Re: mm: deadlock between get_online_cpus/pcpu_alloc

[Mel Gorman]

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