Re: [PATCH 4/4] locking: Introduce smp_cond_acquire()

From: Linus Torvalds <torvalds@linux-foundation.org>
To: Paul McKenney <paulmck@linux.vnet.ibm.com>
Cc: Will Deacon <will.deacon@arm.com>,
	Peter Zijlstra <peterz@infradead.org>,
	Ingo Molnar <mingo@kernel.org>, Oleg Nesterov <oleg@redhat.com>,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
	boqun.feng@gmail.com, Jonathan Corbet <corbet@lwn.net>,
	Michal Hocko <mhocko@kernel.org>,
	David Howells <dhowells@redhat.com>
Subject: Re: [PATCH 4/4] locking: Introduce smp_cond_acquire()
Date: Mon, 2 Nov 2015 17:25:32 -0800	[thread overview]
Message-ID: <CA+55aFxii+-g_=Etsi6wXsZMJOn0eQWdZ+Kwc9hLqBCwfDBiJw@mail.gmail.com> (raw)
In-Reply-To: <20151103011406.GB29027@linux.vnet.ibm.com>

On Mon, Nov 2, 2015 at 5:14 PM, Paul E. McKenney
<paulmck@linux.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