp_acquire__after_ctrl_dep

On Tue, Jun 07, 2016 at 07:01:07PM +0100, Will Deacon wrote:
> On Tue, Jun 07, 2016 at 08:23:15AM -0700, Paul E. McKenney wrote:
> > On Tue, Jun 07, 2016 at 04:59:02PM +0200, Hannes Frederic Sowa wrote:
> > > Sorry, to follow-up again on this. Will Deacon's comments were about
> > > conditional-move instructions, which this compiler-option would prevent,
> > > as far as I can see it.
> > 
> > According to this email thread, I believe that this works the other
> > way around:
> > 
> > http://thread.gmane.org/gmane.linux.kernel/1721993
> > 
> > That parameter prevents the compiler from converting a conditional
> > store into an unconditional store, which would be really problematic.
> > Give the current kernel build, I believe that the compiler really is
> > within its rights to use conditional-move instructions as shown above.
> > But I again must defer to Will Deacon on the details.
> 
> A multi_v7_defconfig build of mainline certainly spits out conditional
> store instructions, but I have no idea whether these correspond to
> WRITE_ONCE or not:
> 
> $ objdump -d vmlinux | grep 'str\(eq\|ne\)' | wc -l
> 7326
> 
> At the end of the day, the ARM architecture says you can't rely on this
> being ordered and I can see it happening in practice in the face of
> conditional stores.

Thank you for the info, Will!

							Thanx, Paul