linux-kernel - Re: [ANNOUNCE] 3.0.1-rt11

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Date:	Tue, 6 Sep 2011 20:00:01 -0700
From:	Frank Rowand <frank.rowand@...sony.com>
To:	linux-rt-users <linux-rt-users@...r.kernel.org>
CC:	"Rowand, Frank" <Frank_Rowand@...yusa.com>,
	"paulmck@...ux.vnet.ibm.com" <paulmck@...ux.vnet.ibm.com>,
	Peter Zijlstra <peterz@...radead.org>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Mike Galbraith <efault@....de>, Ingo Molnar <mingo@...e.hu>,
	Venkatesh Pallipadi <venki@...gle.com>
Subject: Re: [ANNOUNCE] 3.0.1-rt11

On 09/06/11 19:53, Frank Rowand wrote:
> On 08/26/11 16:55, Paul E. McKenney wrote:
>> On Wed, Aug 24, 2011 at 04:58:49PM -0700, Frank Rowand wrote:
>>> On 08/13/11 03:53, Peter Zijlstra wrote:
>>>>
>>>> Whee, I can skip release announcements too!
>>>>
>>>> So no the subject ain't no mistake its not, 3.0.1-rt11 is there for the
>>>> grabs.
> 
> < snip >
> 
>>> I have a consistent (every boot) hang on boot.  With a few
>>> hacks to get console output, I get the
>>>
>>>   rcu_preempt_state detected stalls on CPUs/tasks
> 
> < snip >
> 
>>> This is an ARM NaviEngine (out of tree, so I also have applied
>>> a series of pages for platform support).
>>>
>>> CONFIG_PREEMPT_RT_FULL is set.  Full config is attached.
> 
> I have also replicated the problem on the ARM RealView (in tree) and
> without the RT patches.
> 
>>
>> Hmmm...  The last few that I have seen that looked like this were
>> due to my messing up rcutorture so that the RCU-boost testing kthreads
>> ran CPU-bound at real-time priority.
>>
>> Is it possible that something similar is happening on your system?
>>
>>                                                         Thanx, Paul
> 
> The problem ended up being caused by the allowed cpus mask being set
> to all possible cpus for the ksoftirqd on the secondary processors.
> So the RCU softirq was never executing on cpu 2.
> 
> I'll test the following patch on 3.1 tomorrow.


And the following patch is some clean up for code that is in the
RT patch.  I do not know if this is needed, but after making the
changes in my first patch it seemed reasonable to add some extra
checks here, just in case softirq_check_pending_idle() gets
called in the window before the variable ksoftirqd gets set.

Signed-off-by: Frank Rowand <frank.rowand@...sony.com>
---
 kernel/softirq.c |   25 	14 +	11 -	0 !
 1 file changed, 14 insertions(+), 11 deletions(-)

Index: b/kernel/softirq.c
===================================================================
--- a/kernel/softirq.c
+++ b/kernel/softirq.c
@@ -87,18 +87,21 @@ void softirq_check_pending_idle(void)
 		struct task_struct *tsk;
 
 		tsk = __get_cpu_var(ksoftirqd);
-		/*
-		 * The wakeup code in rtmutex.c wakes up the task
-		 * _before_ it sets pi_blocked_on to NULL under
-		 * tsk->pi_lock. So we need to check for both: state
-		 * and pi_blocked_on.
-		 */
-		raw_spin_lock(&tsk->pi_lock);
+		if (tsk) {
+			/*
+			 * The wakeup code in rtmutex.c wakes up the task
+			 * _before_ it sets pi_blocked_on to NULL under
+			 * tsk->pi_lock. So we need to check for both: state
+			 * and pi_blocked_on.
+			 */
+			raw_spin_lock(&tsk->pi_lock);
+
+			if (!tsk->pi_blocked_on &&
+			    !(tsk->state == TASK_RUNNING))
+				warnpending = 1;
 
-		if (!tsk->pi_blocked_on && !(tsk->state == TASK_RUNNING))
-			warnpending = 1;
-
-		raw_spin_unlock(&tsk->pi_lock);
+			raw_spin_unlock(&tsk->pi_lock);
+		}
 	}
 
 	if (warnpending) {

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