Re: [PATCH] alarmtimer: add .remove_dev hook to put device

From: John Stultz
Date: Wed Jul 18 2012 - 19:23:45 EST

On 07/18/2012 04:16 PM, John Stultz wrote:
On 07/04/2012 09:13 AM, Shawn Guo wrote:
The following is a test sequence that installs a rtc module, remove it
and installs it again.

$ insmod rtc-snvs.ko
snvs_rtc 20cc034.snvs-rtc-lp: rtc core: registered 20cc034.snvs-rtc-lp as rtc0
$ hwclock
Thu Jul 5 08:53:35 2012 0.000000 seconds
$ rmmod rtc-snvs.ko
$ insmod rtc-snvs.ko
snvs_rtc 20cc034.snvs-rtc-lp: rtc core: registered 20cc034.snvs-rtc-lp as rtc1
$ hwclock
hwclock: can't open '/dev/misc/rtc': No such file or directory

The device is registered as rtc0 for the first time insmod, while it
becomes rtc1 with the later insmod.

It's root caused by alarmtimer which never puts the device even when
the rtc is removed. The patch adds a .remove_dev hook to have device
properly put, so that above insmod/rmmod sequence can the rtc device
registered in a consistent behavior.

Signed-off-by: Shawn Guo <shawn.guo@xxxxxxxxxx>
kernel/time/alarmtimer.c | 12 ++++++++++++
1 files changed, 12 insertions(+), 0 deletions(-)

diff --git a/kernel/time/alarmtimer.c b/kernel/time/alarmtimer.c
index aa27d39..39e8773 100644
--- a/kernel/time/alarmtimer.c
+++ b/kernel/time/alarmtimer.c
@@ -96,6 +96,17 @@ static int alarmtimer_rtc_add_device(struct device *dev,
return 0;

+static void alarmtimer_rtc_remove_device(struct device *dev,
+ struct class_interface *class_intf)
+ unsigned long flags;
+ spin_lock_irqsave(&rtcdev_lock, flags);
+ rtcdev = NULL;
+ put_device(dev);
+ spin_unlock_irqrestore(&rtcdev_lock, flags);

Looking at this a little closer, you need to validate that the device being removed is the actual rtcdev before setting it to null.

Otherwise if there's two rtc drivers, and the 1st is the current rtcdev, you'll clear rtcdev if the second rtc driver is removed.

I'll go ahead and fix this up for you.

Actually, this change opens up a bunch of other races, as any caller of alarmtimer_get_rtcdev() could have the rtcdevice removed under it.

We'll need to have proper reference counting w/ get/put calls, probably also adding a alarmtimer_put_rtcdev() interface.

So for now I'm dropping this from my tree. Do you think you might be able to take another stab at this?

Sorry again for the delayed review here.


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