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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
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