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

[Sebastian Andrzej Siewior]

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.

> > >         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.

Sebastian