On 10/10/2012 12:25 PM, Jan Kiszka wrote: > On 2012-10-10 12:07, Gilles Chanteperdrix wrote: >> On 10/10/2012 12:04 PM, Jan Kiszka wrote: >>> On 2012-10-10 11:23, Gilles Chanteperdrix wrote: >>>> On 10/10/2012 11:01 AM, Jan Kiszka wrote: >>>>> On 2012-10-10 10:58, Gilles Chanteperdrix wrote: >>>>>> On 10/10/2012 10:10 AM, Jan Kiszka wrote: >>>>>>> On 2012-10-10 10:04, Gilles Chanteperdrix wrote: >>>>>>>> On 10/10/2012 09:56 AM, Jan Kiszka wrote: >>>>>>>>> On 2012-10-10 09:51, Gilles Chanteperdrix wrote: >>>>>>>>>> On 10/10/2012 09:38 AM, Thierry Bultel wrote: >>>>>>>>>>> Hi Gilles, >>>>>>>>>>> >>>>>>>>>>> Many thanks, >>>>>>>>>>> The first patch does not work, the second does. >>>>>>>>>>> I think the reason for 1st patch why is that in rtcan_virt, we have >>>>>>>>>>> >>>>>>>>>>> rtdm_lock_get_irqsave(&rtcan_recv_list_lock, lock_ctx); >>>>>>>>>>> rtdm_lock_get(&rtcan_socket_lock); >>>>>>>>>>> >>>>>>>>>>> ... >>>>>>>>>>> ---> rtcan_rcv(rx_dev, &skb); >>>>>>>>>>> .... >>>>>>>>>>> >>>>>>>>>>> rtdm_lock_put(&rtcan_socket_lock); >>>>>>>>>>> rtdm_lock_put_irqrestore(&rtcan_recv_list_lock, lock_ctx); >>>>>>>>>>> >>>>>>>>>>> and rtcan_rcv->rtcan_rcv_deliver->rtdm_sem_up(&sock->recv_sem); >>>>>>>>>>> >>>>>>>>>>> thus the same re-scheduling stuff with interrupts locked. >>>>>>>>>>> >>>>>>>>>>> Are you not not afraid of side effects with the second patch, >>>>>>>>>>> since you change the overall behaviour ? >>>>>>>>>>> Won't you prefer a only locally modified rtcan_virt ? >>>>>>>>>> >>>>>>>>>> We should ask Jan's opinion. In any case, if we adopt the second >>>>>>>>>> patch, >>>>>>>>>> we might want to try and reduce the overhead of xnpod_unlock_sched. >>>>>>>>>> >>>>>>>>> >>>>>>>>> We were signaling the semaphore while holding a spin lock? That's a >>>>>>>>> clear bug. Your patch is aligning rtcan to the pattern we are also >>>>>>>>> using >>>>>>>>> in RTnet. We just need to make sure (haven't looked at the full >>>>>>>>> context >>>>>>>>> yet) that sock remains valid even after dropping the lock(s). >>>>>>>> >>>>>>>> The second patch idea was to lock the scheduler while spinlocks are >>>>>>>> held, so that posting a semaphore while holding a spin lock is no >>>>>>>> longer >>>>>>>> a bug. >>>>>>> >>>>>>> Sounds a bit hacky, >>>>>> >>>>>> Well, that is what the linux kernel does. >>>>>> >>>>>> but I think we have this pattern >>>>>>> (RTDM_EXECUTE_ATOMICALLY) >>>>>> >>>>>> RTDM_EXECUTE_ATOMICALLY is a bit of a misnomer, if you do: >>>>>> RTDM_EXECUTE_ATOMICALLY(foo(); rtdm_sem_up(); bar()); >>>>>> foo() and bar() are not executed atomically if sem_up wakes up another >>>>>> thread. >>>>>> >>>>>> So, I do not see how RTDM_EXECUTE_ATOMICALLY solves the issue we are >>>>>> talking about. >>>>> >>>>> RTDM_EXECUTE_ATOMICALLY holds the nucleus lock across the encapsulated >>>>> code, executing it atomically as rescheduling is postponed until the end >>>>> of the block. >>>> >>>> Err... no. Absolutely not. >>> >>> Err... absolutely right. >>> >>> The good news is: we don't need to worry about such kind of locking. In >>> rtcan_raw_recvmsg, the socket is locked via the RTDM context as we are >>> in a handler. So it won't disappear when we drop the lock, and your >>> first patch is fine. >> >> Which one? The first one does not seem to work because the rtdm locks >> seem to be nested. The second one would probably need to find a way to >> reduce the overhead of xnpod_unlock_sched(). What can be done, however, >> is adding a call to xnpod_lock_sched()/xnpod_unlock_sched() in >> RTDM_EXECUTE_ATOMICALLY. > > Oh, I'm seeing the locking forest in rtcan now. I suppose rtcan wasn't > used much on SMP so far. That looks indeed unresolvable without a > semantical change to rtdm_lock/unlock. > > But then we really need something as light-weight as preempt_enable/disable. >
This is not as lightweight as it might be given that we pair a flag and a counter to achieve this (which saves one data reference in xnpod_schedule() though), but this is a start: http://git.xenomai.org/?p=xenomai-2.6.git;a=commit;h=aed4dfce9967e45ef7e8a8da4b6c90267ea81497 So, I'm setting __xnpod_lock_sched() and __xnpod_unlock_sched() in stone in the nucleus API to manipulate the sched locking counter from a context where the nucleus lock is already held, so that RTDM can rely on this for RTDM_EXECUTE_ATOMICALLY(). -- Philippe. _______________________________________________ Xenomai mailing list [email protected] http://www.xenomai.org/mailman/listinfo/xenomai
