linux-kernel - Re: [PATCH] Document that wake_up(), complete() and co. imply a full memory barrier

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Date:	Wed, 22 Apr 2009 16:12:06 +0100
From:	David Howells <dhowells@...hat.com>
To:	Ingo Molnar <mingo@...e.hu>
Cc:	dhowells@...hat.com, Oleg Nesterov <oleg@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Trond.Myklebust@...app.com, serue@...ibm.com, steved@...hat.com,
	viro@...iv.linux.org.uk,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Nick Piggin <nickpiggin@...oo.com.au>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] Document that wake_up(), complete() and co. imply a full memory barrier

Ingo Molnar <mingo@...e.hu> wrote:

> No. They dont generally imply a full memory barrier versus any
> arbitrary prior (or following) memory access.
>
> try_to_wake_up() has an smp_wmb() so it is a write memory barrier
> (but not necessarily a read memory barrier). Otherwise there are
> spinlocks there but spinlocks are not explicit 'full memory
> barriers'.

Blech.  That's a good point LOCK...UNLOCK does not imply a full barrier.

So we can't assume that complete(), wake_up() and co. imply any barriers.

All we can assume is that try_to_wake_up() implies a write barrier, but we
can't assume that that will be called via __wake_up_common().

David
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