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

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Message-ID: <20181023120441.GB10251@jagdpanzerIV>
Date:   Tue, 23 Oct 2018 21:04:41 +0900
From:   Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
To:     Petr Mladek <pmladek@...e.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>,
        Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
Subject: Re: [RFC][PATCHv2 1/4] panic: avoid deadlocks in re-entrant console
 drivers

On (10/23/18 20:54), Sergey Senozhatsky wrote:
> 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().

Seems that s390 is the only arch which defines its own bust_spinlocks().
Not sure why... Just to play games with console_loglevel?

---

void bust_spinlocks(int yes)
{
	if (yes) {
		oops_in_progress = 1;
	} else {
		int loglevel_save = console_loglevel;
		console_unblank();
		oops_in_progress = 0;
		/*
		 * OK, the message is on the console.  Now we call printk()
		 * without oops_in_progress set so that printk will give klogd
		 * a poke.  Hold onto your hats...
		 */
		console_loglevel = 15;
		printk(" ");
		console_loglevel = loglevel_save;
	}
}

---

The "printk(" "); without oops_in_progress" part is a bit worrisome.
This thing technically can deadlock. Unless s390 has no NMI panic().

	-ss