Re: [PATCH v2 4/4] mm/slub: Fix sysfs shrink circular locking dependency

[Waiman Long <longman@redhat.com>]

On 4/27/20 8:13 PM, Qian Cai wrote:
>
>> On Apr 27, 2020, at 7:56 PM, Waiman Long <longman@redhat.com> wrote:
>>
>> A lockdep splat is observed by echoing "1" to the shrink sysfs file
>> and then shutting down the system:
>>
>> [  167.473392] Chain exists of:
>> [  167.473392]   kn->count#279 --> mem_hotplug_lock.rw_sem --> slab_mutex
>> [  167.473392]
>> [  167.484323]  Possible unsafe locking scenario:
>> [  167.484323]
>> [  167.490273]        CPU0                    CPU1
>> [  167.494825]        ----                    ----
>> [  167.499376]   lock(slab_mutex);
>> [  167.502530]                                lock(mem_hotplug_lock.rw_sem);
>> [  167.509356]                                lock(slab_mutex);
>> [  167.515044]   lock(kn->count#279);
>> [  167.518462]
>> [  167.518462]  *** DEADLOCK ***
>>
>> It is because of the get_online_cpus() and get_online_mems() calls in
>> kmem_cache_shrink() invoked via the shrink sysfs file. To fix that, we
>> have to use trylock to get the memory and cpu hotplug read locks. Since
>> hotplug events are rare, it should be fine to refuse a kmem caches
>> shrink operation when some hotplug events are in progress.
> I don’t understand how trylock could prevent a splat. The fundamental issue is that in sysfs slab store case, the locking order (once trylock succeed) is,
>
> kn->count —> cpu/memory_hotplug
>
> But we have the existing reverse chain everywhere.
>
> cpu/memory_hotplug —> slab_mutex —> kn->count
>
The sequence that was prevented by this patch is "kn->count --> 
mem_hotplug_lock.rwsem". This sequence isn't directly in the splat. Once 
this link is broken, the 3-lock circular loop cannot be formed. Maybe I 
should modify the commit log to make this point more clear.

Cheers,
Longman