Re: mm: deadlock between get_online_cpus/pcpu_alloc

[Michal Hocko]

On Tue 07-02-17 10:23:31, Vlastimil Babka wrote:
> On 02/07/2017 09:48 AM, Michal Hocko wrote:
> > On Mon 06-02-17 22:05:30, Mel Gorman wrote:
> >>> Unfortunately it does not seem to help.
> >>
> >> I'm a little stuck on how to best handle this. get_online_cpus() can
> >> halt forever if the hotplug operation is holding the mutex when calling
> >> pcpu_alloc. One option would be to add a try_get_online_cpus() helper which
> >> trylocks the mutex. However, given that drain is so unlikely to actually
> >> make that make a difference when racing against parallel allocations,
> >> I think this should be acceptable.
> >>
> >> Any objections?
> >>
> >> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> >> index 3b93879990fd..a3192447e906 100644
> >> --- a/mm/page_alloc.c
> >> +++ b/mm/page_alloc.c
> >> @@ -3432,7 +3432,17 @@ __alloc_pages_direct_reclaim(gfp_t gfp_mask, unsigned int order,
> >>  	 */
> >>  	if (!page && !drained) {
> >>  		unreserve_highatomic_pageblock(ac, false);
> >> -		drain_all_pages(NULL);
> >> +
> >> +		/*
> >> +		 * Only drain from contexts allocating for user allocations.
> >> +		 * Kernel allocations could be holding a CPU hotplug-related
> >> +		 * mutex, particularly hot-add allocating per-cpu structures
> >> +		 * while hotplug-related mutex's are held which would prevent
> >> +		 * get_online_cpus ever returning.
> >> +		 */
> >> +		if (gfp_mask & __GFP_HARDWALL)
> >> +			drain_all_pages(NULL);
> >> +
> > 
> > This wouldn't work AFAICS. If you look at the lockdep splat, the path
> > which reverses the locking order (takes pcpu_alloc_mutex prior to
> > cpu_hotplug.lock is bpf_array_alloc_percpu which is GFP_USER and thus
> > __GFP_HARDWALL.
> > 
> > I believe we shouldn't pull any dependency on the hotplug locks inside
> > the allocator. This is just too fragile! Can we simply drop the
> > get_online_cpus()? Why do we need it, anyway? Say we are racing with the
> 
> It was added after I noticed in review that queue_work_on() has a
> comment that caller must ensure that cpu can't go away, and wondered
> about it.

Ohh, I haven't noticed the comment. Thanks for pointing it out. I still
do not see what would a missing get_online_cpus mean for queuing.

> Also noted that a similar lru_add_drain_all() does it too.
> 
> > cpu offlining. I have to check the code but my impression was that WQ
> > code will ignore the cpu requested by the work item when the cpu is
> > going offline. If the offline happens while the worker function already
> > executes then it has to wait as we run with preemption disabled so we
> > should be safe here. Or am I missing something obvious?
> 
> Tejun suggested an alternative solution to avoiding get_online_cpus() in
> this thread:
> https://lkml.kernel.org/r/<20170123170329.GA7820@xxxxxxxxxxxxxxx>

OK, so we have page_alloc_cpu_notify which also does drain_pages so all
we have to do to make sure they do not race is to synchronize there.

-- 
Michal Hocko
SUSE Labs