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Date: Wed, 22 Apr 2009 17:19:09 +0200
From: Ingo Molnar <mingo@elte.hu>
To: David Howells <dhowells@redhat.com>
Cc: Oleg Nesterov <oleg@redhat.com>, Andrew Morton <akpm@linux-foundation.org>,
       Trond.Myklebust@netapp.com, serue@us.ibm.com, steved@redhat.com,
       viro@zeniv.linux.org.uk,
       "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>,
       Nick Piggin <nickpiggin@yahoo.com.au>, linux-kernel@vger.kernel.org
Subject: Re: [PATCH] Document that wake_up(), complete() and co. imply a
	full memory barrier
Message-ID: <20090422151909.GA18224@elte.hu>
References: <20090415162712.342d4c07.akpm@linux-foundation.org> <1239649429.16771.9.camel@heimdal.trondhjem.org> <20090413181733.GA10424@redhat.com> <32260.1239658818@redhat.com> <20090413214852.GA1127@redhat.com> <1239659841.16771.26.camel@heimdal.trondhjem.org> <20090413222451.GA2758@redhat.com> <14561.1239873018@redhat.com> <21239.1240407420@redhat.com> <4664.1240413126@redhat.com>
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* David Howells <dhowells@redhat.com> wrote:

> Ingo Molnar <mingo@elte.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