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 16:56:58 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Oleg Nesterov <oleg@...hat.com>
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


* Oleg Nesterov <oleg@...hat.com> wrote:

> 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

yes - but still that commit is only wrt. the ->state check.

> 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).

Note that the patch David sent says "full memory barrier", not "full 
memory barrier wrt. task->state":

+ (*) wake_up(), try_to_wake_up() and co. imply a full memory barrier.
+
+ (*) complete() and co. imply a full memory barrier.

These statements are not true in that form, as this code does not 
imply a full memory barrier. It does imply one on task->state 
_alone_ (and a couple of other wq-internal variables it happens to 
read for sure).

But even that one isnt entirely true in the two sub-cases i noted: 
the !wq case (which can happen in object state teardown) and the 
special ->func handler (which can happen in custom wakeup code a'la 
eventpoll).

So adding a comment that says "this is a full memory barrier" is 
simply not true to that extent, and is easily misunderstood. Adding 
"this is a fully memory barrier for task->state dependent data flow" 
would be more correct. (with a 'as long as wq is not NULL, and as 
long as the code using this isnt overriding ->func)

Agreed?

	Ingo
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