linux-kernel - Re: [PATCH v4] mm/memory_hotplug: Fix remove

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Message-Id: <0BE8F7EF-01DC-47BD-899B-11FB8B40EB0A@lca.pw>
Date:   Sat, 11 Jan 2020 08:55:59 -0500
From:   Qian Cai <cai@....pw>
To:     David Hildenbrand <david@...hat.com>
Cc:     Dan Williams <dan.j.williams@...el.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        stable <stable@...r.kernel.org>,
        Vishal Verma <vishal.l.verma@...el.com>,
        Pavel Tatashin <pasha.tatashin@...een.com>,
        Michal Hocko <mhocko@...e.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Linux MM <linux-mm@...ck.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Greg KH <gregkh@...uxfoundation.org>
Subject: Re: [PATCH v4] mm/memory_hotplug: Fix remove_memory() lockdep splat



> On Jan 11, 2020, at 6:03 AM, David Hildenbrand <david@...hat.com> wrote:
> 
> So I just remember why I think this (and the previously reported done
> for ACPI DIMMs) are false positives. The actual locking order is
> 
> onlining/offlining from user space:
> 
> kn->count -> device_hotplug_lock -> cpu_hotplug_lock -> mem_hotplug_lock
> 
> memory removal:
> 
> device_hotplug_lock -> cpu_hotplug_lock -> mem_hotplug_lock -> kn->count
> 
> 
> This looks like a locking inversion - but it's not. Whenever we come via
> user space we do a mutex_trylock(), which resolves this issue by backing
> up. The device_hotplug_lock will prevent
> 
> I have no clue why the device_hotplug_lock does not pop up in the
> lockdep report here. Sounds wrong to me.
> 
> I think this is a false positive and not stable material.

The point is that there are other paths does kn->count —> cpu_hotplug_lock without needing device_hotplug_lock to race with memory removal.

kmem_cache_shrink_all+0x50/0x100 (cpu_hotplug_lock.rw_sem/mem_hotplug_lock.rw_sem)
shrink_store+0x34/0x60
slab_attr_store+0x6c/0x170
sysfs_kf_write+0x70/0xb0
kernfs_fop_write+0x11c/0x270 ((kn->count)
 __vfs_write+0x3c/0x70
 vfs_write+0xcc/0x200
ksys_write+0x7c/0x140
system_call+0x5c/0x6