linux-kernel - RE: [PATCH 1/2] genirq: Fix the possible synchronize

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Date:	Fri, 21 Feb 2014 12:52:56 +0100 (CET)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	"Liu, Chuansheng" <chuansheng.liu@...el.com>
cc:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"Wang, Xiaoming" <xiaoming.wang@...el.com>
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!

I'm inclined to remove the waitqueue_active() alltogether. It's
creating more headache than it's worth.

Thanks,

	tglx
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