Re: [PATCH v4] mm/memory_hotplug: Fix remove_memory() lockdep splat

[Dan Williams <dan.j.williams@intel.com>]

On Sat, Jan 11, 2020 at 6:52 AM David Hildenbrand <david@redhat.com> wrote:
>
> On 11.01.20 15:25, David Hildenbrand wrote:
> >
> >
> >> Am 11.01.2020 um 14:56 schrieb Qian Cai <cai@lca.pw>:
> >>
> >> 
> >>
> >>> On Jan 11, 2020, at 6:03 AM, David Hildenbrand <david@redhat.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
> >>
> >
> > But not the lock of the memory devices, or am I missing something?
> >
>
> To clarify:
>
> memory unplug will remove e.g., /sys/devices/system/memory/memoryX/,
> which has a dedicated kn->count AFAIK
>
> If you do a "echo 1 > /sys/kernel/slab/X/shrink", you would not lock the
> kn->count of /sys/devices/system/memory/memoryX/, but the one of some
> slab thingy.
>
> The only scenario I could see is if remove_memory_block_devices() will
> not only remove /sys/devices/system/memory/memoryX/, but also implicitly
> e.g., /sys/kernel/slab/X/. If that is the case, then this is indeed not
> a false positive, but something rather hard to trigger (which would
> still classify as stable material).

Yes, already agreed to drop stable.

However, the trylock does not solve the race it just turns the
blocking wait to a spin wait, but the subsequent 5ms sleep does make
the theoretical race nearly impossible, Thanks for pointing that out.

The theoretical race is still a problem because it hides future
lockdep violations, but I otherwise can't point to whether the
kn->count in question is a false positive concern for an actual
deadlock or not. Tracking that down is possible, but not something I
have time for at present.