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. _______________________________________________ Xenomai-core mailing list Xenomai-core@gna.org https://mail.gna.org/listinfo/xenomai-core