linux-kernel - Re: [PATCH 4/4] locking: Introduce smp_cond

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Message-ID: <20151102204037.GX17308@twins.programming.kicks-ass.net>
Date:	Mon, 2 Nov 2015 21:40:37 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	David Howells <dhowells@...hat.com>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Ingo Molnar <mingo@...nel.org>,
	Oleg Nesterov <oleg@...hat.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Paul McKenney <paulmck@...ux.vnet.ibm.com>,
	boqun.feng@...il.com, Jonathan Corbet <corbet@....net>,
	Michal Hocko <mhocko@...nel.org>,
	Will Deacon <will.deacon@....com>
Subject: Re: [PATCH 4/4] locking: Introduce smp_cond_acquire()

On Mon, Nov 02, 2015 at 08:36:49PM +0000, David Howells wrote:
> Linus Torvalds <torvalds@...ux-foundation.org> wrote:
> 
> > > +       smp_read_barrier_depends(); /* ctrl */  \
> > > +       smp_rmb(); /* ctrl + rmb := acquire */  \
> 
> Doesn't smp_rmb() imply an smp_read_barrier_depends() anyway?  In
> memory-barriers.txt, it says:
> 
>      Read memory barriers imply data dependency barriers, and so can
>      substitute for them.

Yes, I noted that in a follow up email, Alpha implements both as MB. So
just the smp_rmb() is sufficient.

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