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    SubjectRe: [PATCH -mm 1/2] irq_work, Use llist in irq_work
    On 09/01/2011 03:58 PM, Peter Zijlstra wrote:
    > On Thu, 2011-09-01 at 11:20 +0800, Huang Ying wrote:
    >> On 09/01/2011 09:46 AM, Huang Ying wrote:
    >>>>> -static void __irq_work_queue(struct irq_work *entry)
    >>>>> +static void __irq_work_queue(struct irq_work *work)
    >>>>> {
    >>>>> - struct irq_work *next;
    >>>>> + struct irq_work_list *irq_work_list;
    >>>>>
    >>>>> - preempt_disable();
    >>>>> + irq_work_list = &get_cpu_var(irq_work_lists);
    >>>>>
    >>>>> - do {
    >>>>> - next = __this_cpu_read(irq_work_list);
    >>>>> - /* Can assign non-atomic because we keep the flags set. */
    >>>>> - entry->next = next_flags(next, IRQ_WORK_FLAGS);
    >>>>> - } while (this_cpu_cmpxchg(irq_work_list, next, entry) != next);
    >>>>> + llist_add(&work->llnode, &irq_work_list->llist);
    >>>>>
    >>>>> /* The list was empty, raise self-interrupt to start processing. */
    >>>>> - if (!irq_work_next(entry))
    >>>>> + if (!test_and_set_bit(LIST_NONEMPTY_BIT, &irq_work_list->flags))
    >>>>> arch_irq_work_raise();
    >>>>
    >>>> So why can't you simply test work->llnode->next?
    >>>
    >>> Yes. That is better. Even if there may be a small race window, it is
    >>> not a big issue to raise one extra self interrupt seldom.
    >>
    >> Remember something about this. I didn't test work->llnode->next here
    >> because I didn't want expose the implementation details like that here.
    >> How about make llist_add() return whether list is empty before adding?
    >> Because it will be an inline function, that should be optimized out if
    >> the caller do not need the information.
    >
    > You could also use llist_empty() although that brings me to that
    > ACCESS_ONCE thing in there, what's the point?

    Something as follow with llist_empty() seems not work.

    empty = llist_empty(irq_work_list);
    llist_add(&work->llnode, irq_work_list);
    if (empty)
    arch_irq_work_raise();

    Because irq_work IRQ handler or timer IRQ handler may be executed just
    before "llist_add(&work->llnode, irq_work_list)", so that, although
    "empty == false", arch_irq_work_raise() still should be executed.

    Can you tell me how to that with llist_empty()?


    For ACCESS_ONCE, Mathiew suggest me to add it,

    Hi, Mathiew,

    Can you explain why ACCESS_ONCE should be used here?

    Best Regards,
    Huang Ying


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