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    SubjectRe: [ANNOUNCE] mdb-2.6.27-rc2-ia32-08-07-08.patch
    Jeremy Fitzhardinge wrote:
    > Stefan Richter wrote:
    >> jmerkey@wolfmountaingroup.com wrote:
    >>> ftp://ftp.wolfmountaingroup.org/pub/mdb/mdb-2.6.27-rc2-ia32-08-07-08.patch
    [...]
    >> The accessors rspin_lock() and rspin_try_lock() peek into spinlock_t and
    >> may therefore not be fully portable. Also, they and rspin_unlock()
    >> don't look SMP safe:
    >>
    >>
    >>> +//
    >>> +// returns 0 - atomic lock occurred, processor assigned
    >>> +// 1 - recusive count increased
    >>> +//
    >>> +
    >>> +unsigned long rspin_lock(volatile rlock_t *rlock)
    >>> +{
    >>> +#if defined(CONFIG_SMP)
    >>> + register unsigned long proc = get_processor_id();
    >>> + register unsigned long retCode;
    >>> +
    >>> + if (rlock->lock.raw_lock.slock && rlock->processor == proc)
    >>>
    >
    > Ticket locks will almost always have a non-zero slock. It doesn't
    > indicate anything about the locked/unlocked state. But this looks like
    > it's effectively doing a trylock:
    >
    > if (!spin_trylock(rlock) && rlock->processor == proc) {
    > rlock->count++;
    > ...
    > } else {
    > rlock->processor = proc;
    > ...
    > }

    Right. This implemention also looks free of race conditions, provided that

    - rspin_lock, rspin_try_lock, and rspin_unlock are only called in
    contexts with disabled preemption and disabled local interrupts,

    - rspin_unlock() rewrites rlock->processor to "no CPU" before
    it drops the lock. (The implementation in
    mdb-2.6.27-rc2-ia32-08-07-08.patch does so.)

    BTW, the rspin_try_lock() in that patch wrong: It always returns 0
    instead of having three branches of execution which return 0/1/-1.
    --
    Stefan Richter
    -=====-==--- =--- -=--=
    http://arcgraph.de/sr/


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