linux-kernel - Re: [PATCH] printk deadlocks if called with runqueue lock held

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Message-ID: <alpine.LFD.1.00.0801171724520.2957@woody.linux-foundation.org>
Date:	Thu, 17 Jan 2008 17:31:49 -0800 (PST)
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Steven Rostedt <rostedt@...dmis.org>
cc:	LKML <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Alexey Dobriyan <adobriyan@...nvz.org>,
	Kirill Korotaev <dev@...ru>, Ingo Molnar <mingo@...e.hu>
Subject: Re: [PATCH] printk deadlocks if called with runqueue lock held



On Thu, 17 Jan 2008, Steven Rostedt wrote:
>
> Calling printk with interrupts disabled should only be done for
> emergencies and debugging anyway.

Agreed, I think this is sane.

> And with this patch, my code ran fine ;-)

Patch looks fine, but I'm saddened:

> +	/*
> +	 * If we try to wake up klogd while printing with the runqueue lock
> +	 * held, this will deadlock. We don't have access to the runqueue
> +	 * lock from here, but just checking for interrupts disabled
> +	 * should be enough.
> +	 */
> +	if (!irqs_disabled() && wake_klogd)
>  		wake_up_klogd();

..because we actually *have* the old hardware irq enable/disable value in 
in that "flags" parameter we just restored, so it's more than a bit sad to 
then ask again whether they were disabled or not!

IOW, I think this should be

	if (raw_irqs_disabled_flags(flags) && wake_klogd)
		wake_up_klogd();

Of course, not all architectures seem to suport that thing (it's currently 
only used by the CONFIG_TRACE_IRQFLAGS config option).

Damn. So close, yet so far away.

			Linus
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