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    SubjectRe: [PATCH] remove BKL from driverfs
    Patrick Mochel wrote:
    >>>I see no reason to hold the BKL in your situation. I replaced it with
    >>>i_sem in some places and just plain removed it in others. I believe
    >>>that you get all of the protection that you need from dcache_lock in
    >>>the dentry insert and activate. Can you prove me wrong?
    >>No, and I'm not about to try very hard. It appears that the place you
    >>removed it should be fine. In driverfs_unlink, you replace it with i_sem.
    >>ramfs, which driverfs mimmicks, doesn't hold any lock during unlink. It
    >>seems it could be removed altogether.
    > Actually, taking i_sem is completely wrong. Look at vfs_unlink() in
    > fs/namei.c:
    > down(&dentry->d_inode->i_sem);
    > if (d_mountpoint(dentry))
    > error = -EBUSY;
    > else {
    > error = dir->i_op->unlink(dir, dentry);
    > if (!error)
    > d_delete(dentry);
    > }
    > up(&dentry->d_inode->i_sem);
    > Then, in driverfs_unlink:
    > struct inode *inode = dentry->d_inode;
    > down(&inode->i_sem);
    > You didn't test this on file removal did you? A good way to verify that
    > you have most of your bases covered is to plug/unplug a USB device a few
    > times. I learned that one from Greg, and it's caught several bugs.

    I need to get some USB devices that work in Linux!

    > Anyway, I say that the lock can be removed altogether. Ditto for mknod as
    > well.

    I agree. It looks like vfs_unlink() provides all of the protection
    that is needed. No BKL, no more i_sem uses. I'm asking Al Viro about
    driverfs_get_inode(). It looks like it will be safe and correct to
    use i_sem there, but stay tuned, it may not be necessary at all.

    Dave Hansen

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