Re: [RFC][PATCH v8 1/2] printk: Make printk() completely async

Petr Mladek
2016-04-04T09:41:13Z

On Thu 2016-03-31 13:52:50, Jan Kara wrote:
> On Thu 31-03-16 13:12:29, Petr Mladek wrote:
> > > #if defined CONFIG_PRINTK
> > > +static int printk_kthread_func(void *data)
> > > +{
> > > + while (1) {
> > > + set_current_state(TASK_INTERRUPTIBLE);
> > > + if (!need_flush_console)
> > > + schedule();
> > > +
> > > + __set_current_state(TASK_RUNNING);
> >
> >
> > We still must do here:
> >
> > need_flush_console = false;
> >
> > Othrerwise, we might start "busy" cycling. cosole_unlock()
> > sometimes returns earlly, e.g. when console_suspended is set
> > or !can_use_console() returns true.
> >
> > Sigh, the handling of "need_flush_console" is a bit strange.
> > Part of the logic depends on logbuf_lock and the other part
> > must be lockless.
>
> Frankly, I think we are overcomplicating this. What we really need to detect
> in printk_kthread_func() is whether someone appended something to the console
> since we woken up. Sure, console_unlock() may have already printed that
> and we would unnecessarily make one more loop over console_lock() and
> console_unlock() but who cares...
>
> So what about having printk_kthread_func() like:
>
> while (1) {
> set_current_state(TASK_INTERRUPTIBLE);
> if (!need_flush_console)
> schedule();
> __set_current_state(TASK_RUNNING);
> need_flush_console = false;
> console_lock();
> console_unlock();
> }
>
> In vprintk_emit() we do:
>
> if (!in_panic && printk_kthread) {
> /* Offload printing to a schedulable context. */
> need_flush_console = true;
> wake_up_process(printk_kthread);
> } else {
> ...
>
> This guarantees that after message was appended to the buffer in
> vprintk_emit(), the message got either printed by console_unlock() or
> printk_kthread is in TASK_RUNNING state and will call console_unlock() once
> scheduled. It also guarantees that printk_kthread_func() won't loop forever
> when there's nothing to print. And that is all we need...
>
> I think the simplicity of this is worth the possible extra loops in
> printk_kthread_func().

I do not have strong opinion about this. I agree that the simplicity
of your proposal is nice. You are much more experienced kernel
developer. If you say that the potential extra loop is fine, I am
fine with it as well :-)

Best Regards,
Petr