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

[Heiko Carstens]

On Mon, Feb 27, 2017 at 05:20:31PM +0100, Michal Hocko wrote:
> [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.

To me it rather looks like bfc8c90139eb ("mem-hotplug: implement
get/put_online_mems") introduced quite subtle and probably wrong locking
rules.

The patch introduced mem_hotplug_begin() in order to have something like
cpu_hotplug_begin() for memory. Note that for cpu hotplug all
cpu_hotplug_begin() calls are serialized by cpu_maps_update_begin().

Especially this makes sure that active_writer can only be changed by one
process. (See also Dan's commit which introduced the lock_device_hotplug()
calls: https://marc.info/?l=linux-kernel&m=148693912419972&w=2 )

If you look at the above commit bfc8c90139eb: there is nothing like
cpu_maps_update_begin() for memory. And therefore it's possible to have
concurrent writers to active_writer.

It looks like now lock_device_hotplug() is supposed to be the new
cpu_maps_update_begin() for memory. But.. this looks like a mess, unless I
read the code completely wrong ;)

> 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@xxxxxxxxxxxxxxxxxx>
> > ---
> >  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
>