Re: [PATCH] mm: slub: fix a deadlock warning in kmem_cache_destroy

[Vlastimil Babka]

+CC Clark

On 1/17/22 10:33, Sebastian Andrzej Siewior wrote:
> On 2022-01-17 16:32:46 [+0800], Xin Long wrote:
>> another issue. From the code analysis, this issue does exist on the
>> upstream kernel, though I couldn't build an upstream RT kernel for the
>> testing.
> 
> This should also reproduce in v5.16 since the commit in question is
> there.

Yeah. I remember we had some issues with the commit during development, but
I'd hope those were resolved and the commit that's ultimately merged got the
fixes, see this subthread:

https://lore.kernel.org/all/0b36128c-3e12-77df-85fe-a153a714569b@xxxxxxxxxxx/

>> > >         CPU0                        CPU1
>> > >         ----                        ----
>> > >   cpus_read_lock()
>> > >                                    kn->active++
>> > >                                    cpus_read_lock() [a]
>> > >   wait until kn->active == 0
>> > >
>> > > Although cpu_hotplug_lock is a RWSEM, [a] will not block in there. But as
>> > > lockdep annotations are added for cpu_hotplug_lock, a deadlock warning
>> > > would be detected:
> 
> The cpu_hotplug_lock is a per-CPU RWSEM. The lock in [a] will block if
> there is a writer pending.
> 
>> > >   ======================================================
>> > >   WARNING: possible circular locking dependency detected
>> > >   ------------------------------------------------------
>> > >   dmsetup/1832 is trying to acquire lock:
>> > >   ffff986f5a0f9f20 (kn->count#144){++++}-{0:0}, at: kernfs_remove+0x1d/0x30
>> > >
>> > >   but task is already holding lock:
>> > >   ffffffffa43817c0 (slab_mutex){+.+.}-{3:3}, at: kmem_cache_destroy+0x2a/0x120
>> > >
> 
> I tried to create & destroy a cryptarget which creates/destroy a cache
> via bio_put_slab(). Either the callchain is different or something else
> is but I didn't see a lockdep warning.

RHEL-8 kernel seems to be 4.18, unless RT uses a newer one. Could be some
silently relevant backport is missing? How about e.g. 59450bbc12be ("mm,
slab, slub: stop taking cpu hotplug lock") ?

> Sebastian