Re: printk meeting at LPC

[Daniel Vetter <daniel.vetter@ffwll.ch>]

On Sun, Sep 15, 2019 at 3:48 PM John Ogness <john.ogness@linutronix.de> wrote:
>
> On 2019-09-13, Daniel Vetter <daniel.vetter@ffwll.ch> wrote:
> >> 2. A kernel thread will be created for each registered console, each
> >> responsible for being the sole printers to their respective
> >> consoles. With this, console printing is _fully_ decoupled from
> >> printk() callers.
> >
> > Is the plan to split the console_lock up into a per-console thing? Or
> > postponed for later on?
>
> AFAICT, the only purpose for a console_lock would be to synchronize
> between the console printing kthread and some other component that wants
> to write to that same device. So a per-console console_lock should be
> the proper solution. However, I will look into the details. My main
> concerns about this are the suspend/resume logic and the code sitting
> behind /dev/console. I will share details once I've sorted it all out.
>
> >> 6. A new may-sleep function pr_flush() will be made available to wait
> >> for all previously printk'd messages to be output on all consoles
> >> before proceeding. For example:
> >>
> >>     pr_cont("Running test ABC... ");
> >>     pr_flush();
> >>
> >>     do_test();
> >>
> >>     pr_cont("PASSED\n");
> >>     pr_flush();
> >
> > Just crossed my mind: Could/should we lockdep-annotate pr_flush (take
> > a lockdep map in there that we also take around the calls down into
> > console drivers in each of the console printing kthreads or something
> > like that)? Just to avoid too many surprises when people call pr_flush
> > from within gpu drivers and wonder why it doesn't work so well.
>
> Why would it not work so well? Basically the task calling pr_flush()
> will monitor the lockless iterators of the various consoles until _all_
> have hit/passed the latest sequence number from the time of the call.

Classic deadlock like the below: Some thread:

mutex_lock(A);
pr_flush();
mutex_unlock(A);

And in the normal console write code also needs do to mutex_lock(A);
mutex_unlock(A); somewhere.

> > Although with this nice plan we'll take the modeset paths fully out of
> > the printk paths (even for normal outputs) I hope, so should be a lot
> > more reasonable.
>
> You will be running in your own preemptible kthread, so any paths you
> take should be safe.
>
> > From gpu perspective this all sounds extremely good and first
> > realistic plan that might lead us to an actually working bsod on
> > linux.
>
> Are you planning on basing the bsod stuff on write_atomic() (which is
> used after entering an emergency state) or on the kmsg_dump facility? I
> would expect kmsg_dump might be more appropriate, unless there are
> concerns that the machine will die before getting that far (i.e. there
> is a lot that happens between when an OOPS begins and when kmsg_dumpers
> are invoked).

Yeah I think kms_dump is what the current patches use. From the fbcon
pov the important bit here is the clearly split out write_atomic, so
that we can make sure we never try to do anything stupid from special
contexts. Aside from the printing itself we also have all kinds of fun
stuff like unblank_screen() and console_unblank() currently in the
panic path still.

-Daniel
--
Daniel Vetter
Software Engineer, Intel Corporation
+41 (0) 79 365 57 48 - http://blog.ffwll.ch