[CXP] Discussing the RTDM specification

Jan Kiszka jan.kiszka at siemens.com
Mon Jan 11 11:48:53 CET 2021

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


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