Messages in this thread | | | From | Dmitry Torokhov <> | Subject | Re: [PATCH] sysfs: differentiate between locking links and non-links | Date | Wed, 10 Feb 2010 09:55:51 -0800 |
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On Wednesday 10 February 2010 09:36:32 am Eric W. Biederman wrote: > Neil Brown <neilb@suse.de> writes: > > 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). > > You are correct. Not until the file handles are closed but until all > users of the underyling methods are complete. > > > 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 for this. > > Separating out symlinks and treating them differently because they can not > cause problems is definitely worth doing. We never take an active > reference in the symlink code so we can never block waiting for symlinks > to be deleted. > > > We block when deleting files in sysfs (and proc and sysctl). If we > did not block we could follow pointers into modules that are being > deleted, or those methods that are running could access data > structures that we want to tear down (perhaps there is a lock we want > to kfree). Blocking in sysfs is to simplify the life of the callers. > Unfortunately for a handful of callers it complicates things.
Exactly. Before Tejun changed sysfs to provide guarantee that no show/store methods are still running, nor new references to the corresponding kobject will be acquired through sysfs after sysfs_remove_file() returns, you had to jump through million of hoops at subsystem level to work with lifetime rules and work around the fact that kobjects could outlive your module.
I was glad to see bunch of ugly code in serio, gameport and input go and I do not want it coming back ;)
> > If you want to compare this to regular files think of what sysfs is > doing as a combined remove and revoke. The remove is easy. Revoke > is just plane difficult. > > Eric
-- Dmitry
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