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

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Date:   Thu, 25 Oct 2018 18:05:39 +0900
From:   Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
To:     Petr Mladek <pmladek@...e.com>
Cc:     Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>,
        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 (10/25/18 10:29), Petr Mladek wrote:
> 
> Yes, klogd is not a big deal. I just think that the bust_spinlocks()
> ping-pong would just confuse the code.

I agree; that's why I put some comments there.

> It might be better to keep the spinlocks busted and make sure that we do
> not cause regressions by not calling bust_spinlocks(0).

Sure, I understand. One reason to keep bust_spinlocks(0) there was "invoke
arch-specific bust_spinlocks(0), which might do something that common
bust_spinlocks() wouldn't do". Without going into details if any arch actually
does anything "special" in bust_spinlocks(0). Another reason was - this patch
looks like a -stable material to me; especially given that we have panic()
deadlock reports now. So I wanted to have a one liner which will not change
things for arch-s that re-define bust_spinlocks() and, at the same time,
fix the deadlock. Other than that I'm all for keeping spinlocks busted all
the time and just doing:

---
#ifdef CONFIG_VT
		unblank_screen();
#endif
		console_unblank();
---

in panic().

BTW, speaking of s390 bust_spinlocks(). It seems that starting from 4.21
all arch-s will use common bust_spinlocks() [1].

[..]
> > 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().

Agreed.

[1] lkml.kernel.org/r/20181025081108.GB26561@...ris

	-ss