Messages in this thread | | | Date | Tue, 23 Oct 2018 20:54:33 +0900 | From | Sergey Senozhatsky <> | Subject | Re: [RFC][PATCHv2 1/4] panic: avoid deadlocks in re-entrant console drivers |
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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
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