linux-kernel - Re: [PATCH 1/6 v2] sched: Init idle->on_rq in init

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Message-ID: <20140211155115.GR27965@twins.programming.kicks-ass.net>
Date:	Tue, 11 Feb 2014 16:51:15 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Thomas Gleixner <tglx@...utronix.de>
Cc:	Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/6 v2] sched: Init idle->on_rq in init_idle()

On Tue, Feb 11, 2014 at 04:34:38PM +0100, Thomas Gleixner wrote:
> I could slap myself for not writing a proper changelog right away. It
> took me some time to figure out why it was added in the first place,
> why it's not longer necessary and why I kept it.

:-)

Thanks!

---
Subject: sched: Init idle->on_rq in init_idle()
From: Thomas Gleixner <tglx@...utronix.de>
Date: Fri, 7 Feb 2014 20:58:37 +0100

We stumbled in RT over a SMP bringup issue on ARM where the
idle->on_rq == 0 was causing try_to_wakeup() on the other cpu to run
into nada land.

After adding that idle->on_rq = 1; I was able to find the root cause
of the lockup: the idle task on the newly woken up cpu was fiddling
with a sleeping spinlock, which is a nono.

I kept the init of idle->on_rq to keep the state consistent and to
avoid another long lasting debug session.

As a side note, the whole debug mess could have been avoided if
might_sleep() would have yelled when called from the idle task. That's
fixed with patch 2/6 - and that one actually has a changelog :)

Cc: Ingo Molnar <mingo@...hat.com>
Signed-off-by: Thomas Gleixner <tglx@...utronix.de>
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Signed-off-by: Peter Zijlstra <peterz@...radead.org>
Link: http://lkml.kernel.org/r/1391803122-4425-2-git-send-email-bigeasy@linutronix.de
---
 kernel/sched/core.c |    1 +
 1 file changed, 1 insertion(+)

--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -4443,6 +4443,7 @@ void init_idle(struct task_struct *idle,
 	rcu_read_unlock();
 
 	rq->curr = rq->idle = idle;
+	idle->on_rq = 1;
 #if defined(CONFIG_SMP)
 	idle->on_cpu = 1;
 #endif
--
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