Re: Q: smp.c && barriers (Was: [PATCH 1/4] generic-smp: remove single ipi fallback for smp_call_function_many())

[Oleg Nesterov <oleg@redhat.com>]

On 02/16, Peter Zijlstra wrote:
>
> On Mon, 2009-02-16 at 23:02 +0100, Oleg Nesterov wrote:
> > On 02/16, Peter Zijlstra wrote:
> > >
> > > On Mon, 2009-02-16 at 22:32 +0100, Oleg Nesterov wrote:
> > > > > I was about to write a response, but found it to be a justification for
> > > > > the read_barrier_depends() at the end of the loop.
> > > >
> > > > I forgot to mention I don't understand the read_barrier_depends() at the
> > > > end of the loop as well ;)
> > >
> > > Suppose cpu0 adds to csd to cpu1:
> > >
> > >
> > >  cpu0:                 cpu1:
> > >
> > > add entry1
> > > mb();
> > > send ipi
> > >                       run ipi handler
> > >                       read_barrier_depends()
> > >                       while (!list_empty())    [A]
> > >                         do foo
> > >
> > > add entry2
> > > mb();
> > > [no ipi -- we still observe entry1]
> > >
> > >                         remove foo
> > >                         read_barrier_depends()
> > >                       while (!list_empty())      [B]
> >
> > Still can't understand.
> >
> > cpu1 (generic_smp_call_function_single_interrupt) does
> > list_replace_init(q->lock), this lock is also taken by
> > generic_exec_single().
> >
> > Either cpu1 sees entry2 on list, or cpu0 sees list_empty()
> > and sends ipi.
>
> cpu0:		cpu1:
>
> spin_lock_irqsave(&dst->lock, flags);
> ipi = list_empty(&dst->list);
> list_add_tail(&data->list, &dst->list);
> spin_unlock_irqrestore(&dst->lock, flags);
>
> ipi ----->
>
> 		while (!list_empty(&q->list)) {
>                 	unsigned int data_flags;
>
>                 	spin_lock(&q->lock);
>                		list_replace_init(&q->list, &list);
> 	                spin_unlock(&q->lock);
>
>
> Strictly speaking the unlock() is semi-permeable, allowing the read of
> q->list to enter the critical section, allowing us to observe an empty
> list, never getting to q->lock on cpu1.

Hmm. If we take &q->lock, then we alread saw !list_empty() ?

And the question is, how can we miss list_empty() == F before spin_lock().

> > > The read_barrier_depends() matches the mb() on the other cpu, without
> > > which the 'new' entry might not be observed.
> >
> > And that mb() looks unneeded too. Again, because
> > generic_smp_call_function_single_interrupt() takes call_single_queue.lock
> > before it uses "data".

to clarify, I meant it is not needed unless we are going to send the IPI.
IOW, I think we can do

	if (ipi) {
		/* Make the list addition visible before sending the ipi. */
		wmb();
		arch_send_call_function_single_ipi(cpu);
	}

> > Even if I missed something (very possible), then I can't understand
> > why we need rmb() only on alpha.
>
> Because only alpha is insane enough to do speculative reads? Dunno
> really :-)

Perhaps...

It would be nice to have a comment which explains how can we miss the
first addition without read_barrier_depends(). And why only on alpha.

And arch/alpha/kernel/smp.c:handle_ipi() does mb() itself...

Confused.

Oleg.