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? -- Philippe.
