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

Joel


--

The zen have a saying:
"When you learn how to listen, ANYONE can be your teacher."

Joel Becker
Principal Software Developer
Oracle
E-mail: joel.becker@oracle.com
Phone: (650) 506-8127


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