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SubjectRe: [PATCH 7/8] padata: Flush the padata queues actively
On Thu, Apr 29, 2010 at 04:11:12PM -0700, Andrew Morton wrote:
> On Thu, 29 Apr 2010 14:44:26 +0200
> Steffen Klassert <steffen.klassert@secunet.com> wrote:
>
> > +static void padata_flush_queues(struct parallel_data *pd)
> > +{
> > + int cpu;
> > + struct padata_queue *queue;
> > +
> > + for_each_cpu(cpu, pd->cpumask) {
> > + queue = per_cpu_ptr(pd->queue, cpu);
> > + flush_work(&queue->pwork);
> > + }
> > +
> > + del_timer_sync(&pd->timer);
> > +
> > + if (atomic_read(&pd->reorder_objects))
> > + padata_reorder(pd);
>
> padata_reorder() can fail to do anything, if someone else is holding
> pd->lock. What happens then?
>

padata does not accept new objects for parallelization if padata_flush_queues
is called. The way of the data objects throught the padata queues is

--> parallelization queue -> reorder queue -> serialization queue -->

So padata_flush_queues processes the objects in the parallelization queue
by doing flush_work(&queue->pwork). Then we delete the timer and wait on a
potentially running timer function. We are not accepting new objects
and the parallelization queue is empty, so the lock must be free then.

>
> > + for_each_cpu(cpu, pd->cpumask) {
> > + queue = per_cpu_ptr(pd->queue, cpu);
> > + flush_work(&queue->swork);
> > + }
> > + BUG_ON(atomic_read(&pd->refcnt) != 0);
> > +}
>
> Are we safe against cpu hot-unplug in this code?

padata_flush_queues is called after a call to get_online_cpus in all but
one cases. I just noticed that I forgot to add the
get_online_cpus/put_online_cpus in padata_free. I'll update the
get_online_cpus/put_online_cpus patch accordingly, then it should be save in
all cases.


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