linux-kernel - RE: [PATCH 2/3] alarmtimer: Using the alarmtimer_get

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Date:	Fri, 2 Nov 2012 03:41:14 +0000
From:	"Liu, Chuansheng" <chuansheng.liu@...el.com>
To:	Thomas Gleixner <tglx@...utronix.de>
CC:	"john.stultz@...aro.org" <john.stultz@...aro.org>,
	"gregkh@...uxfoundation.org" <gregkh@...uxfoundation.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: RE: [PATCH 2/3] alarmtimer: Using the alarmtimer_get_rtcdev for all
 posix clock interface



> -----Original Message-----
> From: Thomas Gleixner [mailto:tglx@...utronix.de]
> Sent: Friday, November 02, 2012 6:56 AM
> To: Liu, Chuansheng
> Cc: john.stultz@...aro.org; gregkh@...uxfoundation.org;
> linux-kernel@...r.kernel.org
> Subject: Re: [PATCH 2/3] alarmtimer: Using the alarmtimer_get_rtcdev for all
> posix clock interface
> 
> On Thu, 1 Nov 2012, Chuansheng Liu wrote:
> >
> > Some posix clock interface directly use the variable rtcdev,
> > cleanup it here by alarmtimer_get_rtcdev().
> >
> > Signed-off-by: liu chuansheng <chuansheng.liu@...el.com>
> > ---
> >  kernel/time/alarmtimer.c |   13 ++++++-------
> >  1 files changed, 6 insertions(+), 7 deletions(-)
> >
> > diff --git a/kernel/time/alarmtimer.c b/kernel/time/alarmtimer.c
> > index 4fc17cb..5490fa8 100644
> > --- a/kernel/time/alarmtimer.c
> > +++ b/kernel/time/alarmtimer.c
> > @@ -86,11 +86,10 @@ static int alarmtimer_rtc_add_device(struct device
> *dev,
> >  		return -1;
> >
> >  	mutex_lock(&rtcdev_mutex);
> > -	if (!rtcdev) {
> > -		rtcdev = rtc;
> > -		/* hold a reference so it doesn't go away */
> > -		get_device(dev);
> > -	}
> > +	rtcdev = rtc;
> > +	/* hold a reference so it doesn't go away */
> > +	get_device(dev);
> > +
> 
> Brilliant.
> 
> rtcdev = NULL
> 
> CPU0				CPU 1
> 
> alarmtimer_rtc_add_device(A)
> 	if (rtcdev)		alarmtimer_rtc_add_device(B)
> 	     return -EBUSY;		if (rtcdev)
> 	mutex_lock();				return -EBUSY;
> 	rtcdev = A;			mutex_lock();
> 	mutex_unlock;
> 
> bla = alarmtimer_rtc_get_device()
> 
> So bla = A
>       				  	mutex_lock() returns
>       				  	rtcdev = B;
> 					mutex_unlock();
> 
> The next call to alarmtimer_rtc_get_device() will return B. Not what
> you want. Maybe you want that, but then your patch is missing an
> explanation why you want that and why this would be a desired
> behaviour.
Thanks your pointing out, I am wrong.
> 
> Thanks,
> 
> 	tglx
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