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 16:39:30 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	Ingo Molnar <mingo@...e.hu>
Cc:	David Howells <dhowells@...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

On 04/22, Ingo Molnar wrote:
>
> * David Howells <dhowells@...hat.com> wrote:
>
> > Oleg Nesterov <oleg@...hat.com> wrote:
> >
> > > > That's an interesting question.  Should wake_up() imply a barrier of any
> > > > sort, I wonder.  Well, __wake_up() does impose a barrier as it uses a
> > > > spinlock, but I wonder if that's sufficient.
> > >
> > > wake_up() does imply the barrier. Note the smp_wmb() in try_to_wake_up().
> > > And in fact this wmb() implies mb(), because spin_lock() itself is STORE,
> > > and the futher LOADs can't leak up before spin_lock().
> > >
> > > But afaics, this doesn't matter? prepare_to_wait() sets
> > > task->state under wait_queue_head_t->lock and wake_up() takes
> > > this look too, so we can't miss the event.
> > >
> > > Or I completely misunderstood the issue...
> >
> > The problem is not what wake_up() and co. do, it's what you are
> > allowed to assume that they do.
> >
> > However, I think you're right, and that we can assume they imply a
> > full memory barrier.  To this end, I've attached a patch to
> > document this.
> >
> > David
> > ---
> > From: David Howells <dhowells@...hat.com>
> > Subject: [PATCH] Document that wake_up(), complete() and co. imply a full memory barrier
> >
> > Add to the memory barriers document to note that wake_up(), complete() and
> > co. all imply a full memory barrier.
>
> 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'.

Yes. But please look at the changelog in

	 "Add memory barrier semantics to wake_up() & co"
	 04e2f1741d235ba599037734878d72e57cb302b5

However, I must admit, I don't understand how to document the semantics
correctly. This wmb() before spin_lock() ensures we don't read task->state
before previous STOREs. This is what we care about, and this is what
I meant when I said "this wmb() implies mb()".

So, I think that try_to_wake_up() implies that the LOADS after it can't be
reordered with STOREs before it (and wmb() of course).

Oleg.

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