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

Sergey Senozhatsky
2016-04-01T01:06:39Z

Hello Petr,

On (03/31/16 13:12), Petr Mladek wrote:
> > + * Set printing kthread sleep condition early, under the
> > + * logbuf_lock, so it (if RUNNING) will go to console_lock()
> > + * and spin on logbuf_lock.
> > + */
> > + if (!in_panic && printk_kthread && !need_flush_console)
> > + need_flush_console = true;
>
> I would remove the if-statement and always set it:
>
> + It does not matter if we set it in panic. It will not affect
> anything.

hm... yes, you're right.

> + The check for printk_kthread is racy. It might be false here
> and it might be true later when check whether to wakeup
> the kthread or try to get console directly.

which is fine, isn't it? we will wakeup the printing kthread, it will
console_lock()/console_unlock() (regardless the state of need_flush_console).
printing thread checks need_flush_console only when it decides whether
it shall schedule.

> + We might set it true even when it was true before.
>
> I think that this was an attempt to avoid a spurious wake up.
> But we solve it better way now.

we also may have 'printk.synchronous = 1', which will purposelessly
dirty need_flush_console from various CPUs from every printk /* and
upon every return from console_unlock() */; that's why I added both
printk_kthread and !need_flush_console (re-dirty already dirtied)
checks.

> > raw_spin_lock(&logbuf_lock);
> > retry = console_seq != log_next_seq;
> > + if (!retry && printk_kthread)
> > + need_flush_console = false;
>
> Similar here. I remove the if-statement and always set it. We will
> either retry or it should be false anyway.

well, 'printk.synchronous = 1'. seems that `!retry' check can be
dropped, I agree.

a side nano-note,
apart from 'printk.synchronous = 1', we also can have !printk_kthread
because kthread_run(printk_kthread_func) failed. it's quite unlikely,
but still.

[..]
> > + 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;

oh, wow! that's a major mistake. thanks a lot for catching this!
shame on me.

> /*
> * Avoid an infinite loop when console_unlock() cannot
> * access consoles, e.g. because of a suspend. We
> * could get asleep here. Someone else will call
> * consoles if conditions change.
> */

looks ok.

> Also another name might help. If we set it false here, the value
> does describe a global state. The variable describes if this
> kthread needs to flush the console. So, more precise name would be
>
> printk_kthread_need_flush_console

yes, makes sense.

> I think that we are close. I really like the current state of
> the patch and how minimalistic it is.

thanks for your review.

I'll re-spin.

-ss