From mboxrd@z Thu Jan 1 00:00:00 1970 Message-ID: <48B7EE28.3070101@domain.hid> Date: Fri, 29 Aug 2008 14:40:08 +0200 From: Jan Kiszka MIME-Version: 1.0 References: <48B5592B.1090005@domain.hid> <48B55F7C.5030901@domain.hid> <48B56685.4060500@domain.hid> <48B570AF.4090900@domain.hid> <48B57281.2090109@domain.hid> <48B57626.8070404@domain.hid> <48B576F2.5010409@domain.hid> <48B57BE0.8000701@domain.hid> <48B57D32.60504@domain.hid> <48B599DD.6070306@domain.hid> <48B5A4AB.3030909@domain.hid> <48B5B9FC.2050900@domain.hid> <48B5D8EC.90009@domain.hid> <48B6776E.6030502@domain.hid> <48B6984B.80804@domain.hid> <48B79A05.7060203@domain.hid> <48B79EAB.8000109@domain.hid> <48B7A3AF.6010802@domain.hid> <48B7A550.2030001@domain.hid> <48B7C31B.1050605@domain.hid> <48B7C438.9020000@domain.hid> <48B7D1F1.2080205@domain.hid> <48B7D39A.9030207@domain.hid> <48B7EBEC.3050809@domain.hid> In-Reply-To: <48B7EBEC.3050809@domain.hid> Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: 7bit Subject: Re: [Xenomai-core] [RFC][PATCH 2/3] Switch to handle-based fast mutex owners List-Id: "Xenomai life and development \(bug reports, patches, discussions\)" List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: rpm@xenomai.org Cc: xenomai-core Philippe Gerum wrote: > Jan Kiszka wrote: >> Philippe Gerum wrote: >>> Gilles Chanteperdrix wrote: >>>> Jan Kiszka wrote: >>>>> Gilles Chanteperdrix wrote: >>>>>> Jan Kiszka wrote: >>>>>>> Gilles Chanteperdrix wrote: >>>>>>>> Jan Kiszka wrote: >>>>>>>>> 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). >>>>>>>> I do not see why. If mutex lock can lock without a syscall, the same >>>>>>>> goes for trylock. >>>>>>> Lock stealing requires the slow path. >>>>>> Ah ok. I thought you mean that trylock had to go systematically through >>>>>> syscall. >>>>>> >>>>>> As for lock stealing, I already said that it was not tested in the >>>>>> current test. >>>>> In fact, that bug is also present the current native skin (SVN, 2.4.x >>>>> should suffer as well). Probably also current 2.4.x posix is affected, >>>>> at least from a first glance. >>>> Well, yes, now that you mention it, calling xnsynch_sleep_on does not >>>> seem obvious for the implementation of a trylock. >>>> >>> Don't even try calling xnsynch_sleep_on with a nonblocking-type timeout. >> Works nicely today (using (XN_NONBLOCK, XN_RELATIVE)) due to early >> timeout checks in xntimer_start. You just have to translate XNTIMEO into >> EWOULDBLOCK on return. >> > > It's very fundamentally crappy. Please don't do that. xnsynch_sleep_on() must do > what it is supposed to mean. So we need xnsynch_try_to_steal + lots of new special-case code in the skins to re-implement what is already there? No, I can't imagine that this is what you want to see in the end. Then lets better rename xnsynch_sleep_on to something like xnsynch_acquire - will be logically needed anyway when we push fast mutex maintenance into it. Jan -- Siemens AG, Corporate Technology, CT SE 2 Corporate Competence Center Embedded Linux