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

[Qian Cai <cai@lca.pw>]

> 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