On 2011-07-11 21:51, Gilles Chanteperdrix wrote:
> On 07/11/2011 09:16 PM, Jan Kiszka wrote:
>> On 2011-07-11 21:10, Jan Kiszka wrote:
>>> On 2011-07-11 20:53, Gilles Chanteperdrix wrote:
>>>> On 07/08/2011 06:29 PM, GIT version control wrote:
>>>>> @@ -2528,6 +2534,22 @@ static inline void do_taskexit_event(struct 
>>>>> task_struct *p)
>>>>>   magic = xnthread_get_magic(thread);
>>>>>   xnlock_get_irqsave(&nklock, s);
>>>>> +
>>>>> + gksched = thread->gksched;
>>>>> + if (gksched) {
>>>>> +         xnlock_put_irqrestore(&nklock, s);
>>>> Are we sure irqs are on here? Are you sure that what is needed is not an
>>>> xnlock_clear_irqon?
>>> We are in the context of do_exit. Not only IRQs are on, also preemption.
>>> And surely no nklock is held.
>>>> Furthermore, I do not understand how we
>>>> "synchronize" with the gatekeeper, how is the gatekeeper garanteed to
>>>> wait for this assignment?
>>> The gatekeeper holds the gksync token while it's active. We request it,
>>> thus we wait for the gatekeeper to become idle again. While it is idle,
>>> we reset the queued reference - but I just realized that this may tramp
>>> on other tasks' values. I need to add a check that the value to be
>>> null'ified is actually still ours.
>> Thinking again, that's actually not a problem: gktarget is only needed
>> while gksync is zero - but then we won't get hold of it anyway and,
>> thus, can't cause any damage.
> Well, you make it look like it does not work. From what I understand,
> what you want is to set gktarget to null if a task being hardened is
> destroyed. But by waiting for the semaphore, you actually wait for the
> harden to be complete, so setting to NULL is useless. Or am I missing
> something else?

Setting to NULL is probably unneeded but still better than rely on the
gatekeeper never waking up spuriously and then dereferencing a stale

The key element of this fix is waitng on gksync, thus on the completion
of the non-RT part of the hardening. Actually, this part usually fails
as the target task received a termination signal at this point.


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