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

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Date:	Mon, 16 Feb 2009 23:02:14 +0100
From:	Oleg Nesterov <oleg@...hat.com>
To:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Cc:	Jens Axboe <jens.axboe@...cle.com>,
	Suresh Siddha <suresh.b.siddha@...el.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Nick Piggin <npiggin@...e.de>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Ingo Molnar <mingo@...e.hu>,
	Rusty Russell <rusty@...tcorp.com.au>,
	Steven Rostedt <rostedt@...dmis.org>,
	linux-kernel@...r.kernel.org
Subject: Re: Q: smp.c && barriers (Was: [PATCH 1/4] generic-smp: remove
	single ipi fallback for smp_call_function_many())

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.

> 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".


Even if I missed something (very possible), then I can't understand
why we need rmb() only on alpha.

Oleg.

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