On 10/10/2012 03:18 PM, Philippe Gerum wrote:
> On 10/10/2012 03:16 PM, Gilles Chanteperdrix wrote:
>> On 10/10/2012 03:09 PM, Philippe Gerum wrote:
>>> On 10/10/2012 03:01 PM, Gilles Chanteperdrix wrote:
>>>> On 10/10/2012 02:57 PM, Philippe Gerum wrote:
>>>>> On 10/10/2012 02:55 PM, Gilles Chanteperdrix wrote:
>>>>>> On 10/10/2012 02:41 PM, Philippe Gerum wrote:
>>>>>>> On 10/10/2012 02:33 PM, Gilles Chanteperdrix wrote:
>>>>>>>> On 10/10/2012 02:09 PM, Philippe Gerum wrote:
>>>>>>>>> On 10/10/2012 01:30 PM, Gilles Chanteperdrix wrote:
>>>>>>>>>> On 10/10/2012 12:54 PM, Philippe Gerum wrote:
>>>>>>>>>>
>>>>>>>>>>> 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().
>>>>>>>>>>
>>>>>>>>>>
>>>>>>>>>> The problem of xnpod_unlock_sched from my point of view is this
>>>>>>>>>> section
>>>>>>>>>> of code:
>>>>>>>>>>
>>>>>>>>>> xnsched_set_self_resched(curr->sched);
>>>>>>>>>> xnpod_schedule();
>>>>>>>>>>
>>>>>>>>>> It means that we will go to the full blown __xnpod_schedule when
>>>>>>>>>> unlocking the top-most spinlock.
>>>>>>>>>>
>>>>>>>>>> I guess what I meant is that we should have a scheduler bit that is
>>>>>>>>>> simply tested in xnpod_schedule, but then we loose the ability for
>>>>>>>>>> the
>>>>>>>>>> threads with the scheduler locked to suspend themselves. So, maybe we
>>>>>>>>>> should define another xnpod_lock/unlock pair, maybe something like
>>>>>>>>>> xnpod_preempt_disablle()/xnpod_preempt_enabled().
>>>>>>>>>>
>>>>>>>>>>
>>>>>>>>>
>>>>>>>>> xnpod_lock_sched() really means xnpod_preempt_disable() in RTOS
>>>>>>>>> parlance.
>>>>>>>>
>>>>>>>> Yes, but the reason why we have to go to __xnpod_schedule in
>>>>>>>> xnpod_unlock_sched() is to allow thread to be suspended with the
>>>>>>>> scheduler locked (you added that at some point for the vxworks
>>>>>>>> emulator,
>>>>>>>> if I remember correctly). But without that need, we can put the XNLOCK
>>>>>>>> bit in the sched->status structure, and simply test that bit with all
>>>>>>>> the others in xnpod_schedule(), and forget about the need to call
>>>>>>>> xnsched_set_self_resched() in xnpod_unlock_sched(). That is all I
>>>>>>>> meant.
>>>>>>>>
>>>>>>>
>>>>>>> You may have to reschedule after an IRQ has interrupted the lock owner
>>>>>>> inside the locked section, in which case you have to make sure to
>>>>>>> reschedule when dropping the sched lock.
>>>>>>
>>>>>> Well, in that case the resched bit will have been set already by the irq
>>>>>> handler calling xnpod_resume_thread, or other service, you do not have
>>>>>> to force it.
>>>>>>
>>>>>
>>>>> Yes, and this is why you have to call __xnpod_schedule() regardless.
>>>>>
>>>> no, xnpod_schedule is enough, it will only call __xnpod_schedule() if
>>>> the resched bit is set.
>>>>
>>>
>>> We are not discussing about the same issue, I'm afraid. We can't
>>> optimize the schedlock path via a scheduler flag since we have to care
>>> about lock nesting. Since the only sane option there is to hold such
>>> counter in the current thread context, the optimization is 100% in the
>>> way we access this information.
>>>
>>
>> Again, if we do not have to allow threads to suspend while holding the
>> scheduler lock, the preempt_count also can be in the sched structure.
>>
>
> Sure, but this point is moot. We just _can't_ do that.
>
Except if we invent another scheduler lock service, simply for the
purpose of spinlocks where suspending the spinlock holder does not make
sense anyway.
--
Gilles.
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