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    SubjectRe: [rfc][patch] store-free path walking

    On Wed, 7 Oct 2009, Nick Piggin wrote:
    > OK, I have a really basic patch that does store-free path walking
    > (except on the final element).


    > dbench is pretty nasty still because it seems to do a lot of stupid
    > things like reading from /proc/mounts all the time.

    You should largely forget about dbench, it can certainly be a useful
    benchmark, but at the same time it's certainly not a _meaningful_ one.
    There are better things to try.

    > The seqlock in the dentry is for getting consistent name,len pointer,
    > and also not giving a false positive if a rename has partially
    > overwritten the name string (false negatives are always fine in the
    > lock free lookup path but false positives are not). Possibly we
    > could make do with a per-sb seqlock for this (or just rename_lock).

    My plan was always to just use rename_lock, and only do it at the outer
    level (and do it for both lookup failures _and_ for the success case).
    Your approach is _way_ more conservative than I would have done, and also
    potentially much slower due to the seqlock-per-path-component thing.

    Remember: seqlocks are almost free on x86, but they can be reasonably
    expensive in other places.

    Hmm. Regardless, this very much does look like what I envisioned, apart
    from details like that. And maybe your per-dentry seqlock is the right
    choice. On x86, it certainly doesn't have the performance issues it could
    have in other places.

    I'd like Al to take a look, if he's around.


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