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    SubjectRe: [PATCH][RFC] %pd - for printing dentry name
    On Tue, Feb 02, 2010 at 07:09:08AM +0000, Al Viro wrote:
    > On Mon, Feb 01, 2010 at 10:53:41PM -0800, Paul E. McKenney wrote:
    > > Here is an approximation that might inspire someone to come up with a
    > > real solution.
    > >
    > > One approach would be to store the name length with the name, so that
    > > struct qstr loses the "len" field, and so that its "name" field points
    > > to a struct that has a "len" field followed by an array of const
    > > unsigned char. That way, the name and length are closely associated.
    > > When you pick up a struct qstr's "name" pointer, you are guaranteed to
    > > get a length that matches the name.
    > >
    > > Unfortunately:
    > >
    > > o In theory, this leaves the length of the dentry unchanged, but
    > > alignment is a problem on 64-bit systems. Also, the long names
    > > gain an extra four bytes.
    > That one is not a big deal.


    > > o If you get a pointer to the d_iname small-name field, rename
    > > might still change the name out from under you. This could in
    > > theory be fixed by refusing to re-use the d_iname field until
    > > an RCU grace period had elapsed (using an external structure
    > > instead). In practice, not sure if this is really a reasonable
    > > approach.
    > That, OTOH, is - most of dentries use inline name and external one is
    > really a rarely used fallback. Making it a common case isn't nice.

    It is possible to move it back inline after a grace period, so that the
    external name would be in use for only a few milliseconds after the
    rename, but that of course adds more complexity.

    > There's another practical problem - a lot of code uses qstr fields and
    > patch will be painful; I couldn't care less about the out-of-tree code,
    > but it's a flagday change and in-tree patch size is not something to
    > sneeze at - I've been crawling through all that code for the last couple
    > of days and there's a lot of it.


    > Trying to play with seqlock-based solutions sounds more promising; I've
    > missed it completely and I'm half-asleep right now, so I'll try to take
    > a look at that after I get some sleep.

    Certainly sounds worth a try.

    Thanx, Paul

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