Re: [PATCH -rt 4/5] workqueue: hotplug fix

[Yong Zhang <yong.zhang0@gmail.com>]

On Wed, Oct 19, 2011 at 09:14:28AM +0200, Thomas Gleixner wrote:
> On Sun, 16 Oct 2011, Yong Zhang wrote:
> > --- a/kernel/workqueue.c
> > +++ b/kernel/workqueue.c
> > @@ -3195,30 +3195,34 @@ static int __devinit workqueue_cpu_up_callback(struct notifier_block *nfb,
> >  	}
> >  
> >  	/* some are called w/ irq disabled, don't disturb irq status */
> > -	spin_lock_irqsave(&gcwq->lock, flags);
> >  
> >  	switch (action) {
> >  	case CPU_UP_PREPARE:
> > +		spin_lock_irqsave(&gcwq->lock, flags);
> >  		BUG_ON(gcwq->first_idle);
> >  		gcwq->first_idle = new_worker;
> > +		spin_unlock_irqrestore(&gcwq->lock, flags);
> >  		break;
> >  
> >  	case CPU_UP_CANCELED:
> > +		spin_lock_irqsave(&gcwq->lock, flags);
> >  		destroy_worker(gcwq->first_idle);
> >  		gcwq->first_idle = NULL;
> > +		spin_unlock_irqrestore(&gcwq->lock, flags);
> >  		break;
> >  
> >  	case CPU_ONLINE:
> > +		spin_lock_irqsave(&gcwq->lock, flags);
> >  		spin_unlock_irq(&gcwq->lock);
> >  		kthread_bind(gcwq->first_idle->task, cpu);
> >  		spin_lock_irq(&gcwq->lock);
> >  		gcwq->flags |= GCWQ_MANAGE_WORKERS;
> >  		start_worker(gcwq->first_idle);
> >  		gcwq->first_idle = NULL;
> > +		spin_unlock_irqrestore(&gcwq->lock, flags);
> >  		break;
> >  	}
> >  
> > -	spin_unlock_irqrestore(&gcwq->lock, flags);
> 
> This part of the patch is pretty pointless.

But CPU_STARTING is called with irq disabled, and it will take
the lock unconditionally. Thus below warning is triggered:
BUG: sleeping function called from invalid context at kernel/rtmutex.c:645
in_atomic(): 1, irqs_disabled(): 1, pid: 0, name: kworker/0:0
Pid: 0, comm: kworker/0:0 Not tainted 3.0.6-rt17-00284-gb76d419 #3
Call Trace:
 [<c06e3b5d>] ? printk+0x1d/0x20
 [<c01390b6>] __might_sleep+0xe6/0x110
 [<c06e633c>] rt_spin_lock+0x1c/0x30
 [<c06cd85b>] workqueue_cpu_up_callback+0x56/0xf3
 [<c06e9d65>] notifier_call_chain+0x45/0x60
 [<c0171cfe>] __raw_notifier_call_chain+0x1e/0x30
 [<c014c9b4>] __cpu_notify+0x24/0x40
 [<c014c9ec>] cpu_notify+0x1c/0x20
 [<c06e1d43>] notify_cpu_starting+0x1e/0x20
 [<c06e0aad>] smp_callin+0xfb/0x10e
 [<c06e0ad9>] start_secondary+0x19/0xd7

Thanks,
Yong