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

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Message-ID: <20181025082947.gznv5roj3zxzlw3t@pathway.suse.cz>
Date:   Thu, 25 Oct 2018 10:29:47 +0200
From:   Petr Mladek <pmladek@...e.com>
To:     Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
Cc:     linux-kernel@...r.kernel.org, Steven Rostedt <rostedt@...dmis.org>,
        Daniel Wang <wonderfly@...gle.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Alan Cox <gnomes@...rguk.ukuu.org.uk>,
        Jiri Slaby <jslaby@...e.com>,
        Peter Feiner <pfeiner@...gle.com>,
        linux-serial@...r.kernel.org,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>
Subject: Re: [RFC][PATCHv2 1/4] panic: avoid deadlocks in re-entrant console
 drivers

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