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SubjectRe: [PATCH 2/2] configfs: Rework configfs_depend_item() locking and make lockdep happy
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]
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