From mboxrd@z Thu Jan 1 00:00:00 1970 Message-ID: <48B7F8CD.30208@domain.hid> Date: Fri, 29 Aug 2008 15:25:33 +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> <48B7EE28.3070101@domain.hid> <48B7F550.7000801@domain.hid> In-Reply-To: <48B7F550.7000801@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: >>> 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. >> > > What I don't want is to see code running a routine called "sleep_on" knowing > that it may not sleep. What I don't want either, is to go through > xnsynch_sleep_on -> xnpod_suspend_thread -> xntimer_start -> > xnsynch_forget_sleeper to get things right with !PIP objects, albeit the caller > knows from the beginning that it does not want to sleep. So much for the fast > path to trylocking. OK, point taken. But as this is a generic API issue, I would like to see it fixed in the generic code, not in some to-be-duplicated branches in all the skins. My point is that our timeout API nicely defines the case XN_NONBLOCK now, throughout all layers. We just need to pass the arguments as-is down, the core will handle them. Let's do optimizations at the lowest possible layer, i.e. in xnsynch_sleep_on (or whatever it may be called better). A simple 'if' before suspend should be enough. And it will keep the code small. > >> 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. >> > > This would not fix issue #2. xnsynch_try_acquire() that would attempt stealing > is the way to go. It's even possible that some code could be shared between that > routine, and the resource stealing support currently provided by > xnsynch_sleep_on(). Only resource acquisition routines that enforce exclusive > ownership would be impacted, not each and every xnsynch_sleep_on caller. For now it will be a PIP show, but atomic_cmpxchg-based semaphores might be an xnsynch extension in the future as well. Jan --- Siemens AG, Corporate Technology, CT SE 2 Corporate Competence Center Embedded Linux