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

Sergey Senozhatsky
2018-10-23T11:54:40Z

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.

Thanks for taking a look.
As of "weird" part I have some explanations:

> @@ -233,17 +233,14 @@ void panic(const char *fmt, ...)
> if (_crash_kexec_post_notifiers)
> __crash_kexec(NULL);
>
> - bust_spinlocks(0);
> -
[..]
> - debug_locks_off();
> +#ifdef CONFIG_VT
> + unblank_screen();
> +#endif
> console_flush_on_panic();
>
> if (!panic_blink)

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. However, a bit of a bigger
argument:
__attribute__((weak)) suggests that bust_spinlocks() is arch-dependent
and it's up to arch to do some extra stuff there [if needed]. So that's
why I decided to keep bust_spinlocks(0) in panic() and, thus, call into
arch-specific code (or common bust_spinlocks); then bump oops_in_progress
so serial consoles become re-entrant and finally call
console_flush_on_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.

Wrt to uart port we usually have "bool locked" flag and unlock
port->lock only if we locked it:

{
if (oops_in_progress)
locked = spin_trylock_irqsave(&port->lock, flags);

...

if (locked)
spin_unlock_irqrestore(&port->lock, flags);
}

Wrt to console_sem we have

{
if (oops_in_progress)
if (!down_trylock_console_sem() != 0)
return;
...
console_unlock();
}

So the locks that we care about in this particular patch (console sem
and port->lock) probably should not see any locking imbalance.

If you have strong opinion then we can have debug_locks_off() change
as part of this patch. But maybe I'd prefer to have it as a separate
patch. What do you think?

-ss