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

[Peter Zijlstra <a.p.zijlstra@chello.nl>]

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.