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    SubjectRe: [PATCH 01/20 -v5] printk - dont wakeup klogd with interrupts disabled

    On Wed, 23 Jan 2008, Daniel Walker wrote:

    >
    > On Wed, 2008-01-23 at 11:02 -0500, Steven Rostedt wrote:
    >
    > > + if (!irqs_disabled() && wake_klogd)
    > > wake_up_klogd();
    >
    > This causes a regression .. When printk is called during an OOPS in
    > kernels without this change then the OOPS will get logged, since the
    > logging process (klogd) is woken to handle the messages.. If you apply
    > this change klogd doesn't wakeup, and hence doesn't log the oops.. So if
    > you remove the wakeup here you have to add it someplace else to maintain
    > the logging ..
    >
    > (I'm not theorizing here, I have defects logged against this specific
    > piece of code..)

    It wont get woken up anyway. Did you look at wake_up_klogd?

    void wake_up_klogd(void)
    {
    if (!oops_in_progress && waitqueue_active(&log_wait))
    wake_up_interruptible(&log_wait);
    }

    So if oops_in_progress is set, then it still wont get woken. Perhaps it
    got woken some other way? Or is oops_in_progress not set in these oops?

    One other solution is to make the runqueue locks visible externally. Like:

    in sched.c:

    int runqueue_is_locked(void)
    {
    int cpu = get_cpu();
    struct rq *rq = cpu_rq(cpu);
    int ret;
    ret = spin_is_locked(&rq->lock);
    put_cpu();
    return ret;
    }
    And in printk we could do:

    if (wake_klogd && !runqueue_is_locked())
    wake_up_klogd();
    This probably is the cleanest solution since it simply prevents the
    deadlock from occurring.

    -- Steve



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