Jan Kiszka <jan.kis...@siemens.com> writes:
> On 07.04.22 13:16, Philippe Gerum wrote: >> >> Jan Kiszka <jan.kis...@siemens.com> writes: >> >>> On 06.04.22 17:56, Philippe Gerum via Xenomai wrote: >>>> From: Philippe Gerum <r...@xenomai.org> >>>> >>>> The current implementation does not atomically consume+clear the event >>>> set to be received by the waiter(s), which makes it useless for >>>> anything but a plain one-time latch due to the race window this opens >>>> with a consume[A]->signal[B]->clear[A] sequence. >>>> >>>> To address this issue, let's provide the auto-clear feature with >>>> __cobalt_event_wait(). >>>> >>>> This change affects the ABI by adding the auto-clear mode as an opt-in >>>> feature, enabled by passing COBALT_EVENT_AUTOCLEAR to >>>> cobalt_event_init(). >>>> >>> >>> Makes sense, but shouldn't autoclear be rather the default then? Which >>> users are affected? None in-tree so far? >>> >> >> There is one user in copperplate: >> https://source.denx.de/Xenomai/xenomai/-/blob/28158391258eea52650856bef5d3ed6ebaaf813b/lib/copperplate/eventobj.c#L87 >> >> which indirectly affects rt_event_signal() from the alchemy API: >> https://source.denx.de/Xenomai/xenomai/-/blob/28158391258eea52650856bef5d3ed6ebaaf813b/lib/alchemy/event.c#L453 >> >> Nobody raised the issue so far with alchemy, which is why I refrained >> from turning the autoclear mode on by default so far. This is debatable, >> since no documentation explains the limitation on usage caused by not >> having the autoclear mode set. >> > > So, the pattern via Alchemy would be > > Thread A Thread B > > rt_event_wait() > rt_event_signal() > rt_event_clear() > > That would force users to perform a state check via a side-channel after > clearing the event to avoid starting to waiting if the condition was met > again. > > OK, but how could users request the new mode in rt_event_create? There > is not even a EV_AUTOCLEAR flag for it. Do you have more patches pending? > The alternative I see is: - assume that some people might be expecting the current - fragile to say the least - behavior, which means that we should add a flag to rt_event_create() in order to enable the auto-clear mode for all others. - consider the current behavior as broken beyond recognition, and force in the auto-clear mode for all alchemy events, at the expense of requiring the folks who have been using a side-channel to paper over the current misdesign, to fix their stuff the right way, based on the auto-clear behavior. IMHO, everyone would be better off with #2, because it would just work in all cases. The side-channel would simply become a useless but innocuous noise if present. Which way should we go is debatable, this is why I did not issue any patch changing the alchemy interface yet. -- Philippe.
