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    SubjectRe: [announce] vfs-scale git tree update
    On Wed, Jan 12, 2011 at 5:13 AM, Linus Torvalds
    <> wrote:
    > [ Added Ian to cc - the BUG_ON() in d_set_d_op() triggers on autofs4,
    > see lkml thread for more details if you care ]
    > On Tue, Jan 11, 2011 at 9:57 AM, Alex Elder <> wrote:
    >> Looking at it quickly, I don't think that would matter for
    >> the case at hand.  I.e., that might be safer but it doesn't
    >> address the fact that these fields are getting initialized
    >> multiple times.
    > You're right - without looking closer or thinking about it any more, I
    > was thinking mkdir etc would create the new dentry, but it can easily
    > be a pre-existing negative one. So it's not like mkdir is always going
    > to be something new that can just be initialized.
    > So yeah, no race or anything like that required - just regular acceses.
    > That said, if we make sure that the d_op gets cleared (or the dentry
    > dropped) when something turns negative, and a negative lookup never
    > sets d_op, it should all basically work.
    > Can you figure out the exact sequence that causes this? Is it enough
    > to just mount something once?

    Doh, sorry I probably didn't re-test autofs4 since putting that BUG_ON
    in there. Now it's trivial to revert the d_set_d_op function to the same
    raciness that was "allowed" before (simply by resetting the d_flags bits).
    There is no new fundamental race that the patch introduced there...
    But I think it is much cleaner to just fix up all filesystems, and will
    potentially close obscure races.

    Often the best way is to just set the same d_ops for negative and
    positive dentries, and have the operations handle both cases.

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