On Tue, Feb 07, 2017 at 12:43:27PM +0100, Michal Hocko wrote:
> > Right. The unbind operation can set a mask that is any allowable CPU and
> > the final process_work is not done in a context that prevents
> > preemption.
> >
> > diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> > index 3b93879990fd..7af165d308c4 100644
> > --- a/mm/page_alloc.c
> > +++ b/mm/page_alloc.c
> > @@ -2342,7 +2342,14 @@ void drain_local_pages(struct zone *zone)
> >
> > static void drain_local_pages_wq(struct work_struct *work)
> > {
> > + /*
> > + * Ordinarily a drain operation is bound to a CPU but may be unbound
> > + * after a CPU hotplug operation so it's necessary to disable
> > + * preemption for the drain to stabilise the CPU ID.
> > + */
> > + preempt_disable();
> > drain_local_pages(NULL);
> > + preempt_enable_no_resched();
> > }
> >
> > /*
> [...]
> > @@ -6711,7 +6714,16 @@ static int page_alloc_cpu_dead(unsigned int cpu)
> > {
> >
> > lru_add_drain_cpu(cpu);
> > +
> > + /*
> > + * A per-cpu drain via a workqueue from drain_all_pages can be
> > + * rescheduled onto an unrelated CPU. That allows the hotplug
> > + * operation and the drain to potentially race on the same
> > + * CPU. Serialise hotplug versus drain using pcpu_drain_mutex
> > + */
> > + mutex_lock(&pcpu_drain_mutex);
> > drain_pages(cpu);
> > + mutex_unlock(&pcpu_drain_mutex);
>
> You cannot put sleepable lock inside the preempt disbaled section...
> We can make it a spinlock right?
>
The CPU down callback can hold a mutex and at least he SLUB callback
already does so. That gives
page_alloc_cpu_dead
mutex_lock
drain_pages
mutex_unlock
drain_all_pages
mutex_lock
queue workqueue
drain_local_pages_wq
preempt_disable
drain_local_pages
drain_pages
preempt_enable
flush queues
mutex_unlock
I must be blind or maybe it's rushing between multiple concerns but which
sleepable lock is of concern?
--
Mel Gorman
SUSE Labs
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