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

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Date:	Tue, 26 Jan 2010 09:50:57 +0100 (CET)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Martin Schwidefsky <schwidefsky@...ibm.com>
cc:	Jason Wessel <jason.wessel@...driver.com>,
	linux-kernel@...r.kernel.org, kgdb-bugreport@...ts.sourceforge.net,
	mingo@...e.hu, 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 Tue, 26 Jan 2010, Martin Schwidefsky wrote:
> On Mon, 25 Jan 2010 22:26:39 -0600
> Jason Wessel <jason.wessel@...driver.com> 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
> > 
> > CC: Thomas Gleixner <tglx@...utronix.de>
> > CC: Martin Schwidefsky <schwidefsky@...ibm.com>
> > CC: John Stultz <johnstul@...ibm.com>
> > CC: Andrew Morton <akpm@...ux-foundation.org>
> > CC: Magnus Damm <damm@...l.co.jp>
> > Signed-off-by: Jason Wessel <jason.wessel@...driver.com>
> 
> The first question I would ask is why does the kernel deadlock? Can we
> have a backchain of a deadlock please?

The problem arises when the kernel is stopped inside the watchdog code
with watchdog_lock held. When kgdb restarts execution then it touches
the watchdog to avoid that TSC gets marked unstable.
 
> Hmm, there are all kinds of races if the watchdog code gets interrupted
> by the kernel debugger. Wouldn't it be better to just disable the
> watchdog while the kernel debugger is active? 

No, we can keep it and in most cases it clocksource_touch_watchdog()
helps to keep TSC alive. A simple "if (!trylock) return;" should solve
the deadlock problem for kgdb without opening a can of worms.

Thanks,

	tglx
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