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    SubjectRe: [PATCH] Fix a race in pid generation that causes pids to be reused immediately.

    On Wed, 9 Jun 2010, Ingo Molnar wrote:

    > * Salman <> wrote:
    > >
    > > A program that repeatedly forks and waits is susceptible to having the
    > > same pid repeated, especially when it competes with another instance of the
    > > same program. This is really bad for bash implementation. Furthermore, many shell
    > > scripts assume that pid numbers will not be used for some length of time.
    > >
    > > Thanks to Ted Tso for the key ideas of this implementation.
    > Looks rather interesting. (Cleanliness-wise i'd suggest to split out the while
    > loop into a helper function.)

    I have to say that usually I can look at a patch and see what it does.

    This time I had absolutely no clue.

    There was a whole big context missing: that original load of "last_pid"
    into "last" at the top of the function, far outside the patch. And in
    this case I don't think splitting out the trivial cmpxchg() loop would
    help that problem - that would just make the "load original" part of the
    big picture be even further away from the final "replace if same" part,
    and I think _that_ is a much more critical part of the subtleties there.

    So I had to read the patch _and_ go read the code it patched, in order to
    at all understand what it did. I think the patch explanation should have
    done it, and I also think this would need a bit comment at the top.

    [ In fact, I'd argue that the _old_ code would have needed a big comment
    at the top about last_pid usage, but i somebody had done that, they'd
    probably also have seen the race while explaning how the code worked ;]

    So having looked at the patch and the code, I agree with the patch, but
    I'd like some more explanation to go with it.

    [ Or Ted's version: as mentioned, I don't think the complexity is actually
    in the final cmpxchg loop itself, but in the bigger picture, so I don't
    think the differences between Ted's and Salman's versions are that big ]


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