[CXP] Discussing the RTDM specification

Jan Kiszka jan.kiszka at siemens.com
Mon Jan 11 14:18:24 CET 2021


On 11.01.21 14:14, Philippe Gerum wrote:
> 
> Jan Kiszka <jan.kiszka at siemens.com> writes:
> 
>> On 09.01.21 18:01, Philippe Gerum wrote:
>>>
>>> Jan Kiszka <jan.kiszka at siemens.com> writes:
>>>
>>>> On 23.12.20 11:40, Philippe Gerum wrote:
>>>>>
>>>>> Jan Kiszka <jan.kiszka at siemens.com> writes:
>>>>>
>>>>>> On 18.12.20 15:19, Philippe Gerum via Xenomai wrote:
>>>>>>>
>>>>>>> This wiki page [1] contains a draft proposal about specifying which
>>>>>>> services from the current RTDM interface should be part of the Common
>>>>>>> Xenomai Platform. Some proposals for deprecation stand out:
>>>>>>>
>>>>>>> - I suspect that only very few RTDM drivers are actually handling
>>>>>>>   requests from other kernel-based drivers in real world applications,
>>>>>>>   at least not enough to justify RTDM codifying these rare cases into a
>>>>>>>   common interface (rtdm_open, rtdm_read, rtdm_write etc).
>>>>>>>
>>>>>>>   In other words, although I would agree that a few particular drivers
>>>>>>>   might want to export a couple of services to kernel-based clients in
>>>>>>>   order to provide them some sort of backchannel, it seems wrong to
>>>>>>>   require RTDM drivers to provide a kernel interface which would match
>>>>>>>   their user interface in the same terms. For these specific cases, ad
>>>>>>>   hoc code in these few drivers should be enough.
>>>>>>>
>>>>>>>   Besides, I believe that most kernel->kernel request paths implemented
>>>>>>>   by in-tree RTDM drivers have never been tested for years, if ever.
>>>>>>>   Meanwhile, this kernel->kernel API introduces a basic exception case
>>>>>>>   into many RTDM and driver code paths, e.g. for differentiating kernel
>>>>>>>   vs user buffers, for only very few use cases.
>>>>>>>
>>>>>>>   For these reasons, I would suggest to deprecate the kernel->kernel API
>>>>>>>   from RTDM starting from 3.3, excluding it from the CXP in the same
>>>>>>>   move.
>>>>>>
>>>>>> That's fine with me. The idea was once that something like bus drivers
>>>>>> would appear, but that never happened.
>>>>>>
>>>>>>>
>>>>>>> - RTDM_EXECUTE_ATOMICALLY() and related calls relying on the Cobalt big
>>>>>>>   lock must go. For SMP scalability reasons, this big lock was
>>>>>>>   eliminated from the EVL core, which means that all the attached
>>>>>>>   semantics will not hold there. Serializing access to shared resources
>>>>>>>   should be guaranteed by resource-specific locking, not by a giant
>>>>>>>   traffic light like the big lock implements.
>>>>>>
>>>>>> This is more complicated: RTDM_EXECUTE_ATOMICALLY was in fact deprecated
>>>>>> long ago, but users were migrated to cobalt_atomic_enter/leave which may
>>>>>> now make it look like we no longer need this. Maybe this is already the
>>>>>> case when using rtdm_waitqueue*, but let's convert everyone first.
>>>>>
>>>>> Alternatively, In-tree v3 drivers could also keep relying
>>>>> RTDM_EXECUTE_ATOMICALLY, the v4 implementation would be different for
>>>>> them. Bottom line is to exclude from the CXP the whole idea that we may
>>>>> schedule while holding a lock to protect against missed wake ups, in
>>>>> general the very existence of any superlock which would cover everything
>>>>> from top to bottom when serializing. I agree that having v3 converge
>>>>> towards the CXP would be better though.
>>>>>
>>>>
>>>> I'm fine with migrating to a new pattern first, drop that old RTDM
>>>> pattern and declare the new one as migration path. Same for below.
>>>>
>>>>>>
>>>>>>>
>>>>>>> - rtdm_mutex_timedlock() has dubious semantics. Hitting a timeout
>>>>>>>   condition on grabbing a mutex either means that:
>>>>>>>
>>>>>>
>>>>>> I think you are missing the use cases:
>>>>>>
>>>>>> mutex-lock-timed
>>>>>> ...
>>>>>> wait-event-timed
>>>>>> ...
>>>>>> mutex-unlock
>>>>>> (which goes long with timeout sequences)
>>>>>>
>>>>>
>>>>> There is a couple of issues with such use case: first we should never
>>>>> ever sleep with a mutex held, this would trigger SIGDEBUG if done from
>>>>> user ( a [binary] semaphore would at least prevent this problem), but
>>>>> more importantly, how would this pattern allow the event to be signaled
>>>>> given the waiter holds the lock the sender would need to acquire first?
>>>>
>>>> Just look at the existing drivers for the use cases (which obviously
>>>> imply signalling without holding the mutex).
>>>>
>>>
>>> Excluding RTDM_EXECUTE_ATOMICALLY() which has no in-tree user, what
>>> remains is solving the issue for users of the cobalt_atomic_{enter,
>>> leave} pattern, i.e.:
>>>
>>> kernel/drivers/can/rtcan_raw.c
>>> kernel/drivers/can/rtcan_socket.c
>>> kernel/drivers/ipc/bufp.c
>>> kernel/drivers/ipc/iddp.c
>>> kernel/drivers/ipc/rtipc.c
>>> kernel/drivers/ipc/xddp.c
>>> kernel/drivers/net/stack/rtmac/tdma/tdma_dev.c
>>> kernel/drivers/testing/timerbench.c
>>> kernel/drivers/udd/udd.c
>>>
>>> For the call sites listed about, AFAICS we'd need to:
>>>
>>> 1. move any blocking call out of the locking scope, by rewriting these
>>> as wait loops rechecking the condition under lock if/when required. Only
>>> a few would need the latter in fact, as in many cases
>>> cobalt_atomic_leave() immediately follows the blocking call in the code
>>> flow.
>>>
>>> 2. provide _nosched variants for signaling calls
>>> (e.g. rtdm_event_pulse_nosched()) and use them, invoking xnsched_run()
>>> out of lock as appropriate.
>>>
>>> However, I cannot find any code exhibiting the issue with mutexes in
>>> these matches. Do you have an in-tree example of the problem you see to
>>> point me at?
>>>
>>
>> All serial drivers use mutexes with timeout in order to make write
>> operations atomic and permit waiting for free buffers inside that atomic
>> section.
> 
> Ok, but none of these driver sleep with the superlock held, which is the
> pattern I'm looking for at the moment. Sleeping with a mutex is a
> different issue which also requires some fixing, but neither involves
> the RTDM_EXECUTE_ATOMICALLY() or cobalt_atomic_enter/leave() constructs.
> 

It /could/ require it if it was not sleeping with mutexes.

Jan

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