Re: [PATCH v2] mm: fix race between kmem_cache destroy, create and deactivate

[Vladimir Davydov]

On Tue, May 22, 2018 at 01:13:36PM -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
> 
> This issue is due to the lack of real reference counting for the root
> kmem_caches. Currently kmem_cache does have a field named refcount which
> has been used for multiple purposes i.e. shared count, reference count
> and noshare flag. Due to its conflated nature, it can not be used for
> reference counting by other subsystems.
> 
> This patch decoupled the reference counting from shared count and
> noshare flag. The new field 'shared_count' represents the shared count
> and noshare flag while 'refcount' is converted into a real reference
> counter.
> 
> The reference counting is only implemented for root kmem_caches for
> simplicity. The reference of a root kmem_cache is elevated on sharing or
> while its memcg kmem_cache creation or deactivation request is in the
> fly and thus it is made sure that the root kmem_cache is not destroyed
> in the middle. As the reference of kmem_cache is elevated on sharing,
> the 'shared_count' does not need any locking protection as at worst it
> can be out-dated for a small window which is tolerable.

I wonder if we could fix this problem without introducing reference
counting for kmem caches (which seems a bit of an overkill to me TBO),
e.g. by flushing memcg_kmem_cache_wq before root cache destruction?