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

    > This is essentially the same struct as mine. I used the pid of the task,
    > where you use the address of the task. You use an atomic count, whereas
    > I used an ordinary int, guarded by the embedded spinlock.
    > > #define nestlock_lock(snl) \
    > > do { \
    > > if ((snl)->uniq == current) { \
    > That would be able to read uniq while it is being written by something
    > else (which it can, according to the code below). It needs protection.

    No it does not, look better.

    > > atomic_inc(&(snl)->count); \
    > OK, that's the same.
    > > } else { \
    > > spin_lock(&(snl)->lock); \
    > > atomic_inc(&(snl)->count); \
    > > (snl)->uniq = current; \
    > Hmm .. else we wait for the lock, and then set count and uniq, while
    > somebody else may have entered and be reading it :-). You exit with

    Nope, think about a case were it breaks. False negatives are not possible
    because it is set by the same task and false positives either.

    > Well, it's not assembler either, but at least it's easily comparable
    > with the nonrecursive version. It's essentially got an extra if and
    > an inc in the lock. That's all.

    Well, there's a little difference. In case of contention, you loop with
    your custom try lock while I used the optimized asm code inside spin_lock.
    But again, I believe we didn't lose anything with the removal of this code
    (nested/recursive locks) from the kernel.

    - Davide

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