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Date: Fri, 21 Feb 2014 14:05:57 +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:
> > -----Original Message-----
> > From: Thomas Gleixner [mailto:tglx@linutronix.de]
> > Sent: Friday, February 21, 2014 7:53 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:
> > > > > > 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?
> > > Not be clear here:)
> > > The atomic_read has no barrier.
> > >
> > > Found commit 6cb2a21049b89 has one similar smp_mb() calling before
> > > waitqueue_active() on one X86 CPU.
> > 
> > Indeed, you are completely right. Great detective work!
> Thanks your encouraging.
> 
> > 
> > I'm inclined to remove the waitqueue_active() alltogether. It's
> > creating more headache than it's worth.
> If I am understanding well, removing the checking of waitqueue_active(),
> and call wakeup() directly which will check list with spinlock protection.

Correct.
 
> If so, I can prepare one patch for it:)

Appreciated.

	tglx
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