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    Subject[PATCH 0/2] del_timer_sync: proof of concept
    Andrew Morton wrote:
    >
    > If we're prepared to rule that a timer handler is not allowed to do
    > add_timer_on() then a recurring timer is permanently pinned to a CPU, isn't
    > it?
    >
    > That should make things simpler?

    In that case I think that both problems (race and scalability)
    can be solved without increasing sizeof(timer_list).

    What if timer_list had ->pending field? Then we can do:

    timer_pending:
    return timer->pending;

    __mod_timer:
    internal_add_timer(new_base, timer);
    timer->base = new_base;
    timer->pending = 1;

    __run_timers:
    list_del(&timer->entry);
    set_running_timer(base, timer);

    /* do not change timer->base */
    timer->pending = 0;

    spin_unlock(base->lock);
    timer->function();

    del_timer:
    if (!timer->pending)
    return 0;
    base = timer->base;
    ...

    del_timer_sync:
    base = timer->base;
    if (!base)
    return 0;
    spin_lock(base->lock);

    if (base != timer->base)
    goto del_again;
    if (base->running_timer == timer)
    goto del_again;

    if (timer->pending)
    list_del(&timer->entry);

    timer->pending = 0;
    timer->base = NULL;

    The ->pending flag could live in the least significant bit of
    timer->base, this way we:
    1. do not waste the space
    2. can read/write base+pending atomically

    These patches are incomplete/suboptimal, just a proof of concept.

    Oleg.
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