Re: [Xenomai-core] [PULL] native: Fix msendq fastlock leakage

[Jan Kiszka <jan.kiszka@domain.hid>]

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On 2011-06-23 13:11, Gilles Chanteperdrix wrote:
> 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.

At least native tasks (but I bet that's true for al skins) aren't part
of the XNSHADOW_CLIENT_DETACH cleanup procedure.

> 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?

Will have a look.

Thanks,
Jan


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