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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Message-Id: <1234820704.30178.396.camel@laptop>
Date:	Mon, 16 Feb 2009 22:45:04 +0100
From:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
To:	Oleg Nesterov <oleg@...hat.com>
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 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]


The read_barrier_depends() matches the mb() on the other cpu, without
which the 'new' entry might not be observed.

So it turns out the initial one is needed as well.

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