On 06/23/2011 11:37 AM, Jan Kiszka wrote:
> On 2011-06-20 19:07, Jan Kiszka wrote:
>> On 2011-06-19 15:00, Gilles Chanteperdrix wrote:
>>> On 06/19/2011 01:17 PM, Gilles Chanteperdrix wrote:
>>>> On 06/19/2011 12:14 PM, Gilles Chanteperdrix wrote:
>>>>> I am working on this ppd cleanup issue again, I am asking for help to
>>>>> find a fix in -head for all cases where the sys_ppd is needed during
>>>>> some cleanup.
>>>>>
>>>>> The problem is that when the ppd cleanup is invoked:
>>>>> - we have no guarantee that current is a thread from the Xenomai
>>>>> application;
>>>>> - if it is, current->mm is NULL.
>>>>>
>>>>> So, associating the sys_ppd to either current or current->mm does not
>>>>> work. What we could do is pass the sys_ppd to all the other ppds cleanup
>>>>> handlers, this would fix cases such as freeing mutexes fastlock, but
>>>>> that does not help when the sys_ppd is needed during a thread deletion
>>>>> hook.
>>>>>
>>>>> I would like to find a solution where simply calling xnsys_ppd_get()
>>>>> will work, where we do not have an xnsys_ppd_get for each context, such
>>>>> as for instance xnsys_ppd_get_by_mm/xnsys_ppd_get_by_task_struct,
>>>>> because it would be too error-prone.
>>>>>
>>>>> Any idea anyone?
>>>>
>>>> The best I could come up with: use a ptd to store the mm currently
>>>> being cleaned up, so that xnshadow_ppd_get continues to work, even
>>>> in the middle of a cleanup.
>>>
>>> In order to also get xnshadow_ppd_get to work in task deletion hooks
>>> (which is needed to avoid the issue at the origin of this thread), we
>>> also need to set this ptd upon shadow mapping, so it is still there
>>> when reaching the task deletion hook (where current->mm may be NULL).
>>> Hence the patch:
>>>
>>> diff --git a/ksrc/nucleus/shadow.c b/ksrc/nucleus/shadow.c
>>> index b243600..6bc4210 100644
>>> --- a/ksrc/nucleus/shadow.c
>>> +++ b/ksrc/nucleus/shadow.c
>>> @@ -65,6 +65,11 @@ int nkthrptd;
>>> EXPORT_SYMBOL_GPL(nkthrptd);
>>> int nkerrptd;
>>> EXPORT_SYMBOL_GPL(nkerrptd);
>>> +int nkmmptd;
>>> +EXPORT_SYMBOL_GPL(nkmmptd);
>>> +
>>> +#define xnshadow_mmptd(t) ((t)->ptd[nkmmptd])
>>> +#define xnshadow_mm(t) ((struct mm_struct *)xnshadow_mmptd(t))
>>
>> xnshadow_mm() can now return a no longer existing mm. So no user of
>> xnshadow_mm should ever dereference that pointer. Thus we better change
>> all that user to treat the return value as a void pointer e.g.
>>
>>>
>>> struct xnskin_slot {
>>> struct xnskin_props *props;
>>> @@ -1304,6 +1309,8 @@ int xnshadow_map(xnthread_t *thread, xncompletion_t
>>> __user *u_completion,
>>> * friends.
>>> */
>>> xnshadow_thrptd(current) = thread;
>>> + xnshadow_mmptd(current) = current->mm;
>>> +
>>> rthal_enable_notifier(current);
>>>
>>> if (xnthread_base_priority(thread) == 0 &&
>>> @@ -2759,7 +2766,15 @@ static void detach_ppd(xnshadow_ppd_t * ppd)
>>>
>>> static inline void do_cleanup_event(struct mm_struct *mm)
>>> {
>>> + struct task_struct *p = current;
>>> + struct mm_struct *old;
>>> +
>>> + old = xnshadow_mm(p);
>>> + xnshadow_mmptd(p) = mm;
>>> +
>>> ppd_remove_mm(mm, &detach_ppd);
>>> +
>>> + xnshadow_mmptd(p) = old;
>>
>> I don't have the full picture yet, but that feels racy: If the context
>> over which we clean up that foreign mm is also using xnshadow_mmptd,
>> other threads in that process may dislike this temporary change.
>>
>>> }
>>>
>>> RTHAL_DECLARE_CLEANUP_EVENT(cleanup_event);
>>> @@ -2925,7 +2940,7 @@ EXPORT_SYMBOL_GPL(xnshadow_unregister_interface);
>>> xnshadow_ppd_t *xnshadow_ppd_get(unsigned muxid)
>>> {
>>> if (xnpod_userspace_p())
>>> - return ppd_lookup(muxid, current->mm);
>>> + return ppd_lookup(muxid, xnshadow_mm(current) ?: current->mm);
>>>
>>> return NULL;
>>> }
>>> @@ -2960,8 +2975,9 @@ int xnshadow_mount(void)
>>> sema_init(&completion_mutex, 1);
>>> nkthrptd = rthal_alloc_ptdkey();
>>> nkerrptd = rthal_alloc_ptdkey();
>>> + nkmmptd = rthal_alloc_ptdkey();
>>>
>>> - if (nkthrptd < 0 || nkerrptd < 0) {
>>> + if (nkthrptd < 0 || nkerrptd < 0 || nkmmptd < 0) {
>>> printk(KERN_ERR "Xenomai: cannot allocate PTD slots\n");
>>> return -ENOMEM;
>>> }
>>> diff --git a/ksrc/skins/posix/mutex.c b/ksrc/skins/posix/mutex.c
>>> index 6ce75e5..cc86852 100644
>>> --- a/ksrc/skins/posix/mutex.c
>>> +++ b/ksrc/skins/posix/mutex.c
>>> @@ -219,10 +219,6 @@ void pse51_mutex_destroy_internal(pse51_mutex_t *mutex,
>>> xnlock_put_irqrestore(&nklock, s);
>>>
>>> #ifdef CONFIG_XENO_FASTSYNCH
>>> - /* We call xnheap_free even if the mutex is not pshared; when
>>> - this function is called from pse51_mutexq_cleanup, the
>>> - sem_heap is destroyed, or not the one to which the fastlock
>>> - belongs, xnheap will simply return an error. */
>>
>> I think this comment is not completely obsolete. It still applies /wrt
>> shared/non-shared.
>>
>>> xnheap_free(&xnsys_ppd_get(mutex->attr.pshared)->sem_heap,
>>> mutex->synchbase.fastlock);
>>> #endif /* CONFIG_XENO_FASTSYNCH */
>>>
>>>
>>
>> If we can resolve that potential race, this looks like a nice solution.
>
> We still have to address that ordering issue I almost forgot:
> do_cleanup_event runs before do_task_exit_event when terminating the
> last task. The former destroys the sem heap, the latter fires the delete
> hook which then tries to free msendq.fastlock to an invalid heap.
>
> Should be fixable by setting sem_heap NULL in the ppd on destroy and
> skipping the fastlock release in __task_delete_hook if the heap pointer
> is found like that.
I do not think this can be a problem, as the do_cleanup_event will also
destroy the threads. Anyway, I just pushed a branch "u_mode" on the
xenomai-gch git with all the work based on this mmptd, could you try and
pull it to see if you sill have this cleanup/task_exit issue?
>
> Jan
>
--
Gilles.
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