Subject: Re: [PATCH] powerpc: irq work racing with timer interrupt can result in timer interrupt hang

From: Benjamin Herrenschmidt <[email-masked]>
Date: Sat, 10 May 2014 14:26:33 +1000

On Fri, 2014-05-09 at 15:22 +0530, Preeti U Murthy wrote:
> in __timer_interrupt() outside the _else_ loop? This will ensure that no
> matter what, before exiting timer interrupt handler we check for pending
> irq work.

We still need to make sure that set_next_event() doesn't move the
dec beyond the next tick if there is a pending timer... maybe we
can fix it like this:

static int decrementer_set_next_event(unsigned long evt,
				      struct clock_event_device *dev)
{
	__get_cpu_var(decrementers_next_tb) = get_tb_or_rtc() + evt;

	/* Don't adjust the decrementer if some irq work is pending */
	if (!test_irq_work_pending())
		set_dec(evt);

	return 0;
}

Along with a single occurrence of:

	if (test_irq_work_pending())
		set_dec(1);

At the end of __timer_interrupt(), outside if the current else {}
case, this should work, don't you think ?

What about this completely untested patch ?

diff --git a/arch/powerpc/kernel/time.c b/arch/powerpc/kernel/time.c
index 122a580..ba7e83b 100644
--- a/arch/powerpc/kernel/time.c
+++ b/arch/powerpc/kernel/time.c
@@ -503,12 +503,13 @@ void __timer_interrupt(void)
                now = *next_tb - now;
                if (now <= DECREMENTER_MAX)
                        set_dec((int)now);
-               /* We may have raced with new irq work */
-               if (test_irq_work_pending())
-                       set_dec(1);
                __get_cpu_var(irq_stat).timer_irqs_others++;
        }
 
+       /* We may have raced with new irq work */
+       if (test_irq_work_pending())
+               set_dec(1);
+
 #ifdef CONFIG_PPC64
        /* collect purr register values often, for accurate calculations */
        if (firmware_has_feature(FW_FEATURE_SPLPAR)) {
@@ -813,15 +814,11 @@ static void __init clocksource_init(void)
 static int decrementer_set_next_event(unsigned long evt,
                                      struct clock_event_device *dev)
 {
-       /* Don't adjust the decrementer if some irq work is pending */
-       if (test_irq_work_pending())
-               return 0;
        __get_cpu_var(decrementers_next_tb) = get_tb_or_rtc() + evt;
-       set_dec(evt);
 
-       /* We may have raced with new irq work */
-       if (test_irq_work_pending())
-               set_dec(1);
+       /* Don't adjust the decrementer if some irq work is pending */
+       if (!test_irq_work_pending())
+               set_dec(evt);
 
        return 0;
 }