[PATCH v3] mm: fix race between kmem_cache destroy, create and deactivate

[Shakeel Butt <shakeelb@google.com>]

The memcg kmem cache creation and deactivation (SLUB only) is
asynchronous. If a root kmem cache is destroyed whose memcg cache is in
the process of creation or deactivation, the kernel may crash.

Example of one such crash:
	general protection fault: 0000 [#1] SMP PTI
	CPU: 1 PID: 1721 Comm: kworker/14:1 Not tainted 4.17.0-smp
	...
	Workqueue: memcg_kmem_cache kmemcg_deactivate_workfn
	RIP: 0010:has_cpu_slab
	...
	Call Trace:
	? on_each_cpu_cond
	__kmem_cache_shrink
	kmemcg_cache_deact_after_rcu
	kmemcg_deactivate_workfn
	process_one_work
	worker_thread
	kthread
	ret_from_fork+0x35/0x40

To fix this race, on root kmem cache destruction, mark the cache as
dying and flush the workqueue used for memcg kmem cache creation and
deactivation.

Signed-off-by: Shakeel Butt <shakeelb@google.com>
---
Changelog since v2:
- Instead of refcount, flush the workqueue

Changelog since v1:
- Added more documentation to the code
- Renamed fields to be more readable

---
 include/linux/slab.h |  1 +
 mm/slab_common.c     | 21 ++++++++++++++++++++-
 2 files changed, 21 insertions(+), 1 deletion(-)

diff --git a/include/linux/slab.h b/include/linux/slab.h
index 9ebe659bd4a5..71c5467d99c1 100644
--- a/include/linux/slab.h
+++ b/include/linux/slab.h
@@ -658,6 +658,7 @@ struct memcg_cache_params {
 			struct memcg_cache_array __rcu *memcg_caches;
 			struct list_head __root_caches_node;
 			struct list_head children;
+			bool dying;
 		};
 		struct {
 			struct mem_cgroup *memcg;
diff --git a/mm/slab_common.c b/mm/slab_common.c
index b0dd9db1eb2f..a3496375f5ea 100644
--- a/mm/slab_common.c
+++ b/mm/slab_common.c
@@ -136,6 +136,7 @@ void slab_init_memcg_params(struct kmem_cache *s)
 	s->memcg_params.root_cache = NULL;
 	RCU_INIT_POINTER(s->memcg_params.memcg_caches, NULL);
 	INIT_LIST_HEAD(&s->memcg_params.children);
+	s->memcg_params.dying = false;
 }
 
 static int init_memcg_params(struct kmem_cache *s,
@@ -608,7 +609,7 @@ 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)
+	if (memcg->kmem_state != KMEM_ONLINE || root_cache->memcg_params.dying)
 		goto out_unlock;
 
 	idx = memcg_cache_id(memcg);
@@ -712,6 +713,9 @@ void slab_deactivate_memcg_cache_rcu_sched(struct kmem_cache *s,
 	    WARN_ON_ONCE(s->memcg_params.deact_fn))
 		return;
 
+	if (s->memcg_params.root_cache->memcg_params.dying)
+		return;
+
 	/* pin memcg so that @s doesn't get destroyed in the middle */
 	css_get(&s->memcg_params.memcg->css);
 
@@ -823,11 +827,24 @@ static int shutdown_memcg_caches(struct kmem_cache *s)
 		return -EBUSY;
 	return 0;
 }
+
+static void flush_memcg_workqueue(struct kmem_cache *s)
+{
+	mutex_lock(&slab_mutex);
+	s->memcg_params.dying = true;
+	mutex_unlock(&slab_mutex);
+
+	flush_workqueue(memcg_kmem_cache_wq);
+}
 #else
 static inline int shutdown_memcg_caches(struct kmem_cache *s)
 {
 	return 0;
 }
+
+static inline void flush_memcg_workqueue(struct kmem_cache *s)
+{
+}
 #endif /* CONFIG_MEMCG && !CONFIG_SLOB */
 
 void slab_kmem_cache_release(struct kmem_cache *s)
@@ -845,6 +862,8 @@ void kmem_cache_destroy(struct kmem_cache *s)
 	if (unlikely(!s))
 		return;
 
+	flush_memcg_workqueue(s);
+
 	get_online_cpus();
 	get_online_mems();
 
-- 
2.17.0.921.gf22659ad46-goog