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Date:   Mon, 17 Jan 2022 10:33:59 +0100
From:   Sebastian Andrzej Siewior <bigeasy@linutronix.de>
To:     Xin Long <lucien.xin@gmail.com>
Cc:     Hyeonggon Yoo <42.hyeyoo@gmail.com>,
        LKML <linux-kernel@vger.kernel.org>, linux-mm@kvack.org,
        Christoph Lameter <cl@linux.com>,
        Pekka Enberg <penberg@kernel.org>,
        David Rientjes <rientjes@google.com>,
        Joonsoo Kim <iamjoonsoo.kim@lge.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Vlastimil Babka <vbabka@suse.cz>,
        Antoine Tenart <atenart@kernel.org>
Subject: Re: [PATCH] mm: slub: fix a deadlock warning in kmem_cache_destroy
Message-ID: <YeU4B46F+oFUBRLE@linutronix.de>
References: <388098b2c03fbf0a732834fc01b2d875c335bc49.1642170196.git.lucien.xin@gmail.com>
 <YeO8pcs866Iu2iJX@ip-172-31-30-232.ap-northeast-1.compute.internal>
 <CADvbK_eY=3Gf79MkvK72Nh86ysN4eoFei0k1jg0frg22GgToGA@mail.gmail.com>
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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