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Date: Fri, 21 Feb 2014 12:10:35 +0100 (CET)
From: Thomas Gleixner <tglx@linutronix.de>
To: "Liu, Chuansheng" <chuansheng.liu@intel.com>
cc: "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "Wang, Xiaoming" <xiaoming.wang@intel.com>
Subject: RE: [PATCH 1/2] genirq: Fix the possible synchronize_irq()
 wait-forever
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On Fri, 21 Feb 2014, Liu, Chuansheng wrote:
> Hello Thomas,
> 
> > -----Original Message-----
> > From: Thomas Gleixner [mailto:tglx@linutronix.de]
> > Sent: Friday, February 21, 2014 6:34 PM
> > To: Liu, Chuansheng
> > Cc: linux-kernel@vger.kernel.org; Wang, Xiaoming
> > Subject: RE: [PATCH 1/2] genirq: Fix the possible synchronize_irq() wait-forever
> > 
> > On Fri, 21 Feb 2014, Liu, Chuansheng wrote:
> > > But feels there is another case which the synchronize_irq waited there
> > forever,
> > > it is no waking up action from irq_thread().
> > >
> > > CPU0                                  CPU1
> > > disable_irq()                         irq_thread()
> > >   synchronize_irq()
> > >     wait_event()
> > >      adding the __wait into the queue  wake_threads_waitq
> > >        test threads_active==0
> > > 					 atomic_dec_and_test(threads_active) 1 -- > 0
> > >
> > waitqueue_active(&desc->wait_for_threads)
> > >                                   <== Here without smp_mb(), CPU1
> > maybe detect
> > >                                       the queue is still empty??
> > >      schedule()
> > >
> > > It will cause although the threads_active is 0, but irq_thread() didn't do the
> > waking up action.
> > > Is it reasonable? Then maybe we can add one smp_mb() before
> > waitqueue_active.
> > 
> > I think you have a point there, but not on x86 wherre the atomic_dec
> > and the spinlock on the queueing side are full barriers. For non-x86
> > there is definitely a potential issue.
> > 
> But even on X86, spin_unlock has no full barrier, the following scenario:
> CPU0                   CPU1
> spin_lock               
>                        atomic_dec_and_test 
> insert into queue         
> spin_unlock
>                        checking waitqueue_active

But CPU0 sees the 0, right?

Thanks,

	tglx
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