Re: mm: deadlock between get_online_cpus/pcpu_alloc

[Michal Hocko]

On Tue 07-02-17 12:43:27, Michal Hocko wrote:
> On Tue 07-02-17 11:34:35, Mel Gorman wrote:
> > On Tue, Feb 07, 2017 at 11:35:52AM +0100, Michal Hocko wrote:
> > > On Tue 07-02-17 10:28:09, Mel Gorman wrote:
> > > > On Tue, Feb 07, 2017 at 10:49:28AM +0100, Vlastimil Babka wrote:
> > > > > On 02/07/2017 10:43 AM, Mel Gorman wrote:
> > > > > > If I'm reading this right, a hot-remove will set the pool POOL_DISASSOCIATED
> > > > > > and unbound. A workqueue queued for draining get migrated during hot-remove
> > > > > > and a drain operation will execute twice on a CPU -- one for what was
> > > > > > queued and a second time for the CPU it was migrated from. It should still
> > > > > > work with flush_work which doesn't appear to block forever if an item
> > > > > > got migrated to another workqueue. The actual drain workqueue function is
> > > > > > using the CPU ID it's currently running on so it shouldn't get confused.
> > > > > 
> > > > > Is the worker that will process this migrated workqueue also guaranteed
> > > > > to be pinned to a cpu for the whole work, though? drain_local_pages()
> > > > > needs that guarantee.
> > > > > 
> > > > 
> > > > It should be by running on a workqueue handler bound to that CPU (queued
> > > > on wq->cpu_pwqs in __queue_work)
> > > 
> > > Are you sure? The comment in kernel/workqueue.c says
> > >          * While DISASSOCIATED, the cpu may be offline and all workers have
> > >          * %WORKER_UNBOUND set and concurrency management disabled, and may
> > >          * be executing on any CPU.  The pool behaves as an unbound one.
> > > 
> > > I might be misreadig but an unbound pool can be handled by workers which
> > > are not pinned on any cpu AFAIU.
> > 
> > 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?

Scratch that! For some reason I thought that cpu notifiers are run in an
atomic context. Now that I am checking the code again it turns out I was
wrong. __cpu_notify uses __raw_notifier_call_chain so this is not an
atomic context. Anyway, shouldn't be it sufficient to disable preemption
on drain_local_pages_wq? The CPU hotplug callback will not preempt us
and so we cannot work on the same cpus, right?

-- 
Michal Hocko
SUSE Labs