linux-kernel - Re: [tip:timers/core] x86: Do not unregister PIT clocksource on PIT oneshot setup/shutdown

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Message-ID: <20090822231457.7139bd00@skybase>
Date:	Sat, 22 Aug 2009 23:14:57 +0200
From:	Martin Schwidefsky <schwidefsky@...ibm.com>
To:	Ingo Molnar <mingo@...e.hu>
Cc:	mingo@...hat.com, hpa@...or.com, linux-kernel@...r.kernel.org,
	johnstul@...ibm.com, tglx@...utronix.de,
	linux-tip-commits@...r.kernel.org
Subject: Re: [tip:timers/core] x86: Do not unregister PIT clocksource on	PIT
 oneshot setup/shutdown

On Sat, 22 Aug 2009 12:35:41 +0200
Ingo Molnar <mingo@...e.hu> wrote:

> even after this fix, -tip testing found yet another circular locking 
> bug, on a 32-bit Core2 laptop:

You got to love lockdep .. another ugly one:
clocksource_register: has clocksource_mutex, get cpu_add_remove_lock
native_cpu_up: has cpu_add_remove_lock, get clocksource_mutex

To register a clocksource the clocksource_mutex is acquired and if
necessary timekeeping_notify is called to install the clocksource as
the timekeeper clock. timekeeping_notify uses stop_machine which needs
to take cpu_add_remove_lock mutex.
Starting a new cpu is done with the cpu_add_remove_lock mutex held.
native_cpu_up checks the tsc of the new cpu and if the tsc is no good
clocksource_change_rating is called. Which needs the clocksource_mutex
and the deadlock is complete.

-- 
blue skies,
   Martin.

"Reality continues to ruin my life." - Calvin.

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