linux-kernel - generic one-shot bug (was Re: sparc generic time / clockevents)

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Message-Id: <20070224.211353.74752521.davem@davemloft.net>
Date:	Sat, 24 Feb 2007 21:13:53 -0800 (PST)
From:	David Miller <davem@...emloft.net>
To:	johnstul@...ibm.com
Cc:	tglx@...utronix.de, linux-kernel@...r.kernel.org,
	peter.keilty@...com
Subject: generic one-shot bug (was Re: sparc generic time / clockevents)

As I suspected, the one-shot code wasn't very well tested and I'd be
the one to debug this thing on sparc64 :-)

When a timer exceeds the timer period, the one-shot handling code does
the following loop:

	for (;;) {
		ktime_t next = ktime_add(dev->next_event, tick_period);

		if (!clockevents_program_event(dev, next, ktime_get()))
			return;
		tick_periodic(cpu);
	}

So it just keeps running tick_periodic() until we "catch up".

Problem is, if clockevents_program_event() gets a "next" time in the
past, the very case where we'll loop, it DOES NOT update
dev->next_event.  It returns the error before doing so.

As a result of this, we'll loop forever here, the softlockup watchdog
will trigger, and the system will wedge completely.

I was getting a softlockup and immediate system hang, so to debug this
I kept a history of the last 8 TSC values when tick_periodic() was
invoked.  At softlockup trigger, I'd dump the log.  And what I saw
were TSC deltas that we so tiny as to be just enough to indicate
tick_periodic() was running in a tight loop :-)

I propose the following fix, which I'm about to test.

Signed-off-by: David S. Miller <davem@...emloft.net>

diff --git a/kernel/time/tick-common.c b/kernel/time/tick-common.c
index 4500e34..0986a2b 100644
--- a/kernel/time/tick-common.c
+++ b/kernel/time/tick-common.c
@@ -77,6 +77,7 @@ static void tick_periodic(int cpu)
 void tick_handle_periodic(struct clock_event_device *dev)
 {
 	int cpu = smp_processor_id();
+	ktime_t next;

 	tick_periodic(cpu);

@@ -86,12 +87,12 @@ void tick_handle_periodic(struct clock_event_device *dev)
 	 * Setup the next period for devices, which do not have
 	 * periodic mode:
 	 */
+	next = ktime_add(dev->next_event, tick_period);
 	for (;;) {
-		ktime_t next = ktime_add(dev->next_event, tick_period);
-
 		if (!clockevents_program_event(dev, next, ktime_get()))
 			return;
 		tick_periodic(cpu);
+		next = ktime_add(next, tick_period);
 	}
 }

-
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