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    SubjectRe: Linux 2.2.19pre2
    On Sun, Dec 24, 2000 at 11:23:33AM +1100, Andrew Morton wrote:
    > Andrea Arcangeli wrote:
    > > 1) could be fixed trivially by making the waitqueue_lock a spinlock, but
    > > this way doesn't solve 2). And if we solve 2) properly than 1) gets fixed as

    BTW (follow up myself), really making the lock a spinlock (not a readwrite
    lock) would fix 2) as well (waitqueue_lock is global in 2.2.x I was thinking at
    the per-waitqueue lock of 2.4.x ;).

    > > well.
    > I don't understand the problem with 2) in 2.2? Every task on both waitqueues
    > gets woken up. Won't it sort itself out OK?

    Not every task if it's a wake-one on both waitqueues. The problem should be the
    same in 2.2.x and 2.4.x. But if such usage makes sense is uncertain...

    > For 2.4, 2) is an issue because we can have tasks on two waitqueues at the
    > same time, with a mix of exclusive and non. Putting a global spinlock
    > into __wake_up_common would fix it, but was described as "horrid" by
    > you-know-who :)

    Yes. And that wouldn't fix the race number 3) below.

    > > I agree the right fix for 2) (and in turn for 1) ) is to count the number of
    > > exclusive wake_up_process that moves the task in the runqueue, if the task was
    > > just in the runqueue we must not consider it as an exclusive wakeup (so in turn
    > > we'll try again to wakeup the next exclusive-wakeup waiter). This will
    > > fix both races. Since the fix is self contained in __wake_up it's fine
    > > for 2.2.19pre3 as well and we can keep using a read_write lock then.
    > I really like this approach. It fixes another problem in 2.4:
    > Example:
    > static struct request *__get_request_wait(request_queue_t *q, int rw)
    > {
    > register struct request *rq;
    > DECLARE_WAITQUEUE(wait, current);
    > add_wait_queue_exclusive(&q->wait_for_request, &wait);
    > for (;;) {
    > __set_current_state(TASK_UNINTERRUPTIBLE);
    > /* WINDOW HERE */
    > spin_lock_irq(&io_request_lock);
    > rq = get_request(q, rw);
    > spin_unlock_irq(&io_request_lock);

    note that the above is racy and can lose a wakeup, 2.4.x needs
    set_current_state there (not __set_current_state): spin_lock isn't a two-way
    barrier, it only forbids stuff ot exit the critical section. So on some
    architecture (not on the alpha for example) the cpu could reorder the code
    this way:

    rq = get_request

    So inverting the order of operations. That needs to be fixed too (luckily
    it's a one liner).

    > if (rq)
    > break;
    > generic_unplug_device(q);
    > schedule();
    > }
    > remove_wait_queue(&q->wait_for_request, &wait);
    > current->state = TASK_RUNNING;
    > return rq;
    > }
    > If this task enters the schedule() and is then woken, and another
    > wakeup is sent to the waitqueue while this task is executing in
    > the marked window, __wake_up_common() will try to wake this
    > task a second time and will then stop looking for tasks to wake.
    > The outcome: two wakeups sent to the queue, but only one task woken.


    And btw such race is new and it must been introduced in late 2.4.0-test1X or
    so, I'm sure it couldn't happen in whole 2.3.x and 2.4.0-testX because the
    wakeup was clearing atomically the exclusive bit from the task->state.

    Still talking about late 2.4.x changes, why add_wait_queue_exclusive gone
    in kernel/fork.c?? That's obviously not the right place :).

    > I haven't thought about it super-hard, but I think that if
    > __wake_up_common's exclusive-mode handling were changed
    > as you describe, so that it keeps on scanning the queue until it has
    > *definitely* moved a task onto the runqueue then this
    > problem goes away.

    Yes, that's true.

    > > Those races of course are orthogonal with the issue we discussed previously
    > > in this thread: a task registered in two waitqueues and wanting an exclusive
    > > wakeup from one waitqueue and a wake-all from the other waitqueue (for
    > > addressing that we need to move the wake-one information from the task struct
    > > to the waitqueue_head and I still think that shoudln't be addressed in 2.2.x,
    > > 2.2.x is fine with a per-task-struct wake-one information)
    > OK by me, as long as people don't uncautiously start using the
    > capability for other things.
    > > Should I take care of the 2.2.x fix, or will you take care of it? I'm not using
    > > the wake-one patch in 2.2.19pre3 because I don't like it (starting from the
    > > useless wmb() in accept) so if you want to take care of 2.2.19pre3 yourself I'd
    > > suggest to apply the wake-one patch against 2.2.19pre3 in my ftp-patch area
    > > first. Otherwise give me an ack and I'll extend myself my wake-one patch to
    > > ignore the wake_up_process()es that doesn't move the task in the runqueue.
    > ack.
    > I'll take another look at the 2.4 patch and ask you to review that
    > when I've finished with the netdevice wetworks, if that's
    > OK.

    OK. Thanks for the help.

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