From mboxrd@z Thu Jan 1 00:00:00 1970 Message-ID: <48B7F550.7000801@domain.hid> Date: Fri, 29 Aug 2008 15:10:40 +0200 From: Philippe Gerum 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> In-Reply-To: <48B7EE28.3070101@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 Reply-To: rpm@xenomai.org List-Id: "Xenomai life and development \(bug reports, patches, discussions\)" List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Jan Kiszka Cc: xenomai-core 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. > 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. > Jan > -- Philippe.