linux-kernel - Re: [trace events] WARNING: CPU: 0 PID: 91 at kernel/sched/core.c:7253 __might

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Message-ID: <20141008154838.GM10832@worktop.programming.kicks-ass.net>
Date:	Wed, 8 Oct 2014 17:48:38 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	Fengguang Wu <fengguang.wu@...el.com>,
	Jet Chen <jet.chen@...el.com>, Su Tao <tao.su@...el.com>,
	Yuanhan Liu <yuanhan.liu@...el.com>, LKP <lkp@...org>,
	linux-kernel@...r.kernel.org
Subject: Re: [trace events] WARNING: CPU: 0 PID: 91 at
 kernel/sched/core.c:7253 __might_sleep()

On Wed, Oct 08, 2014 at 11:08:25AM -0400, Steven Rostedt wrote:
> > Steve, wth is that thing supposed to do?
> > 
> > 	set_current_state(TASK_INTERRUPTIBLE);
> > 	while (!kthread_should_stop())
> > 		schedule();
> > 
> > That looks broken alright. It'll revert to a yield() 'spin' loop if it
> > ever gets a wakeup for anything other than the kthread_stop().
> > 
> > Did you mean to write something like:
> > 
> > 	set_current_state(TASK_INTERRUPTIBLE);
> > 	while (!kthread_should_stop()) {
> > 		schedule();
> > 		set_current_state(TASK_INTERRUPTIBLE);
> > 	}
> > 	__set_current_state(TASK_RUNNING);
> > 
> > ?
> 
> Wow, what a blast from the past. That code hasn't been touched since
> 2009!
> 
> Anyway, all that thread did was call test work on each cpu, and then
> waits to be killed. It should only get a single wake up and that should
> be from the kthread_stop() call. IOW, that loop should never be
> executed more than once.
> 
> What exactly is the bug here?

The bug is as explained, the loop is wrong and will revert to a yield
'spin' loop after a single wakeup.

The debugging that caught it is that you exit the loop without setting
TASK_RUNNING.
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