From mboxrd@z Thu Jan 1 00:00:00 1970 Message-ID: <4DFCA09B.20609@domain.hid> Date: Sat, 18 Jun 2011 14:56:59 +0200 From: Gilles Chanteperdrix MIME-Version: 1.0 References: <4DFB869F.9080006@domain.hid> <4DFB88EC.9090100@domain.hid> <4DFBA305.9000303@domain.hid> <4DFC7C0E.1090700@domain.hid> <4DFC9575.1030904@domain.hid> <4DFC95B7.8070703@domain.hid> In-Reply-To: <4DFC95B7.8070703@domain.hid> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit Subject: Re: [Xenomai-core] [Xenomai-git] Jan Kiszka : nucleus: Fix interrupt handler tails List-Id: Xenomai life and development List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Jan Kiszka Cc: Xenomai core On 06/18/2011 02:10 PM, Jan Kiszka wrote: > On 2011-06-18 14:09, Gilles Chanteperdrix wrote: >> On 06/18/2011 12:21 PM, Jan Kiszka wrote: >>> On 2011-06-17 20:55, Gilles Chanteperdrix wrote: >>>> On 06/17/2011 07:03 PM, Jan Kiszka wrote: >>>>> On 2011-06-17 18:53, Gilles Chanteperdrix wrote: >>>>>> On 06/17/2011 04:38 PM, GIT version control wrote: >>>>>>> Module: xenomai-jki >>>>>>> Branch: for-upstream >>>>>>> Commit: 7203b1a66ca0825d5bcda1c3abab9ca048177914 >>>>>>> URL: http://git.xenomai.org/?p=xenomai-jki.git;a=commit;h=7203b1a66ca0825d5bcda1c3abab9ca048177914 >>>>>>> >>>>>>> Author: Jan Kiszka >>>>>>> Date: Fri Jun 17 09:46:19 2011 +0200 >>>>>>> >>>>>>> nucleus: Fix interrupt handler tails >>>>>>> >>>>>>> Our current interrupt handlers assume that they leave over the same task >>>>>>> and CPU they entered. But commit f6af9b831c broke this assumption: >>>>>>> xnpod_schedule invoked from the handler tail can now actually trigger a >>>>>>> domain migration, and that can also include a CPU migration. This causes >>>>>>> subtle corruptions as invalid xnstat_exectime_t objects may be restored >>>>>>> and - even worse - we may improperly flush XNHTICK of the old CPU, >>>>>>> leaving Linux timer-wise dead there (as happened to us). >>>>>>> >>>>>>> Fix this by moving XNHTICK replay and exectime accounting before the >>>>>>> scheduling point. Note that this introduces a tiny imprecision in the >>>>>>> accounting. >>>>>> >>>>>> I am not sure I understand why moving the XNHTICK replay is needed: if >>>>>> we switch to secondary mode, the HTICK is handled by xnpod_schedule >>>>>> anyway, or am I missing something? >>>>> >>>>> The replay can work on an invalid sched (after CPU migration in >>>>> secondary mode). We could reload the sched, but just moving the replay >>>>> is simpler. >>>> >>>> But does it not remove the purpose of this delayed replay? >>> >>> Hmm, yes, in the corner case of coalesced timed RT task wakeup and host >>> tick over a root thread. Well, then we actually have to reload sched and >>> keep the ordering to catch that as well. >>> >>>> >>>> Note that if you want to reload the sched, you also have to shut >>>> interrupts off, because upon return from xnpod_schedule after migration, >>>> interrupts are on. >>> >>> That would be another severe bug if we left an interrupt handler with >>> hard IRQs enabled - the interrupt tail code of ipipe would break. >>> >>> Fortunately, only xnpod_suspend_thread re-enables IRQs and returns. >>> xnpod_schedule also re-enables but then terminates the context (in >>> xnshadow_exit). So we are safe. >> >> I do not think we are, at least on platforms where context switches >> happen with irqs on. > > Can you sketch a problematic path? On platforms with IPIPE_WANT_PREEMPTIBLE_SWITCH on, all context switches happens with irqs on. So, in particular, the context switch to a relaxed task happens with irqs on. In __xnpod_schedule, we then return from xnpod_switch_to with irqs on, and so return from __xnpod_schedule with irqs on. Maybe in the irq handlers, we should skip the XNHTICK replay, when current_domain is root_domain. -- Gilles.