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

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Message-ID: <20090422151909.GA18224@elte.hu>
Date:	Wed, 22 Apr 2009 17:19:09 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	David Howells <dhowells@...hat.com>
Cc:	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


* David Howells <dhowells@...hat.com> wrote:

> 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().

Yeah, it's all too special-case. We might rely on it in special, 
well-argued cases but we should not document it as a general barrier 
property.

	Ingo
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