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Message-ID: <20090216220214.GA10093@redhat.com>
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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