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SubjectRe: [PATCH] fix unsafe operation in high resolution timer
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On Sun, Dec 26, 2010 at 9:12 PM, Yong Zhang <yong.zhang0@gmail.com> wrote:
> On Sat, Dec 25, 2010 at 10:19:11PM +0800, Hillf Danton wrote:
>> > But for a hrtimer which is not free in its callback, like a
>> > static defined one. the hrtimer could be referenced at the same
>> > time. So here you cann't just delete the two lines.
>>
>> After callback, as you agree, it is hard to determine in the current
>> implementation if the hrtimer is static defined, though another bit
>> could be added, say, in the flag word of hrtimer
>
> Yeah, that is an option, like a flag FREE_IN_CALLBACK/ONESHOT which
> indicate that. Or just let the callback return another value like
> HRTIMER_FREED. But as I said before I'm not sure what's the best way
> to fix that. And maybe there's more suitable method.
>
> BTW, is there any user who free the hrtimer in its callback?
>
>> , so cutting the two
>> lines off is deserved.
>> And more, who will take care of the NORESTART again after callback?
>
> It's not related to NORESTART, just HRTIMER_STATE_CALLBACK.
> hrtimer'strategy somehow depends on HRTIMER_STATE_CALLBACK.
> You can take a look at the caller of hrtimer_callback_running().
>

Then the callback is responsible for signaling correctly.
Hillf
---
--- a/include/linux/hrtimer.h 2010-11-01 19:54:12.000000000 +0800
+++ b/include/linux/hrtimer.h 2010-12-29 23:43:16.000000000 +0800
@@ -44,6 +44,7 @@ enum hrtimer_mode {
enum hrtimer_restart {
HRTIMER_NORESTART, /* Timer is not restarted */
HRTIMER_RESTART, /* Timer must be restarted */
+ HRTIMER_RECLAIM, /* Timer is recycled */
};

/*
--- a/kernel/hrtimer.c 2010-11-01 19:54:12.000000000 +0800
+++ b/kernel/hrtimer.c 2010-12-29 23:45:48.000000000 +0800
@@ -1225,9 +1225,13 @@ static void __run_hrtimer(struct hrtimer
raw_spin_unlock(&cpu_base->lock);
trace_hrtimer_expire_entry(timer, now);
restart = fn(timer);
+ if (restart != HRTIMER_RECLAIM)
trace_hrtimer_expire_exit(timer);
raw_spin_lock(&cpu_base->lock);

+ if (restart == HRTIMER_RECLAIM)
+ return;
+
/*
* Note: We clear the CALLBACK bit after enqueue_hrtimer and
* we do not reprogramm the event hardware. Happens either in

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