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    SubjectRe: [PATCH v5 0/2] printk: Console owner and waiter logic cleanup
    Hi,

    On (01/15/18 12:50), Petr Mladek wrote:
    > On Mon 2018-01-15 11:17:43, Petr Mladek wrote:
    > > PS: Sergey, you have many good points. The printk-stuff is very
    > > complex and we could spend years discussing the perfect solution.
    >
    > BTW: One solution that comes to my mind is based on ideas
    > already mentioned in this thread:
    >
    > void console_unlock(void)
    > {
    > disable_preemtion();
    >
    > while(pending_message) {
    >
    > call_console_drivers();
    >
    > if (too_long_here() && current != printk_kthread) {
    > wake_up_process(printk_kthread())
    >
    > }
    >
    > enable_preemtion();
    > }

    unfortunately disabling preemtion in console_unlock() is a bit
    dangerous :( we have paths that call console_unlock() exactly
    to flush everything (not only new pending messages, but everything)
    that is in logbuf and we cannot return from console_unlock()
    preliminary in that case.

    > bool too_long_here(void)
    > {
    > return should_resched();
    > or
    > return spent_here() > 1 / HZ / 2;
    > or
    > what ever we agree on
    > }
    >
    >
    > int printk_kthread_func(void *data)
    > {
    > while(1) {
    > if (!pending_messaged)
    > schedule();
    >
    > if (console_trylock_spinning())
    > console_unlock();
    >
    > cond_resched();
    > }
    > }

    overall that's very close to what I have in one of my private branches.
    console_trylock_spinning() for some reason does not perform really
    well on my made-up internal printk torture tests. it seems that I
    have a much better stability (no lockups and so on) when I also let
    printk_kthread to sleep on console_sem(). but I will look further.

    -ss

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