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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
    > 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
    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?



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

    Joel Becker
    Principal Software Developer
    Phone: (650) 506-8127

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