On Thu, 2011-06-23 at 19:32 +0200, Gilles Chanteperdrix wrote:
> On 06/23/2011 01:15 PM, Jan Kiszka wrote:
> > 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.
>
> Then it is indeed a problem, as we will have another ppd issue in the
> task deletion callbacks. I did not notice, because the posix skin does
> cleanup the threads, and I simply assumed that every skin did it.
>
> What is the reason for doing this? Are the thread deletion hooks
> supposed to work only when called from the context of the dying thread?
>
When a shadow thread self-deletes, yes. This is a pre-requisite for
xnshadow_unmap to work on the proper context.
--
Philippe.
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