Gilles Chanteperdrix wrote: > Philippe Gerum wrote: >> Gilles Chanteperdrix wrote: >>> Jan Kiszka wrote: >>>> Gilles Chanteperdrix wrote: >>>>> Jan Kiszka wrote: >>>> ... >>>>>> I think I'm getting closer to the issue. Our actual problem comes from >>>>>> the fact that the xnsynch_owner is easily out of sync with the real >>>>>> owner, it even sometimes points to a former owner: >>>>>> >>>>>> Thread A releases a mutex on which thread B pends. It wakes up B, >>>>>> causing it to become the new xnsynch owner, and clears the claimed bit >>>>>> as there are no further sleepers. B returns, and when it wants to >>>>>> release the mutex, it does this happily in user space because claimed is >>>>>> not set. Now the fast lock variable is 'unlocked', while xnsynch still >>>>>> reports B being the owner. This is no problem as the next time two >>>>>> threads fight over this lock the waiter will simply overwrite the >>>>>> xnsynch_owner before it falls asleep. But this "trick" doesn't work for >>>>>> waiters that have been robbed. They will spin inside xnsynch_sleep_on >>>>>> and stumble over this inconsistency. >>>>>> >>>>>> I have two approaches in mind now: First one is something like >>>>>> XNSYNCH_STEALNOINFORM, i.e. causing xnsynch_sleep_on to not set XNROBBED >>>>>> so that the robbed thread spins one level higher in the skin code - >>>>>> which would have to be extended a bit. >>>>> No, the stealing is the xnsynch job. >>>>> >>>>>> Option two is to clear xnsynch_owner once a new owner is about to return >>>>>> from kernel with the lock held while there are no more xnsynch_sleepers. >>>>>> That should work with even less changes and save us one syscall in the >>>>>> robbed case. Need to think about it more, though. >>>>> In fact the only time when the owner is required to be in sync is when >>>>> PIP occurs, and this is guaranteed to work, because when PIP is needed a >>>>> syscall is emitted anyway. To the extent that xnsynch does not even >>>>> track the owner on non PIP synch (which is why the posix skin originally >>>>> forcibly set the synch owner, and it was simply kept to get the fastsem >>>>> stuff working). >>>>> >>>>> Ok. And what about the idea of the xnsynch bit to tell him "hey, the >>>>> owner is tracked in the upper layer, go there to find it". >>>> I'm yet having difficulties to imagine how this should look like when >>>> it's implemented. Would it be simpler than my second idea? >>>> >>>> Anyway, here is a patch (on top of my handle-based lock series) for the >>>> approach that clears xnsynch_owner when there are no waiters. At least >>>> it causes no regression based on your test, but I haven't checked lock >>>> stealing yet. In theory, everything still appears to be fine to me. This >>>> approach basically restores the state we find when some thread just >>>> acquired the lock in user space. >>> Yes, I did not think about the stealing when writing my test, but I >>> think it could be a good idea to add it to the test, especially if you >>> want to port the test to the native API. >>> >>> I let Philippe decide here. He is the one who did the stealing stuff and >>> probably knows better. >>> >> Currently, the xnsynch strongly couples PIP and ownership, which seems to >> impede >> your various proposals. I would suggest to decouple that: the basic property >> of >> some xnsynch that we may want to handle is exclusiveness, then dynamic >> priority >> inheritance is another property, that could stack its own semantics on top of >> exclusiveness. >> >> XNSYNCH_EXCLUSIVE would cover all ownership-related actions, XNSYNCH_PIP >> would >> simply add dynamic priority management. Non exclusive object would not >> require >> any xnsynch_set_owner() handling. >> >> Just to give a clear signal here: I will happily consider any change to the >> xnsynch object that may ease the implementation of fast ownership handling >> (i.e. >> userland-to-userland transfer). The only thing is that such code is very much >> prone to regressions, so a testsuite must come with core changes in that >> area, >> but I guess you know that already. > > Ok. I think unit_mutex.c is a good start. It only lacks testing > XNROBBED.
My colleague sent me an extension. It's native-only so far, but it already pointed out a bug in my try-acquire implementation that should be present in posix as well (trylock must go through the slow path). > And well, cases of more than two waiters are not tested as > well, but I would not think it really matters. It could be extended as > well to allow switching at start time between the posix api and the > native api, after all we do not care much about the overhead of calling > function pointers in a a test application. For now we have two files already, but maybe we can merge them later based on your proposal. > > As for evolution of xnsynch, I am all for it, but it would break > anything implemented before, so it will probably be on Jan's critical path. I have some intermediate version here that breaks up the retry-on-XNROBBED loop, doing it at skin level instead. This is targeting the first customer delivery. I also have two implementations of fast locking now, native and posix, and they don't look that bad /wrt moving quite a few bits into xnsynch - later. Jan PS: The test cases + my /proc/xenomai/registry/usage probably unveiled another pending bug: Native user space tasks are leaking handles, even on vanilla Xenomai SVN (didn't test 2.4.x yet). Will look into this later.
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