linux-kernel - Re: [crash, bisected] Re: clocksource: Resolve cpu hotplug dead lock with TSC unstable

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Date:	Fri, 11 Sep 2009 15:33:05 +0200
From:	Martin Schwidefsky <schwidefsky@...ibm.com>
To:	Ingo Molnar <mingo@...e.hu>
Cc:	Jens Axboe <jens.axboe@...cle.com>,
	John Stultz <johnstul@...ibm.com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Mike Galbraith <efault@....de>,
	Con Kolivas <kernel@...ivas.org>, linux-kernel@...r.kernel.org
Subject: Re: [crash, bisected] Re: clocksource: Resolve cpu hotplug dead
 lock with TSC unstable

On Fri, 11 Sep 2009 09:37:47 +0200
Ingo Molnar <mingo@...e.hu> wrote:

> 
> * Ingo Molnar <mingo@...e.hu> wrote:
> 
> > 
> > * Ingo Molnar <mingo@...e.hu> wrote:
> > 
> > > 
> > > * Jens Axboe <jens.axboe@...cle.com> wrote:
> > > 
> > > > I went to try -tip btw, but it crashes on boot. Here's the 
> > > > backtrace, typed manually, it's crashing in 
> > > > queue_work_on+0x28/0x60.
> > > > 
> > > > Call Trace:
> > > >         queue_work
> > > >         schedule_work
> > > >         clocksource_mark_unstable
> > > >         mark_tsc_unstable
> > > >         check_tsc_sync_source
> > > >         native_cpu_up
> > > >         relay_hotcpu_callback
> > > >         do_forK_idle
> > > >         _cpu_up
> > > >         cpu_up
> > > >         kernel_init
> > > >         kernel_thread_helper
> > > 
> > > hm, that looks like an old bug i fixed days ago via:
> > > 
> > >   00a3273: Revert "x86: Make tsc=reliable override boot time stability checks"
> > > 
> > > Have you tested tip:master - do you still know which sha1?
> > 
> > Ok, i reproduced it on a testbox and bisected it, the crash is 
> > caused by:
> > 
> >  7285dd7fd375763bfb8ab1ac9cf3f1206f503c16 is first bad commit
> >  commit 7285dd7fd375763bfb8ab1ac9cf3f1206f503c16
> >  Author: Thomas Gleixner <tglx@...utronix.de>
> >  Date:   Fri Aug 28 20:25:24 2009 +0200
> > 
> >     clocksource: Resolve cpu hotplug dead lock with TSC unstable
> >     
> >     Martin Schwidefsky analyzed it:
> > 
> > I've reverted it in tip/master for now.
> 
> and that uncovers the circular locking bug that this commit was 
> supposed to fix ...
> 
> Martin?

This patch should fix the obvious problem that the watchdog_work
structure is not yet initialized if the clocksource watchdog is not
running yet.
--
Subject: [PATCH] clocksource: statically initialize watchdog workqueue

From: Martin Schwidefsky <schwidefsky@...ibm.com>

The watchdog timer is started after the watchdog clocksource and at least
one watched clocksource have been registered. The clocksource work element
watchdog_work is initialized just before the clocksource timer is started.
This is too late for the clocksource_mark_unstable call from native_cpu_up.
To fix this use a static initializer for watchdog_work.

Signed-off-by: Martin Schwidefsky <schwidefsky@...ibm.com>
---
 kernel/time/clocksource.c |    5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

Index: linux-2.6/kernel/time/clocksource.c
===================================================================
--- linux-2.6.orig/kernel/time/clocksource.c
+++ linux-2.6/kernel/time/clocksource.c
@@ -123,10 +123,12 @@ static DEFINE_MUTEX(clocksource_mutex);
 static char override_name[32];
 
 #ifdef CONFIG_CLOCKSOURCE_WATCHDOG
+static void clocksource_watchdog_work(struct work_struct *work);
+
 static LIST_HEAD(watchdog_list);
 static struct clocksource *watchdog;
 static struct timer_list watchdog_timer;
-static struct work_struct watchdog_work;
+static DECLARE_WORK(watchdog_work, clocksource_watchdog_work);
 static DEFINE_SPINLOCK(watchdog_lock);
 static cycle_t watchdog_last;
 static int watchdog_running;
@@ -230,7 +232,6 @@ static inline void clocksource_start_wat
 {
 	if (watchdog_running || !watchdog || list_empty(&watchdog_list))
 		return;
-	INIT_WORK(&watchdog_work, clocksource_watchdog_work);
 	init_timer(&watchdog_timer);
 	watchdog_timer.function = clocksource_watchdog;
 	watchdog_last = watchdog->read(watchdog);

-- 
blue skies,
   Martin.

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

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