Messages in this thread | | | Date | Wed, 28 Jan 2009 11:32:47 +0100 | From | Louis Rilling <> | Subject | Re: [PATCH 2/2] configfs: Rework configfs_depend_item() locking and make lockdep happy |
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On 27/01/09 20:13 -0800, Joel Becker wrote: > On Thu, Dec 18, 2008 at 07:00:18PM +0100, Louis Rilling wrote: > > configfs_depend_item() recursively locks all inodes mutex from configfs root to > > the target item, which makes lockdep unhappy. The purpose of this recursive > > locking is to ensure that the item tree can be safely parsed and that the target > > item, if found, is not about to leave. > > > > This patch reworks configfs_depend_item() locking using configfs_dirent_lock. > > Since configfs_dirent_lock protects all changes to the configfs_dirent tree, and > > protects tagging of items to be removed, this lock can be used instead of the > > inodes mutex lock chain. > > This needs that the check for dependents be done atomically with > > CONFIGFS_USET_DROPPING tagging. > > > > Now lockdep looks happy with configfs. > > This looks almost, but not quite right. > In the create path, we do configfs_new_dirent() before we set > sd->s_type. But configfs_new_dirent() attaches sd->s_sibling. So, in > aonther thread, configfs_depend_prep() can traverse this s_sibling > without CONFIGFS_USET_CREATING being set. This turns out to be safe > because CONFIGFS_DIR is also not set - but boy I'd like a comment about > that.
Definitely agreed. I should have written this comment instead of letting you notice this.
> What if we're in mkdir(2) in one thread and another thread is > trying to pin the parent directory? That is, we are inside > configfs_mkdir(parent, new_dentry, mode). The other thread is doing > configfs_depend_item(subsys, parent). With this patch, the other thread > will not take parent->i_mutex. It will happily determine that > parent is part of the tree and bump its s_dependent with no locking. Is > this OK?
Yes this is the expected impact. It is OK because 1) under a same critical section of configfs_dirent_lock, depend_item() checks that CONFIGFS_USET_DROPPING is not set and bumps s_dependent; 2) under a same critical section of configfs_dirent_lock, configfs_rmdir() checks the s_dependent count and tries to set CONFIGFS_USET_DROPPING.
> If it is - isn't this patch good without any other reason? That > is, aside from the issues of lockdep, isn't it better for > configfs_depend_item() to never have to worry about the VFS locks other > than the configfs root?
Yes, this patch may look like an improvement, independently from lockdep. I think that locking is simpler this way, and this also removes the need for configfs_depend_rollback(). Moreover this moves towards the management of configfs_dirents protected by configfs_dirent_lock only. In the end, it's up to you to judge if this is a good direction ;)
Thanks,
Louis
-- Dr Louis Rilling Kerlabs Skype: louis.rilling Batiment Germanium Phone: (+33|0) 6 80 89 08 23 80 avenue des Buttes de Coesmes http://www.kerlabs.com/ 35700 Rennes [unhandled content-type:application/pgp-signature] | |