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Date: Thu, 20 Aug 2009 11:28:20 +0200
From: Martin Schwidefsky <schwidefsky@de.ibm.com>
To: Ingo Molnar <mingo@elte.hu>
Cc: Thomas Gleixner <tglx@linutronix.de>,
       Peter Zijlstra <a.p.zijlstra@chello.nl>,
       john stultz <johnstul@us.ibm.com>, linux-kernel@vger.kernel.org
Subject: Re: [circular locking bug] Re: [patch 00/15] clocksource /
 timekeeping rework V4 (resend V3 + bug fix)
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On Wed, 19 Aug 2009 22:25:54 +0200
Ingo Molnar <mingo@elte.hu> wrote:

> 
> ok, with all the latest patches i re-added these bits to 
> -tip, and it triggered this lockdep assert on a testbox:

Another one :-(

> stack backtrace:
> Pid: 1, comm: swapper Not tainted 2.6.31-rc6-tip-01234-gcc9be0e-dirty #1054
> Call Trace:
>  [<c106f430>] print_usage_bug+0x130/0x180
>  [<c106f5eb>] mark_lock_irq+0x16b/0x260
>  [<c106f240>] ? check_usage_forwards+0x0/0xc0
>  [<c106f7fe>] mark_lock+0x11e/0x3a0
>  [<c106fbff>] mark_irqflags+0x17f/0x190
>  [<c107177a>] __lock_acquire+0x29a/0x520
>  [<c1071a6a>] lock_acquire+0x6a/0xc0
>  [<c10664d7>] ? clocksource_unregister+0x17/0x50
>  [<c175719b>] __mutex_lock_common+0x3b/0x340
>  [<c10664d7>] ? clocksource_unregister+0x17/0x50
>  [<c1757551>] mutex_lock_nested+0x31/0x40
>  [<c10664d7>] ? clocksource_unregister+0x17/0x50
>  [<c10664d7>] clocksource_unregister+0x17/0x50
>  [<c1008b3a>] pit_disable_clocksource+0x2a/0x40
>  [<c1008bb9>] init_pit_timer+0x29/0xb0
>  [<c106825a>] clockevents_set_mode+0x1a/0x50
>  [<c1069a96>] tick_switch_to_oneshot+0x96/0xc0
>  [<c1069ad2>] tick_init_highres+0x12/0x20
>  [<c105e32d>] hrtimer_switch_to_hres+0x4d/0x100
>  [<c105ebbd>] hrtimer_run_pending+0x4d/0x50
>  [<c104bb85>] run_timer_softirq+0x25/0x230

Ok, the cause is that the i8253 pit clocksource code tries to
unregister a clocksource from a timer interrupt. Bad idea with the new
code. Why does the pit clocksource have to >unregister< the clock if
the set_mode callback is called with CLOCK_EVT_MODE_SHUTODWN,
CLOCK_EVT_MODE_UNUSED, or CLOCK_EVT_MODE_ONESHOT? Very strange, I would
argue that the clocksource should never unregister in the set_mode
callback, the timekeeping code should not use the clocksource if it is
unsuitable for e.g. the one shot mode.

-- 
blue skies,
   Martin.

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