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    SubjectRe: [RFD driver-core] Lifetime problems of the current driver model
    On Mon, Apr 02, 2007 at 04:59:43AM +0900, Tejun Heo wrote:
    >
    > Dependencies between sysfs/kobject objects are clearer now and I think
    > I got the locking and referencing correct. This patch immediately
    > fixes 'sysfs attr grabbing the wrong kobject and module' problem -
    > sysfs and module lifetimes are unrelated now. We can drop
    > half-working attr->owner.
    >
    > * A sysfs node no longer hold reference to its kobject. It just
    > attaches itself to the kobject on creation and detaches on deletion.
    >
    > * For a dir node, sysfs_dirent no longer directly points to kobject.
    > It points to sysfs_dir which contains pointer to kobject and a rwsem
    > to protect it.
    >
    > * An open file doesn't hold a reference to kobject. It holds a
    > reference to sysfs_dirent. kobject pointer is verified and
    > show/store are performed with rwsem read-locked. Deletion
    > disconnects the sysfs from its kobject while the rwsem is
    > write-locked. This mechanism replaces buffer orphaning and kobject
    > validation during open.

    Ah, very nice.

    > * attr ops is determined on sysfs node creation not on open. This is
    > a step toward making kobject opaque in sysfs.
    >
    > * bin files are handled similarly but mmap makes the open file hold
    > reference to the kobject till it's closed. Any better ideas?

    This is probably needed, and might be acceptable, and I think we can
    live with it as there is only a very small number of sysfs files that
    fall into this category.

    > * symlink is reimplemented in terms of sysfs_dirents instead of
    > kobjects. sysfs_dirent->s_parent is added and each sysfs_dirent
    > holds reference to its parent. Currently walking up the tree
    > requires read locking and unlocking sysfs_dir at each level. This
    > can be removed if name is added to sysfs_dirent.
    >
    > * As kobject can be disconnected anytime and sysfs code still needs to
    > look follow dentry link in kobject, kobject->dentry is protected by
    > dcache_lock. Once kobject becomes opaque to sysfs, this hack can go
    > away. All in all, making kobject completely opaque in sysfs isn't
    > too far away after this patch although it would require mass code
    > update.

    What would need to be updated after this?

    > What do you think about this approach? If it's acceptable, I'll test
    > further and split the patch into logical steps to get it reviewed
    > better.

    At first glance, I think it looks fine, but Maneesh is the one who
    understands this code better than anyone, so I would like to get his
    opinion on it.

    I think you should start splitting it all up into steps, which will help
    us review it a whole lot easier, as overall, I think this is a very
    worthy goal.

    thanks so much for doing this work,

    greg k-h
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