linux-kernel - Re: [patch 3/3] clockevents: Fix resume logic

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Date:	Sat, 12 May 2007 20:49:32 +0200
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	"Rafael J. Wysocki" <rjw@...k.pl>, Ingo Molnar <mingo@...e.hu>,
	LKML <linux-kernel@...r.kernel.org>,
	John Stultz <johnstul@...ibm.com>, linux-acpi@...r.kernel.org
Subject: Re: [patch 3/3] clockevents: Fix resume logic - updated version

On Sat, 2007-05-12 at 19:01 +0200, Thomas Gleixner wrote:
> On Sat, 2007-05-12 at 09:51 -0700, Andrew Morton wrote:
> > The locking in clocksource_resume_watchdog looks pretty pointless anyway.
> > Can't we just delete it?
> > 
> > The only thing it can race against is, conceivably,
> > 
> >         resumed = watchdog_resumed;
> >         if (unlikely(resumed))
> >                 watchdog_resumed = 0;
> > 
> > which could be solved by using test_and_clear_bit().
> 
> True. I'll redo it.

Here you go.

	tglx

------------------------------->
Subject: clocksource fix lock order in the resume path

lockdep complains about the lock nesting of clocksource and watchdog
lock in the resume path.

Change the resume marker to a bit operation and remove the lock from
this path.

Signed-off-by: Thomas Gleixner <tglx@...utronix.de>

--- a/kernel/time/clocksource.c
+++ b/kernel/time/clocksource.c
@@ -74,7 +74,7 @@ static struct clocksource *watchdog;
 static struct timer_list watchdog_timer;
 static DEFINE_SPINLOCK(watchdog_lock);
 static cycle_t watchdog_last;
-static int watchdog_resumed;
+static unsigned long watchdog_resumed;
 
 /*
  * Interval: 0.5sec Threshold: 0.0625s
@@ -104,9 +104,7 @@ static void clocksource_watchdog(unsigned long data)
 
 	spin_lock(&watchdog_lock);
 
-	resumed = watchdog_resumed;
-	if (unlikely(resumed))
-		watchdog_resumed = 0;
+	resumed = test_and_clear_bit(0, &watchdog_resumed);
 
 	wdnow = watchdog->read();
 	wd_nsec = cyc2ns(watchdog, (wdnow - watchdog_last) & watchdog->mask);
@@ -151,9 +149,7 @@ static void clocksource_watchdog(unsigned long data)
 }
 static void clocksource_resume_watchdog(void)
 {
-	spin_lock(&watchdog_lock);
-	watchdog_resumed = 1;
-	spin_unlock(&watchdog_lock);
+	set_bit(0, &watchdog_resumed);
 }
 
 static void clocksource_check_watchdog(struct clocksource *cs)


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