From: Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com>
To: Petr Mladek <pmladek@suse.com>
Cc: Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com>,
linux-kernel@vger.kernel.org,
Steven Rostedt <rostedt@goodmis.org>,
Daniel Wang <wonderfly@google.com>,
Peter Zijlstra <peterz@infradead.org>,
Andrew Morton <akpm@linux-foundation.org>,
Linus Torvalds <torvalds@linux-foundation.org>,
Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
Alan Cox <gnomes@lxorguk.ukuu.org.uk>,
Jiri Slaby <jslaby@suse.com>, Peter Feiner <pfeiner@google.com>,
linux-serial@vger.kernel.org,
Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
Subject: Re: [RFC][PATCHv2 1/4] panic: avoid deadlocks in re-entrant console drivers
Date: Tue, 23 Oct 2018 20:54:33 +0900 [thread overview]
Message-ID: <20181023115433.GA10251@jagdpanzerIV> (raw)
In-Reply-To: <20181023110751.un2u67bc7dpo4ska@pathway.suse.cz>
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
next prev parent reply other threads:[~2018-10-23 11:54 UTC|newest]
Thread overview: 44+ messages / expand[flat|nested] mbox.gz Atom feed top
2018-10-16 5:04 [RFC][PATCHv2 0/4] less deadlock prone serial consoles Sergey Senozhatsky
2018-10-16 5:04 ` [RFC][PATCHv2 1/4] panic: avoid deadlocks in re-entrant console drivers Sergey Senozhatsky
2018-10-17 4:48 ` Sergey Senozhatsky
2018-10-23 11:07 ` Petr Mladek
2018-10-23 11:54 ` Sergey Senozhatsky [this message]
2018-10-23 12:04 ` Sergey Senozhatsky
2018-10-23 12:12 ` Sergey Senozhatsky
2018-10-25 9:06 ` Petr Mladek
2018-10-25 9:31 ` Sergey Senozhatsky
2018-10-25 8:29 ` Petr Mladek
2018-10-25 9:05 ` Sergey Senozhatsky
2018-10-25 10:10 ` [PATCHv3] " Sergey Senozhatsky
2018-10-25 10:51 ` kbuild test robot
2018-10-25 11:56 ` Sergey Senozhatsky
2018-10-31 12:27 ` Petr Mladek
2018-11-01 1:48 ` Sergey Senozhatsky
2018-11-01 8:08 ` Petr Mladek
2018-11-22 13:12 ` Petr Mladek
2018-12-12 0:53 ` Daniel Wang
2018-12-12 5:23 ` Sergey Senozhatsky
2018-12-12 5:59 ` Daniel Wang
2018-12-12 6:06 ` Sergey Senozhatsky
2018-12-12 6:09 ` Daniel Wang
2018-10-16 5:04 ` [RFC][PATCHv2 2/4] printk: move printk_safe macros to printk header Sergey Senozhatsky
2018-10-16 7:27 ` Peter Zijlstra
2018-10-16 11:40 ` Petr Mladek
2018-10-16 12:17 ` Peter Zijlstra
2018-10-17 10:50 ` Petr Mladek
2018-10-17 14:00 ` Peter Zijlstra
2018-10-22 14:30 ` Petr Mladek
2018-10-16 12:27 ` Sergey Senozhatsky
2018-10-16 12:38 ` Peter Zijlstra
2018-10-16 12:54 ` Peter Zijlstra
2018-10-16 14:21 ` Peter Zijlstra
2018-10-17 4:32 ` Sergey Senozhatsky
2018-10-17 7:57 ` Peter Zijlstra
2018-10-17 13:36 ` Sergey Senozhatsky
2018-10-23 6:25 ` Sergey Senozhatsky
2018-10-16 5:04 ` [RFC][PATCHv2 3/4] serial: introduce uart_port locking helpers Sergey Senozhatsky
2018-12-08 3:12 ` Sergey Senozhatsky
2018-12-12 11:08 ` Greg Kroah-Hartman
2018-10-16 5:04 ` [RFC][PATCHv2 4/4] tty: 8250: switch to " Sergey Senozhatsky
2018-10-16 7:23 ` [RFC][PATCHv2 0/4] less deadlock prone serial consoles Peter Zijlstra
2018-10-16 8:12 ` Sergey Senozhatsky
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