Re: [PATCH] mm, add_memory_resource: hold device_hotplug lock over mem_hotplug_{begin, done}

[Michal Hocko <mhocko@kernel.org>]

[CC Rafael]

I've got lost in the acpi indirection (again). I can see
acpi_device_hotplug calling lock_device_hotplug() but i cannot find a
path down to add_memory() which might call add_memory_resource. But the
patch below sounds suspicious to me. Is it possible that this could lead
to a deadlock. I would suspect that it is the s390 code which needs to
do the locking. But I would have to double check - it is really easy to
get lost there.

On Sun 26-02-17 12:42:44, Sebastian Ott wrote:
> With 4.10.0-10265-gc4f3f22 the following warning is triggered on s390:
> 
> WARNING: CPU: 6 PID: 1 at drivers/base/core.c:643 assert_held_device_hotplug+0x4a/0x58
> [    5.731214] Call Trace:
> [    5.731219] ([<000000000067b8b0>] assert_held_device_hotplug+0x40/0x58)
> [    5.731225]  [<0000000000337914>] mem_hotplug_begin+0x34/0xc8
> [    5.731231]  [<00000000008b897e>] add_memory_resource+0x7e/0x1f8
> [    5.731236]  [<00000000008b8bd2>] add_memory+0xda/0x130
> [    5.731243]  [<0000000000d7f0dc>] add_memory_merged+0x15c/0x178
> [    5.731247]  [<0000000000d7f3a6>] sclp_detect_standby_memory+0x2ae/0x2f8
> [    5.731252]  [<00000000001002ba>] do_one_initcall+0xa2/0x150
> [    5.731258]  [<0000000000d3adc0>] kernel_init_freeable+0x228/0x2d8
> [    5.731263]  [<00000000008b6572>] kernel_init+0x2a/0x140
> [    5.731267]  [<00000000008c3972>] kernel_thread_starter+0x6/0xc
> [    5.731272]  [<00000000008c396c>] kernel_thread_starter+0x0/0xc
> [    5.731276] no locks held by swapper/0/1.
> [    5.731280] Last Breaking-Event-Address:
> [    5.731285]  [<000000000067b8b6>] assert_held_device_hotplug+0x46/0x58
> [    5.731292] ---[ end trace 46480df21194c96a ]---

such an informtion belongs to the changelog

> ----->8
> mm, add_memory_resource: hold device_hotplug lock over mem_hotplug_{begin, done}
> 
> With commit 3fc219241 ("mm: validate device_hotplug is held for memory hotplug")
> a lock assertion was added to mem_hotplug_begin() which led to a warning
> when add_memory() is called. Fix this by acquiring device_hotplug_lock in
> add_memory_resource().
> 
> Signed-off-by: Sebastian Ott <sebott@linux.vnet.ibm.com>
> ---
>  mm/memory_hotplug.c | 2 ++
>  1 file changed, 2 insertions(+)
> 
> diff --git a/mm/memory_hotplug.c b/mm/memory_hotplug.c
> index 1d3ed58..c633bbc 100644
> --- a/mm/memory_hotplug.c
> +++ b/mm/memory_hotplug.c
> @@ -1361,6 +1361,7 @@ int __ref add_memory_resource(int nid, struct resource *res, bool online)
>  		new_pgdat = !p;
>  	}
>  
> +	lock_device_hotplug();
>  	mem_hotplug_begin();
>  
>  	/*
> @@ -1416,6 +1417,7 @@ int __ref add_memory_resource(int nid, struct resource *res, bool online)
>  
>  out:
>  	mem_hotplug_done();
> +	unlock_device_hotplug();
>  	return ret;
>  }
>  EXPORT_SYMBOL_GPL(add_memory_resource);
> -- 
> 2.3.0
> 
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-- 
Michal Hocko
SUSE Labs