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    SubjectRe: recursive spinlocks. Shoot.
    On Sun, 18 May 2003, Peter T. Breuer wrote:

    >
    > Here's a before-breakfast implementation of a recursive spinlock. That
    > is, the same thread can "take" the spinlock repeatedly. This is crude -
    > I just want to focus some attention on the issue (while I go out and
    > have breakfast :'E).
    >
    > The idea is to implement trylock correctly, and then get lock and
    > unlock from that via the standard algebra. lock is a loop doing
    > a trylock until it succeeds. We emerge from a successful trylock
    > with the lock notionally held.
    >
    > The "spinlock" is a register of the current pid, plus a recursion
    > counter, with atomic access. The pid is either -1 (unset, count is
    > zero) or some decent value (count is positive).
    >
    > The trylock will succeed and set the pid if it is currently unset. It
    > will succeed if the pid matches ours, and increment the count of
    > holders.
    >
    > Unlock just decrements the count. When we've unlocked enough times,
    > somebody else can take the lock.

    A looong time ago I gave to someone a recursive spinlock implementation
    that they integrated in the USB code. I don't see it in the latest
    kernels, so I have to guess that they found a better solution to do their
    things. I'm biased to say that it must not be necessary to have the thing
    if you structure your code correctly.



    - Davide

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