Re: [Patch 0/2] sysfs: fix s_active lockdep warning

[Eric W. Biederman]

Cong Wang <amwang@xxxxxxxxxx> writes:

> Eric W. Biederman wrote:
>> Amerigo Wang <amwang@xxxxxxxxxx> writes:
>>
>>> Recently we met a lockdep warning from sysfs during s2ram or cpu hotplug.
>>> As reported by several people, it is something like:
>>>
>>> [ 6967.926563] ACPI: Preparing to enter system sleep state S3
>>> [ 6967.956156] Disabling non-boot CPUs ...
>>> [ 6967.970401]
>>> [ 6967.970408] =============================================
>>> [ 6967.970419] [ INFO: possible recursive locking detected ]
>>> [ 6967.970431] 2.6.33-rc2-git6 #27
>>> [ 6967.970439] ---------------------------------------------
>>> [ 6967.970450] pm-suspend/22147 is trying to acquire lock:
>>> [ 6967.970460]  (s_active){++++.+}, at: [<c10d2941>]
>>> sysfs_hash_and_remove+0x3d/0x4f
>>> [ 6967.970493]
>>> [ 6967.970497] but task is already holding lock:
>>> [ 6967.970506]  (s_active){++++.+}, at: [<c10d4110>]
>>> sysfs_get_active_two+0x16/0x36
>>> [...]
>>>
>>> Eric already provides a patch for this[1], but it still can't fix the
>>> problem. I add the missing part of Eric's patch and send these two patches
>>> together, hopefully we can fix the warning completely.
>>>
>>> 1. http://lkml.org/lkml/2010/1/10/282
>>>
>>>
>>> Reported-by: Benjamin Herrenschmidt <benh@xxxxxxxxxxxxxxxxxxx>
>>> Reported-by: Larry Finger <Larry.Finger@xxxxxxxxxxxx>
>>> Reported-by: Miles Lane <miles.lane@xxxxxxxxx>
>>> Reported-by: Heiko Carstens <heiko.carstens@xxxxxxxxxx>
>>> Signed-off-by: WANG Cong <amwang@xxxxxxxxxx>
>>> Signed-off-by: Eric W. Biederman <ebiederm@xxxxxxxxxxxx>
>>> Cc: Tejun Heo <tj@xxxxxxxxxx>
>>> Cc: Greg Kroah-Hartman <gregkh@xxxxxxx>
>>
>> Thanks for following up on this.
>>
>> I suspect we may want to create a separate class for each sysfs file
>> instead of playing whack-a-mole and creating a subclass each time we
>> have problems.
>>
>> I don't see why the rules for one sysfs file should be the same as for
>> any other sysfs file.
>>
>
> I am confused, we don't know who created sysfs files unless we
> separate them by subclasses, the way of your patch is very straight
> ward.

The assumption is that all entities in a class are used very similarly.
What I was suggesting is that it may make sense, and be simpler to have
a separate __key value and thus place each sysfs file in it's class.

Doing that is a lot more for lockdep to track, but it would not produce
the confusing false positives that we see now.  From the reports I have
seen we may have more that 16 subclasses in sysfs, and it will likely
take us a while to find them all.

Eric

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