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To: Cong Wang <amwang@redhat.com>
Cc: linux-kernel@vger.kernel.org, Tejun Heo <tj@kernel.org>,
       Greg Kroah-Hartman <gregkh@suse.de>, Miles Lane <miles.lane@gmail.com>,
       Heiko Carstens <heiko.carstens@de.ibm.com>,
       Benjamin Herrenschmidt <benh@kernel.crashing.org>,
       Larry Finger <Larry.Finger@lwfinger.net>, akpm@linux-foundation.org
Subject: Re: [Patch 0/2] sysfs: fix s_active lockdep warning
References: <20100129070516.4058.77227.sendpatchset@localhost.localdomain>
	<m1eil9v0j9.fsf@fess.ebiederm.org> <4B629E7F.5020200@redhat.com>
From: ebiederm@xmission.com (Eric W. Biederman)
Date: Fri, 29 Jan 2010 05:44:07 -0800
In-Reply-To: <4B629E7F.5020200@redhat.com> (Cong Wang's message of "Fri\, 29 Jan 2010 16\:38\:23 +0800")
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Cong Wang <amwang@redhat.com> writes:

> Eric W. Biederman wrote:
>> Amerigo Wang <amwang@redhat.com> 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@kernel.crashing.org>
>>> Reported-by: Larry Finger <Larry.Finger@lwfinger.net>
>>> Reported-by: Miles Lane <miles.lane@gmail.com>
>>> Reported-by: Heiko Carstens <heiko.carstens@de.ibm.com>
>>> Signed-off-by: WANG Cong <amwang@redhat.com>
>>> Signed-off-by: Eric W. Biederman <ebiederm@xmission.com>
>>> Cc: Tejun Heo <tj@kernel.org>
>>> Cc: Greg Kroah-Hartman <gregkh@suse.de>
>>
>> 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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