Re: [PATCH v1] mm/memory_hotplug: Don't take the cpu_hotplug_lock

[David Hildenbrand]

On 24.09.19 20:54, Qian Cai wrote:
> On Tue, 2019-09-24 at 17:11 +0200, Michal Hocko wrote:
>> On Tue 24-09-19 11:03:21, Qian Cai wrote:
>> [...]
>>> While at it, it might be a good time to rethink the whole locking over there, as
>>> it right now read files under /sys/kernel/slab/ could trigger a possible
>>> deadlock anyway.
>>>
>>
>> [...]
>>> [ÂÂ442.452090][ T5224] -> #0 (mem_hotplug_lock.rw_sem){++++}:
>>> [ÂÂ442.459748][ T5224]ÂÂÂÂÂÂÂÂvalidate_chain+0xd10/0x2bcc
>>> [ÂÂ442.464883][ T5224]ÂÂÂÂÂÂÂÂ__lock_acquire+0x7f4/0xb8c
>>> [ÂÂ442.469930][ T5224]ÂÂÂÂÂÂÂÂlock_acquire+0x31c/0x360
>>> [ÂÂ442.474803][ T5224]ÂÂÂÂÂÂÂÂget_online_mems+0x54/0x150
>>> [ÂÂ442.479850][ T5224]ÂÂÂÂÂÂÂÂshow_slab_objects+0x94/0x3a8
>>> [ÂÂ442.485072][ T5224]ÂÂÂÂÂÂÂÂtotal_objects_show+0x28/0x34
>>> [ÂÂ442.490292][ T5224]ÂÂÂÂÂÂÂÂslab_attr_show+0x38/0x54
>>> [ÂÂ442.495166][ T5224]ÂÂÂÂÂÂÂÂsysfs_kf_seq_show+0x198/0x2d4
>>> [ÂÂ442.500473][ T5224]ÂÂÂÂÂÂÂÂkernfs_seq_show+0xa4/0xcc
>>> [ÂÂ442.505433][ T5224]ÂÂÂÂÂÂÂÂseq_read+0x30c/0x8a8
>>> [ÂÂ442.509958][ T5224]ÂÂÂÂÂÂÂÂkernfs_fop_read+0xa8/0x314
>>> [ÂÂ442.515007][ T5224]ÂÂÂÂÂÂÂÂ__vfs_read+0x88/0x20c
>>> [ÂÂ442.519620][ T5224]ÂÂÂÂÂÂÂÂvfs_read+0xd8/0x10c
>>> [ÂÂ442.524060][ T5224]ÂÂÂÂÂÂÂÂksys_read+0xb0/0x120
>>> [ÂÂ442.528586][ T5224]ÂÂÂÂÂÂÂÂ__arm64_sys_read+0x54/0x88
>>> [ÂÂ442.533634][ T5224]ÂÂÂÂÂÂÂÂel0_svc_handler+0x170/0x240
>>> [ÂÂ442.538768][ T5224]ÂÂÂÂÂÂÂÂel0_svc+0x8/0xc
>>
>> I believe the lock is not really needed here. We do not deallocated
>> pgdat of a hotremoved node nor destroy the slab state because an
>> existing slabs would prevent hotremove to continue in the first place.
>>
>> There are likely details to be checked of course but the lock just seems
>> bogus.
> 
> Check 03afc0e25f7f ("slab: get_online_mems for
> kmem_cache_{create,destroy,shrink}"). It actually talk about the races during
> memory as well cpu hotplug, so it might even that cpu_hotplug_lock removal is
> problematic?
> 

Which removal are you referring to? get_online_mems() does not mess with
the cpu hotplug lock (and therefore this patch).

-- 

Thanks,

David / dhildenb