linux-kernel - Re: [PATCH 3/4] kgdb,clocksource: Prevent kernel hang in kernel debugger

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Message-ID: <alpine.LFD.2.00.1001260933520.5926@localhost.localdomain>
Date:	Tue, 26 Jan 2010 09:45:10 +0100 (CET)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Jason Wessel <jason.wessel@...driver.com>
cc:	linux-kernel@...r.kernel.org, kgdb-bugreport@...ts.sourceforge.net,
	mingo@...e.hu, Martin Schwidefsky <schwidefsky@...ibm.com>,
	John Stultz <johnstul@...ibm.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Magnus Damm <damm@...l.co.jp>
Subject: Re: [PATCH 3/4] kgdb,clocksource: Prevent kernel hang in kernel
 debugger

On Mon, 25 Jan 2010, Jason Wessel wrote:
> This is a regression fix against: 0f8e8ef7c204988246da5a42d576b7fa5277a8e4
> 
> Spin locks were added to the clocksource_resume_watchdog() which cause
> the kernel debugger to deadlock on an SMP system frequently.
> 
> The kernel debugger can try for the lock, but if it fails it should
> continue to touch the clocksource watchdog anyway, else it will trip
> if the general kernel execution has been paused for too long.
> 
> This introduces an possible race condition where the kernel debugger
> might not process the list correctly if a clocksource is being added
> or removed at the time of this call.  This race is sufficiently rare vs
> having the kernel debugger hang the kernel

I'm not really excited happy about adding a race condition :) 

If you stop the kernel in the middle of the watchdog code
(i.e. watchdog_lock is held) then clocksource_reset_watchdog() is not
really a guarantee to keep the TSC alive.
 
>  void clocksource_touch_watchdog(void)
>  {
> -	clocksource_resume_watchdog();
> +	unsigned long flags;
> +
> +	int got_lock = spin_trylock_irqsave(&watchdog_lock, flags);

So I prefer

	if (!spin_trylock_irqsave(&watchdog_lock, flags))
		return;

If that results in TSC being marked unstable then that is way better
than having a race which might even crash or lock the machine when the
stop happened in the middle of a list_add().

Thanks,

	tglx
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