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. Jan -- Siemens AG, T RDA IOT Corporate Competence Center Embedded Linux
