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

[Vladimir Davydov]

On Tue, May 29, 2018 at 05:12:04PM -0700, Shakeel Butt wrote:
> 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.

> @@ -845,6 +862,8 @@ void kmem_cache_destroy(struct kmem_cache *s)
>  	if (unlikely(!s))
>  		return;
>  
> +	flush_memcg_workqueue(s);
> +

This should definitely help against async memcg_kmem_cache_create(),
but I'm afraid it doesn't eliminate the race with async destruction,
unfortunately, because the latter uses call_rcu_sched():

  memcg_deactivate_kmem_caches
   __kmem_cache_deactivate
    slab_deactivate_memcg_cache_rcu_sched
     call_rcu_sched
                                            kmem_cache_destroy
                                             shutdown_memcg_caches
                                              shutdown_cache
      memcg_deactivate_rcufn
       <dereference destroyed cache>

Can we somehow flush those pending rcu requests?