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SubjectRe: [PATCH] sysfs: differentiate between locking links and non-links
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Greg KH <gregkh@suse.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@suse.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@suse.de>
>
> Nice patch, I'll queue it up for .34.

Note the patch does not compile with lockdep disabled.

Eric


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