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

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Date:	Fri, 21 Feb 2014 11:01:34 +0000
From:	"Liu, Chuansheng" <chuansheng.liu@...el.com>
To:	Thomas Gleixner <tglx@...utronix.de>
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

Hello Thomas,

> -----Original Message-----
> From: Thomas Gleixner [mailto:tglx@...utronix.de]
> Sent: Friday, February 21, 2014 6:34 PM
> To: Liu, Chuansheng
> Cc: linux-kernel@...r.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

Here after inserting into the queue, before waitqueue_active,
there is no mb.

So is it still the case? Thanks.


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