Jan Kiszka <[email protected]> writes:
> On 09.01.21 18:01, Philippe Gerum wrote: >> >> Jan Kiszka <[email protected]> writes: >> >>> On 23.12.20 11:40, Philippe Gerum wrote: >>>> >>>> Jan Kiszka <[email protected]> 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. -- Philippe.
