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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 Thu, Feb 11, 2010 at 01:42:10PM -0800, Eric W. Biederman wrote:
>> 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.
>
> Ugh, why not?
>
> Neil, care to fix this up?


#ifdef CONFIG_DEBUG_LOCK_ALLOC
-#define sysfs_dirent_init_lockdep(sd) \
+#define sysfs_dirent_init_lockdep(sd, type) \
do { \
static struct lock_class_key __key; \
\
- lockdep_init_map(&sd->dep_map, "s_active", &__key, 0); \
+ lockdep_init_map(&sd->dep_map, "s_active_" type, &__key, 0); \
} while(0)
#else
#define sysfs_dirent_init_lockdep(sd) do {} while(0)
^^^^
Eric





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