[PATCH v6 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 | 19 +++++++++++++++----
 1 file changed, 15 insertions(+), 4 deletions(-)

diff --git a/mm/slab_common.c b/mm/slab_common.c
index 09b26673b63f..2914a8f0aa85 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)
 {
@@ -629,6 +630,7 @@ void memcg_create_kmem_cache(struct mem_cgroup *memcg,
 	struct memcg_cache_array *arr;
 	struct kmem_cache *s = NULL;
 	char *cache_name;
+	bool dying;
 	int idx;
 
 	get_online_cpus();
@@ -640,7 +642,13 @@ void memcg_create_kmem_cache(struct mem_cgroup *memcg,
 	 * The memory cgroup could have been offlined while the cache
 	 * creation work was pending.
 	 */
-	if (memcg->kmem_state != KMEM_ONLINE || root_cache->memcg_params.dying)
+	if (memcg->kmem_state != KMEM_ONLINE)
+		goto out_unlock;
+
+	spin_lock_irq(&memcg_kmem_wq_lock);
+	dying = root_cache->memcg_params.dying;
+	spin_unlock_irq(&memcg_kmem_wq_lock);
+	if (dying)
 		goto out_unlock;
 
 	idx = memcg_cache_id(memcg);
@@ -735,14 +743,17 @@ static void kmemcg_cache_deactivate(struct kmem_cache *s)
 
 	__kmemcg_cache_deactivate(s);
 
+	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)
@@ -852,9 +863,9 @@ 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
-- 
2.20.1