linux-kernel - RE: [PATCH 1/3] alarmtimer: Replace the spinlock rtcdev

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Date:	Fri, 2 Nov 2012 03:32:26 +0000
From:	"Liu, Chuansheng" <chuansheng.liu@...el.com>
To:	Thomas Gleixner <tglx@...utronix.de>
CC:	Oliver Neukum <oneukum@...e.de>,
	"john.stultz@...aro.org" <john.stultz@...aro.org>,
	"gregkh@...uxfoundation.org" <gregkh@...uxfoundation.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"Liu, Chuansheng" <chuansheng.liu@...el.com>
Subject: RE: [PATCH 1/3] alarmtimer: Replace the spinlock rtcdev_lock with
 mutex

> It's safe to do in any code path which wants to use rtcdev:
> 
>      if (!rtcdev)
>      	return -ENODEV:
>      do_something(rtcdev);
> 
Really thanks your analysis and sharing, learn something.
And preparing a new patch with your comments for this point.

Subject: [PATCH] alarmtimer: Removing lock at alarmtimer_get_rtcdev()

[From Thomas comments]
"Protecting" the dereferencing of rtcdev with any kind of lock does
not provide any useful protection at all.

Moreover, alarmtimer_rtc_add_device just set the rtc device ONCE,
and the device will never go away and is never going to change, it's
completely pointless to have a lock in alarmtimer_get_rtcdev() at all.

So this patch remove the lock/unlock actions in alarmtimer_get_rtcdev().

Signed-off-by: liu chuansheng <chuansheng.liu@...el.com>
---
 kernel/time/alarmtimer.c |   21 +++++++--------------
 1 files changed, 7 insertions(+), 14 deletions(-)

diff --git a/kernel/time/alarmtimer.c b/kernel/time/alarmtimer.c
index f11d83b..eed3b26 100644
--- a/kernel/time/alarmtimer.c
+++ b/kernel/time/alarmtimer.c
@@ -62,14 +62,7 @@ static DEFINE_SPINLOCK(rtcdev_lock);
  */
 struct rtc_device *alarmtimer_get_rtcdev(void)
 {
-       unsigned long flags;
-       struct rtc_device *ret;
-
-       spin_lock_irqsave(&rtcdev_lock, flags);
-       ret = rtcdev;
-       spin_unlock_irqrestore(&rtcdev_lock, flags);
-
-       return ret;
+       return rtcdev;
 }
 
 
@@ -224,10 +217,10 @@ static int alarmtimer_suspend(struct device *dev)
        freezer_delta = ktime_set(0, 0);
        spin_unlock_irqrestore(&freezer_delta_lock, flags);
 
-       rtc = alarmtimer_get_rtcdev();
        /* If we have no rtcdev, just return */
-       if (!rtc)
+       if (!rtcdev)
                return 0;
+       rtc = rtcdev;
 
        /* Find the soonest timer to expire*/
        for (i = 0; i < ALARM_NUMTYPE; i++) {
@@ -444,7 +437,7 @@ static int alarm_clock_getres(const clockid_t which_clock, struct timespec *tp)
 {
        clockid_t baseid = alarm_bases[clock2alarm(which_clock)].base_clockid;
 
-       if (!alarmtimer_get_rtcdev())
+       if (!rtcdev)
                return -ENOTSUPP;
 
        return hrtimer_get_res(baseid, tp);
@@ -461,7 +454,7 @@ static int alarm_clock_get(clockid_t which_clock, struct timespec *tp)
 {
        struct alarm_base *base = &alarm_bases[clock2alarm(which_clock)];
 
-       if (!alarmtimer_get_rtcdev())
+       if (!rtcdev)
                return -ENOTSUPP;
 
        *tp = ktime_to_timespec(base->gettime());
@@ -479,7 +472,7 @@ static int alarm_timer_create(struct k_itimer *new_timer)
        enum  alarmtimer_type type;
        struct alarm_base *base;
 
-       if (!alarmtimer_get_rtcdev())
+       if (!rtcdev)
                return -ENOTSUPP;
 
        if (!capable(CAP_WAKE_ALARM))
@@ -682,7 +675,7 @@ static int alarm_timer_nsleep(const clockid_t which_clock, int flags,
        int ret = 0;
        struct restart_block *restart;
 
-       if (!alarmtimer_get_rtcdev())
+       if (!rtcdev)
                return -ENOTSUPP;
 
        if (!capable(CAP_WAKE_ALARM))
-- 
1.7.0.4
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