linux-kernel - Re: [PATCH] sysfs: differentiate between locking links and non-links

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Date:	Thu, 11 Feb 2010 13:42:10 -0800
From:	ebiederm@...ssion.com (Eric W. Biederman)
To:	Greg KH <gregkh@...e.de>
Cc:	Neil Brown <neilb@...e.de>, Tejun Heo <tj@...nel.org>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] sysfs: differentiate  between locking links and non-links

Greg KH <gregkh@...e.de> writes:

> On Wed, Feb 10, 2010 at 12:09:33PM +1100, Neil Brown wrote:
>> 
>> Hi,
>>  I've just spent a while sorting out some lockdep complaints triggered
>>  by the recent addition of the "s_active" lockdep annotation in sysfs
>>   (commit 846f99749ab68bbc7f75c74fec305de675b1a1bf)
>> 
>>  Some of them are genuine and I have submitted a fix for those.
>>  Some are, I think, debatable and I get to that is a minute.  I've
>>  submitted a fix for them anyway.
>>  But some are to my mind clearly bogus and I'm hoping that can be
>>  fixed by the change below (or similar).
>>  The 'bogus' ones are triggered by writing to a sysfs attribute file
>>  for which the handler tries to delete a symlink from sysfs.
>>  This appears to be a recursion on s_active as s_active is held while
>>  the handler runs and is again needed to effect the delete.  However
>>  as the thing being deleted is a symlink, it is very clearly a
>>  different object to the thing triggering the delete, so there is no
>>  real loop.
>> 
>>  The following patch splits the lockdep context in two - one for
>>  symlink and one for everything else.  This removes the apparent loop.
>>  (An example report can be seen in
>>      http://bugzilla.kernel.org/show_bug.cgi?id=15142).
>> 
>>  The "debatable" dependency loops happen when writing to one attribute
>>  causes a different attribute to be deleted.  In my (md) case this can
>>  actually cause a deadlock as both the attributes take the same lock
>>  while the handler is running.  This is because deleting the attribute
>>  will block until the all accesses of that attribute have completed (I
>>  think).
>>  However it should be possible to delete a name from sysfs while there
>>  are still accesses pending (it works for normal files!!).  So if
>>  sysfs could be changed to simply unlink the file and leave deletion to
>>  happen when the refcount become zero it would certainly make my life
>>  a lot easier, and allow the removal of some ugly code from md.c.
>>  I don't know sysfs well enough to suggest a patch though.
>> 
>> Thanks,
>> NeilBrown
>> 
>> 
>> 
>> commit 2e502cfe444b68f6ef6b8b2abe83b6112564095b
>> Author: NeilBrown <neilb@...e.de>
>> Date:   Wed Feb 10 09:43:45 2010 +1100
>> 
>>     sysfs: differentiate  between locking links and non-links for sysfs
>>     
>>     symlinks and non-symlink is sysfs are very different.
>>     A symlink can never be locked (active) while an attribute
>>     modification routine is running.  So removing symlink from an
>>     attribute 'store' routine should be permitted without any lockdep
>>     warnings.
>>     
>>     So split the lockdep context for 's_active' in two, one for symlinks
>>     and other for everything else.
>>     
>>     Signed-off-by: NeilBrown <neilb@...e.de>
>
> Nice patch, I'll queue it up for .34.

Note the patch does not compile with lockdep disabled.

Eric
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