Re: [RFC][PATCHv2 1/4] panic: avoid deadlocks in re-entrant console drivers

Petr Mladek
2018-10-25T08:29:47Z

On Tue 2018-10-23 20:54:33, Sergey Senozhatsky wrote:
> On (10/23/18 13:07), Petr Mladek wrote:
> > Though this looks a bit weird.
> >
> > I have just realized that console_unblank() is called by
> > bust_spinlocks(0) and does basically the same as
> > console_flush_on_panic(). Also it does not make much
> > sense wake_up_klogd() there. Finally, it seems to be
> > too late to disable lockdep there.
>
> So I did look at what lib/bust_spinlocks.c does; and I agree that waking
> up klogd makes little sense, on the other hand it just sets per-cpu
> pending bit, so not a big deal. console_unlock() should do there the
> same thing as console_flush_on_panic().

Yes, klogd is not a big deal. I just think that the bust_spinlocks()
ping-pong would just confuse the code. It might be better to keep
the spinlocks busted and make sure that we do not cause regressions
by not calling bust_spinlocks(0).

It tried to solve this by calling the important actions from
bust_spinlocks(0) directly in panic(). I wanted to omit anything
that does not make sense in panic().

> > void __attribute__((weak)) bust_spinlocks(int yes)
> > {
> > if (yes) {
> > + /*
> > + * Some locks might get ignored in the Oops situation
> > + * to get an important work done. Locks debug should
> > + * be disabled to avoid reporting bad unlock balance.
> > + */
> > + debug_locks_off();
> > ++oops_in_progress;
>
> Hmm, I don't think I've seen any reports because of this. From printk/console
> POV the locks which are not taken under oops_in_progress are not released.

Fair enough. Let's keep debug_locks_off() in panic().

Best Regards,
Petr