Messages in this thread | | | Date | Tue, 27 Jan 2009 20:13:53 -0800 | From | Joel Becker <> | Subject | Re: [PATCH 2/2] configfs: Rework configfs_depend_item() locking and make lockdep happy |
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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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