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Date: Thu, 23 Apr 2009 23:24:34 +0200
From: Ingo Molnar <mingo@elte.hu>
To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: David Howells <dhowells@redhat.com>, Oleg Nesterov <oleg@redhat.com>,
       Andrew Morton <akpm@linux-foundation.org>, serue@us.ibm.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] It may not be assumed that wake_up(), finish_wait()
	and co. imply a memory barrier
Message-ID: <20090423212434.GA20556@elte.hu>
References: <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> <5591.1240417398@redhat.com> <21209.1240504344@redhat.com> <22170.1240518945@redhat.com> <alpine.LFD.2.00.0904231410590.3101@localhost.localdomain>
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* Linus Torvalds <torvalds@linux-foundation.org> wrote:

> On Thu, 23 Apr 2009, David Howells wrote:
> > 
> > Well, Ingo's point is it could be left up to the caller of 
> > wake_up() to supply the barrier:

Well, i mainly reacted to your documentation patch which was 
incorrect as it said wake-up implies a _FULL_ memory barrier.

I also suggested that lockless code should have its barriers clearly 
documented and they should not really rely on kernel facilities 
acting as memory barriers.

Then i also suggested that maybe in the future we could remove the 
smp_wmb() from try_to_wake_up(). That was just an afterthought, and 
a rather stupid one at that, as Linus quickly noted :-)

	Ingo