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

[Shakeel Butt <shakeelb@google.com>]

On Sat, May 26, 2018 at 11:58 AM, Vladimir Davydov
<vdavydov.dev@gmail.com> wrote:
> 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?

Thanks I will look into workqueue flushing.