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Date: Fri, 21 Feb 2014 11:33:52 +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:
> 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.

Thanks,

	tglx



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