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

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Date:	Fri, 2 Nov 2012 06:10:45 +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

> 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.
> 

Based on current implementation alarmtimer_rtc_add_device() will just
be called once thru class interface, also &parent->p->mutex is there to protect
the class_intf->add_dev() calling, so we can remove rtcdev_lock.
Is it making sense?


Subject: [PATCH] alarmtimer: Removing the usage of lock rtcdev_lock

alarmtimer_rtc_add_device() will be called once at the time
alarmtimer_init() is called.

alarmtimer_init -->
        alarmtimer_rtc_interface_setup -->
                class_interface_register -->
                        mutex_lock(&parent->p->mutex)
                        ...
                        class_intf->add_dev()/alarmtimer_rtc_add_device
                        ...
                        mutex_unlock(&parent->p->mutex);

But also the mutex parent->p->mutex has protected the add_dev()
callback.

So here removing the lock of rtcdev_lock.

Signed-off-by: liu chuansheng <chuansheng.liu@...el.com>
---
 kernel/time/alarmtimer.c |   13 ++++---------
 1 files changed, 4 insertions(+), 9 deletions(-)

diff --git a/kernel/time/alarmtimer.c b/kernel/time/alarmtimer.c
index eed3b26..660091c 100644
--- a/kernel/time/alarmtimer.c
+++ b/kernel/time/alarmtimer.c
@@ -51,7 +51,6 @@ static struct wakeup_source *ws;
 /* rtc timer and device for setting alarm wakeups at suspend */
 static struct rtc_timer                rtctimer;
 static struct rtc_device       *rtcdev;
-static DEFINE_SPINLOCK(rtcdev_lock);
 
 /**
  * alarmtimer_get_rtcdev - Return selected rtcdevice
@@ -69,7 +68,6 @@ struct rtc_device *alarmtimer_get_rtcdev(void)
 static int alarmtimer_rtc_add_device(struct device *dev,
                                struct class_interface *class_intf)
 {
-       unsigned long flags;
        struct rtc_device *rtc = to_rtc_device(dev);
 
        if (rtcdev)
@@ -80,13 +78,10 @@ static int alarmtimer_rtc_add_device(struct device *dev,
        if (!device_may_wakeup(rtc->dev.parent))
                return -1;
 
-       spin_lock_irqsave(&rtcdev_lock, flags);
-       if (!rtcdev) {
-               rtcdev = rtc;
-               /* hold a reference so it doesn't go away */
-               get_device(dev);
-       }
-       spin_unlock_irqrestore(&rtcdev_lock, flags);
+       rtcdev = rtc;
+       /* hold a reference so it doesn't go away */
+       get_device(dev);
+
        return 0;
 }
 
-- 
1.7.0.4

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