From mboxrd@z Thu Jan 1 00:00:00 1970 Message-ID: <4CD1FABD.1080301@domain.hid> Date: Thu, 04 Nov 2010 01:13:49 +0100 From: Gilles Chanteperdrix MIME-Version: 1.0 References: <4CC82C8D.3080808@domain.hid> <4CC84327.9070202@domain.hid> <4CC92786.3030509@domain.hid> <4CC92902.4040904@domain.hid> <4CC943A2.9020806@domain.hid> <4CC94E0B.9070106@domain.hid> <4CCEF104.7050409@domain.hid> <4CD11AB1.8090407@domain.hid> <4CD13A70.8040702@domain.hid> <4CD14B1E.4000707@domain.hid> <4CD14C92.90901@domain.hid> <4CD14DBC.3060505@domain.hid> <4CD1509A.3000908@domain.hid> <4CD152F3.4080203@domain.hid> <4CD16654.6080704@domain.hid> <4CD18782.7090607@domain.hid> <4CD191EE.7000604@domain.hid> <4CD1936E.50203@domain.hid> <4CD1BA29.9000303@domain.hid> <1288816871.1842.84.camel@domain.hid> <4CD1DC1B.8060407@domain.hid> <4CD1DE12.5010309@domain.hid> <4CD1E890.5010702@domain.hid> <4CD1EC2F.4040603@domain.hid> <4CD1ED16.8030103@domain.hid> <4CD1EDA8.10007@domain.hid> <4CD1F33C.5070208@domain.hid> <4CD1F3F5.5080505@domain.hid> <4CD1F4FE.9020908@domain.hid> <4CD1F69B.9070100@domain.hid> <4CD1F906.1070703@domain.hid> In-Reply-To: <4CD1F906.1070703@domain.hid> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit Subject: Re: [Xenomai-core] Potential problem with rt_eepro100 List-Id: Xenomai life and development List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Jan Kiszka Cc: "xenomai@xenomai.org" Jan Kiszka wrote: > Am 04.11.2010 00:56, Gilles Chanteperdrix wrote: >> Jan Kiszka wrote: >>> Am 04.11.2010 00:44, Gilles Chanteperdrix wrote: >>>> Jan Kiszka wrote: >>>>> Am 04.11.2010 00:18, Gilles Chanteperdrix wrote: >>>>>> Jan Kiszka wrote: >>>>>>> Am 04.11.2010 00:11, Gilles Chanteperdrix wrote: >>>>>>>> Jan Kiszka wrote: >>>>>>>>> Am 03.11.2010 23:11, Jan Kiszka wrote: >>>>>>>>>> Am 03.11.2010 23:03, Jan Kiszka wrote: >>>>>>>>>>> But we not not always use atomic ops for manipulating status bits (but >>>>>>>>>>> we do in other cases where this is no need - different story). This may >>>>>>>>>>> fix the race: >>>>>>>>>> Err, nonsense. As we manipulate xnsched::status also outside of nklock >>>>>>>>>> protection, we must _always_ use atomic ops. >>>>>>>>>> >>>>>>>>>> This screams for a cleanup: local-only bits like XNHTICK or XNINIRQ >>>>>>>>>> should be pushed in a separate status word that can then be safely >>>>>>>>>> modified non-atomically. >>>>>>>>> Second try to fix and clean up the sched status bits. Anders, please >>>>>>>>> test. >>>>>>>>> >>>>>>>>> Jan >>>>>>>>> >>>>>>>>> diff --git a/include/nucleus/pod.h b/include/nucleus/pod.h >>>>>>>>> index 01ff0a7..5987a1f 100644 >>>>>>>>> --- a/include/nucleus/pod.h >>>>>>>>> +++ b/include/nucleus/pod.h >>>>>>>>> @@ -277,12 +277,10 @@ static inline void xnpod_schedule(void) >>>>>>>>> * context is active, or if we are caught in the middle of a >>>>>>>>> * unlocked context switch. >>>>>>>>> */ >>>>>>>>> -#if XENO_DEBUG(NUCLEUS) >>>>>>>>> if (testbits(sched->status, XNKCOUT|XNINIRQ|XNSWLOCK)) >>>>>>>>> return; >>>>>>>>> -#else /* !XENO_DEBUG(NUCLEUS) */ >>>>>>>>> - if (testbits(sched->status, >>>>>>>>> - XNKCOUT|XNINIRQ|XNSWLOCK|XNRESCHED) != XNRESCHED) >>>>>>>>> +#if !XENO_DEBUG(NUCLEUS) >>>>>>>>> + if (!sched->resched) >>>>>>>>> return; >>>>>>>>> #endif /* !XENO_DEBUG(NUCLEUS) */ >>>>>>>> Having only one test was really nice here, maybe we simply read a >>>>>>>> barrier before reading the status? >>>>>>>> >>>>>>> I agree - but the alternative is letting all modifications of >>>>>>> xnsched::status use atomic bitops (that's required when folding all bits >>>>>>> into a single word). And that should be much more costly, specifically >>>>>>> on SMP. >>>>>> What about issuing a barrier before testing the status? >>>>>> >>>>> The problem is not about reading but writing the status concurrently, >>>>> thus it's not about the code you see above. >>>> The bits are modified under nklock, which implies a barrier when >>>> unlocked. Furthermore, an IPI is guaranteed to be received on the remote >>>> CPU after this barrier, so, a barrier should be enough to see the >>>> modifications which have been made remotely. >>> Check nucleus/intr.c for tons of unprotected status modifications. >> Ok. Then maybe, we should reconsider the original decision to start >> fiddling with the XNRESCHED bit remotely. > > ...which removed complexity and fixed a race? Let's better review the > checks done in xnpod_schedule vs. its callers, I bet there is more to > save (IOW: remove the need to test for sched->resched). Not that much complexitiy... and the race was a false positive in debug code, no big deal. At least it worked, and it has done so for a long time. No atomic needed, no barrier, only one test in xnpod_schedule. And a nice invariant: sched->status is always accessed on the local cpu. What else? -- Gilles.