Re: [PATCH v5 0/2] printk: Console owner and waiter logic cleanup

Petr Mladek
2018-01-16T09:08:50Z

On Tue 2018-01-16 14:16:22, Sergey Senozhatsky wrote:
> On (01/15/18 09:51), Petr Mladek wrote:
> > On Sat 2018-01-13 16:31:00, Sergey Senozhatsky wrote:
> > > On (01/12/18 13:55), Petr Mladek wrote:
> > > [..]
> > > > > I'm not fixing console_unlock(), I'm fixing printk(). BTW, all my
> > > > > kernels are CONFIG_PREEMPT (I'm a RT guy), my mind thinks more about
> > > > > PREEMPT kernels than !PREEMPT ones.
> > > >
> > > > I would say that the patch improves also console_unlock() but only in
> > > > non-preemttive context.
> > > >
> > > > By other words, it makes console_unlock() finite in preemptible context
> > > > (limited by buffer size). It might still be unlimited in
> > > > non-preemtible context.
> > >
> > > could you elaborate a bit?
> >
> > Ah, I am sorry, I swapped the conditions. I meant that
> > console_unlock() is finite in non-preemptible context.
>
> by the way. just for the record,
>
> probably there is a way for us to have a task printing more than
> O(logbuf) even in non-preemptible context.
>
> CPU0
>
> vprintk_emit()
> preempt_disable()
> console_unlock()
> {
> for (;;) {
> printk_safe_enter_irqsave()
> call_console_drivers();
> printk_safe_exit_irqrestore()
>
> << IRQ >>
> dump_stack()
> printk()->log_store()
> ....
> printk()->log_store()
> << iret >>
> }
> }
> preempt_enable()

Great catch! And good to know about it when designing further
improvements.

Best Regards,
Petr