kasan: resched in quarantine_remove_cache()

We see reported stalls/lockups in quarantine_remove_cache() on machines
with large amounts of RAM.  quarantine_remove_cache() needs to scan
whole quarantine in order to take out all objects belonging to the
cache.  Quarantine is currently 1/32-th of RAM, e.g.  on a machine with
256GB of memory that will be 8GB.  Moreover quarantine scanning is a
walk over uncached linked list, which is slow.

Add cond_resched() after scanning of each non-empty batch of objects.
Batches are specifically kept of reasonable size for quarantine_put().
On a machine with 256GB of RAM we should have ~512 non-empty batches,
each with 16MB of objects.

Link: http://lkml.kernel.org/r/20170308154239.25440-1-dvyukov@google.com
Signed-off-by: Dmitry Vyukov <dvyukov@google.com>
Acked-by: Andrey Ryabinin <aryabinin@virtuozzo.com>
Cc: Greg Thelen <gthelen@google.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/kasan/quarantine.c

@@ -283,8 +283,15 @@ void quarantine_remove_cache(struct kmem_cache *cache)
 	on_each_cpu(per_cpu_remove_cache, cache, 1);
 
 	spin_lock_irqsave(&quarantine_lock, flags);
-	for (i = 0; i < QUARANTINE_BATCHES; i++)
+	for (i = 0; i < QUARANTINE_BATCHES; i++) {
+		if (qlist_empty(&global_quarantine[i]))
+			continue;
 		qlist_move_cache(&global_quarantine[i], &to_free, cache);
+		/* Scanning whole quarantine can take a while. */
+		spin_unlock_irqrestore(&quarantine_lock, flags);
+		cond_resched();
+		spin_lock_irqsave(&quarantine_lock, flags);
+	}
 	spin_unlock_irqrestore(&quarantine_lock, flags);
 
 	qlist_free_all(&to_free, cache);