On Tue 07-02-17 12:54:48, Vlastimil Babka wrote:
> On 02/07/2017 12:43 PM, Michal Hocko wrote:
> > > 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?
>
> Could we do flush_work() with a spinlock? Sounds bad too.
We surely cannot. I thought the lock would be gone in drain_all_pages,
we would deadlock with the lock there anyway... But it is true that we
would need a way to only allow one caller to get in. This is getting
messier and messier...
> Maybe we could just use the fact that the whole drain happens with disabled
> irq's and obtain the current cpu under that protection?
preempt_disable should be enough, no? The CPU callback is not called
from an IRQ context, right?
---
diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 1ee49474207e..4a9a65479435 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -2343,7 +2343,16 @@ void drain_local_pages(struct zone *zone)
static void drain_local_pages_wq(struct work_struct *work)
{
+ /*
+ * drain_all_pages doesn't use proper cpu hotplug protection so
+ * we can race with cpu offline when the WQ can move this from
+ * a cpu pinned worker to an unbound one. We can operate on a different
+ * cpu which is allright but we also have to make sure to not move to
+ * a different one.
+ */
+ preempt_disable();
drain_local_pages(NULL);
+ preempt_enable();
}
/*
@@ -2379,12 +2388,6 @@ void drain_all_pages(struct zone *zone)
}
/*
- * As this can be called from reclaim context, do not reenter reclaim.
- * An allocation failure can be handled, it's simply slower
- */
- get_online_cpus();
-
- /*
* We don't care about racing with CPU hotplug event
* as offline notification will cause the notified
* cpu to drain that CPU pcps and on_each_cpu_mask
@@ -2423,7 +2426,6 @@ void drain_all_pages(struct zone *zone)
for_each_cpu(cpu, &cpus_with_pcps)
flush_work(per_cpu_ptr(&pcpu_drain, cpu));
- put_online_cpus();
mutex_unlock(&pcpu_drain_mutex);
}
--
Michal Hocko
SUSE Labs
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