[CXP] Discussing the RTDM specification

Philippe Gerum rpm at xenomai.org
Mon Jan 11 14:14:34 CET 2021

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.


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