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

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Date:	Mon, 2 Nov 2015 17:25:32 -0800
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Paul McKenney <paulmck@...ux.vnet.ibm.com>
Cc:	Will Deacon <will.deacon@....com>,
	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...nel.org>,
	Oleg Nesterov <oleg@...hat.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	boqun.feng@...il.com, Jonathan Corbet <corbet@....net>,
	Michal Hocko <mhocko@...nel.org>,
	David Howells <dhowells@...hat.com>
Subject: Re: [PATCH 4/4] locking: Introduce smp_cond_acquire()

On Mon, Nov 2, 2015 at 5:14 PM, Paul E. McKenney
<paulmck@...ux.vnet.ibm.com> wrote:
>
> You would ask that question when I am many thousands of miles from my
> copy of the Alpha reference manual!  ;-)

I don't think I've touched a paper manual in years. Too m uch effort
to index and search. Look here

   http://download.majix.org/dec/alpha_arch_ref.pdf

> There is explicit wording in that manual that says that no multi-variable
> ordering is implied without explicit memory-barrier instructions.

Right. But the whole "read -> conditional -> write" ends up being a
dependency chain to *every* single write that happens after the
conditional.

So there may be no "multi-variable ordering" implied in any individual
access, but despite that a simple "read + conditional" orders every
single store that comes after it. Even if it orders them
"individually".

                Linus
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