[PATCH v7 07/10] mm: synchronize access to kmem_cache dying flag using a spinlock

[Roman Gushchin <guro@fb.com>]

Currently the memcg_params.dying flag and the corresponding
workqueue used for the asynchronous deactivation of kmem_caches
is synchronized using the slab_mutex.

It makes impossible to check this flag from the irq context,
which will be required in order to implement asynchronous release
of kmem_caches.

So let's switch over to the irq-save flavor of the spinlock-based
synchronization.

Signed-off-by: Roman Gushchin <guro@fb.com>
---
 mm/slab_common.c | 15 ++++++++++++---
 1 file changed, 12 insertions(+), 3 deletions(-)

diff --git a/mm/slab_common.c b/mm/slab_common.c
index 9383104651cd..1e5eaf84bf08 100644
--- a/mm/slab_common.c
+++ b/mm/slab_common.c
@@ -130,6 +130,7 @@ int __kmem_cache_alloc_bulk(struct kmem_cache *s, gfp_t flags, size_t nr,
 #ifdef CONFIG_MEMCG_KMEM
 
 LIST_HEAD(slab_root_caches);
+static DEFINE_SPINLOCK(memcg_kmem_wq_lock);
 
 void slab_init_memcg_params(struct kmem_cache *s)
 {
@@ -734,14 +735,22 @@ static void kmemcg_cache_deactivate(struct kmem_cache *s)
 
 	__kmemcg_cache_deactivate(s);
 
+	/*
+	 * memcg_kmem_wq_lock is used to synchronize memcg_params.dying
+	 * flag and make sure that no new kmem_cache deactivation tasks
+	 * are queued (see flush_memcg_workqueue() ).
+	 */
+	spin_lock_irq(&memcg_kmem_wq_lock);
 	if (s->memcg_params.root_cache->memcg_params.dying)
-		return;
+		goto unlock;
 
 	/* pin memcg so that @s doesn't get destroyed in the middle */
 	css_get(&s->memcg_params.memcg->css);
 
 	s->memcg_params.work_fn = __kmemcg_cache_deactivate_after_rcu;
 	call_rcu(&s->memcg_params.rcu_head, kmemcg_rcufn);
+unlock:
+	spin_unlock_irq(&memcg_kmem_wq_lock);
 }
 
 void memcg_deactivate_kmem_caches(struct mem_cgroup *memcg)
@@ -851,9 +860,9 @@ static int shutdown_memcg_caches(struct kmem_cache *s)
 
 static void flush_memcg_workqueue(struct kmem_cache *s)
 {
-	mutex_lock(&slab_mutex);
+	spin_lock_irq(&memcg_kmem_wq_lock);
 	s->memcg_params.dying = true;
-	mutex_unlock(&slab_mutex);
+	spin_unlock_irq(&memcg_kmem_wq_lock);
 
 	/*
 	 * SLAB and SLUB deactivate the kmem_caches through call_rcu. Make
-- 
2.21.0