Re: preempt rcu bug on s390

[Heiko Carstens]

On Sat, Feb 09, 2008 at 06:07:11AM -0800, Paul E. McKenney wrote:
> On Sat, Feb 09, 2008 at 12:34:35PM +0100, Heiko Carstens wrote:
> > Using CONFIG_PREEMPT_RCU and CONFIG_NO_IDLE_HZ on s390 my system always
> > gets stuck when running with more than one cpu.
> > When booting with four cpus I get all four cpus caught withing cpu_idle
> > and not advancing anymore. However there is the init process which is
> > waitung for synchronize_rcu() to complete (lcrash output):
> > 
> > STACK TRACE FOR TASK: 0xf84d968 (swapper)
> > 
> >  STACK:
> >  0 schedule+842 [0x36c956]
> >  1 schedule_timeout+172 [0x36d0e4]
> >  2 wait_for_common+204 [0x36c398]
> >  3 synchronize_rcu+76 [0x567bc]
> >  4 netlink_change_ngroups+150 [0x2b4302]
> >  5 genl_register_mc_group+256 [0x2b6174]
> >  6 genl_init+188 [0x534e44]
> >  7 kernel_init+444 [0x518334]
> >  8 kernel_thread_starter+6 [0x192a6]
> > 
> > If I change the code so that timer ticks won't be disabled everything
> > runs fine. So my guess is that rcu_needs_cpu() doesn't do the right
> > thing for the rcu preemptible case.
> > 
> > Kernel version is git head of today.
> > 
> > Any ideas?
> 
> Does this tree have http://lkml.org/lkml/2008/1/29/208 applied?
> 
> If not, could you please check it out?

It's not applied, however it doesn't change anything. Also the patch
is tied to the dynticks implementation which is differently from
s390's nohz implementation.
I had to add the patch below so it would make at least some sense.
But it doesn't fix the problem.

---
 arch/s390/kernel/time.c |    2 ++
 include/linux/hardirq.h |    2 +-
 kernel/rcupreempt.c     |    2 +-
 3 files changed, 4 insertions(+), 2 deletions(-)

Index: linux-2.6/kernel/rcupreempt.c
===================================================================
--- linux-2.6.orig/kernel/rcupreempt.c
+++ linux-2.6/kernel/rcupreempt.c
@@ -413,7 +413,7 @@ static void __rcu_advance_callbacks(stru
 	}
 }
 
-#ifdef CONFIG_NO_HZ
+#if defined(CONFIG_NO_HZ) || defined(CONFIG_NO_IDLE_HZ)
 
 DEFINE_PER_CPU(long, dynticks_progress_counter) = 1;
 static DEFINE_PER_CPU(long, rcu_dyntick_snapshot);
Index: linux-2.6/arch/s390/kernel/time.c
===================================================================
--- linux-2.6.orig/arch/s390/kernel/time.c
+++ linux-2.6/arch/s390/kernel/time.c
@@ -200,6 +200,7 @@ static void stop_hz_timer(void)
 		if (timer >= jiffies_timer_cc)
 			todval = timer;
 	}
+	rcu_enter_nohz();
 	set_clock_comparator(todval);
 }
 
@@ -213,6 +214,7 @@ static void start_hz_timer(void)
 
 	if (!cpu_isset(smp_processor_id(), nohz_cpu_mask))
 		return;
+	rcu_exit_nohz();
 	account_ticks(get_clock());
 	set_clock_comparator(S390_lowcore.jiffy_timer + CPU_DEVIATION);
 	cpu_clear(smp_processor_id(), nohz_cpu_mask);
Index: linux-2.6/include/linux/hardirq.h
===================================================================
--- linux-2.6.orig/include/linux/hardirq.h
+++ linux-2.6/include/linux/hardirq.h
@@ -109,7 +109,7 @@ static inline void account_system_vtime(
 }
 #endif
 
-#if defined(CONFIG_PREEMPT_RCU) && defined(CONFIG_NO_HZ)
+#if defined(CONFIG_PREEMPT_RCU) && (defined(CONFIG_NO_HZ) || defined(CONFIG_NO_IDLE_HZ))
 extern void rcu_irq_enter(void);
 extern void rcu_irq_exit(void);
 #else
--
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