From mboxrd@z Thu Jan 1 00:00:00 1970 Message-ID: <4E1BEC9F.1020404@domain.hid> Date: Tue, 12 Jul 2011 08:41:35 +0200 From: Gilles Chanteperdrix MIME-Version: 1.0 References: <4E1B469A.8000703@domain.hid> <4E1B4AC0.80506@domain.hid> <4E1B4C19.2070205@domain.hid> <4E1B542B.2010906@domain.hid> <4E1B5638.1050005@domain.hid> <4E1B56E0.20109@domain.hid> <4E1B57D1.1070401@domain.hid> <4E1B5860.1000309@domain.hid> <4E1B5944.5030408@domain.hid> In-Reply-To: <4E1B5944.5030408@domain.hid> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit Subject: Re: [Xenomai-core] [Xenomai-git] Jan Kiszka : nucleus: Fix race between gatekeeper and thread deletion List-Id: Xenomai life and development List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Jan Kiszka Cc: Xenomai core On 07/11/2011 10:12 PM, Jan Kiszka wrote: > On 2011-07-11 22:09, Gilles Chanteperdrix wrote: >> On 07/11/2011 10:06 PM, Jan Kiszka wrote: >>> On 2011-07-11 22:02, Gilles Chanteperdrix wrote: >>>> On 07/11/2011 09:59 PM, Jan Kiszka wrote: >>>>> On 2011-07-11 21:51, Gilles Chanteperdrix wrote: >>>>>> On 07/11/2011 09:16 PM, Jan Kiszka wrote: >>>>>>> On 2011-07-11 21:10, Jan Kiszka wrote: >>>>>>>> On 2011-07-11 20:53, Gilles Chanteperdrix wrote: >>>>>>>>> On 07/08/2011 06:29 PM, GIT version control wrote: >>>>>>>>>> @@ -2528,6 +2534,22 @@ static inline void do_taskexit_event(struct task_struct *p) >>>>>>>>>> magic = xnthread_get_magic(thread); >>>>>>>>>> >>>>>>>>>> xnlock_get_irqsave(&nklock, s); >>>>>>>>>> + >>>>>>>>>> + gksched = thread->gksched; >>>>>>>>>> + if (gksched) { >>>>>>>>>> + xnlock_put_irqrestore(&nklock, s); >>>>>>>>> >>>>>>>>> Are we sure irqs are on here? Are you sure that what is needed is not an >>>>>>>>> xnlock_clear_irqon? >>>>>>>> >>>>>>>> We are in the context of do_exit. Not only IRQs are on, also preemption. >>>>>>>> And surely no nklock is held. >>>>>>>> >>>>>>>>> Furthermore, I do not understand how we >>>>>>>>> "synchronize" with the gatekeeper, how is the gatekeeper garanteed to >>>>>>>>> wait for this assignment? >>>>>>>> >>>>>>>> The gatekeeper holds the gksync token while it's active. We request it, >>>>>>>> thus we wait for the gatekeeper to become idle again. While it is idle, >>>>>>>> we reset the queued reference - but I just realized that this may tramp >>>>>>>> on other tasks' values. I need to add a check that the value to be >>>>>>>> null'ified is actually still ours. >>>>>>> >>>>>>> Thinking again, that's actually not a problem: gktarget is only needed >>>>>>> while gksync is zero - but then we won't get hold of it anyway and, >>>>>>> thus, can't cause any damage. >>>>>> >>>>>> Well, you make it look like it does not work. From what I understand, >>>>>> what you want is to set gktarget to null if a task being hardened is >>>>>> destroyed. But by waiting for the semaphore, you actually wait for the >>>>>> harden to be complete, so setting to NULL is useless. Or am I missing >>>>>> something else? >>>>> >>>>> Setting to NULL is probably unneeded but still better than rely on the >>>>> gatekeeper never waking up spuriously and then dereferencing a stale >>>>> pointer. >>>>> >>>>> The key element of this fix is waitng on gksync, thus on the completion >>>>> of the non-RT part of the hardening. Actually, this part usually fails >>>>> as the target task received a termination signal at this point. >>>> >>>> Yes, but since you wait on the completion of the hardening, the test >>>> if (target &&...) in the gatekeeper code will always be true, because at >>>> this point the cleanup code will still be waiting for the semaphore. >>> >>> Yes, except we will ever wake up the gatekeeper later on without an >>> updated gktarget, ie. spuriously. Better safe than sorry, this is hairy >>> code anyway (hopefully obsolete one day). >> >> The gatekeeper is not woken up by posting the semaphore, the gatekeeper >> is woken up by the thread which is going to be hardened (and this thread >> is the one which waits for the semaphore). > > All true. And what is the point? The point being, would not something like this patch be sufficient? diff --git a/ksrc/nucleus/shadow.c b/ksrc/nucleus/shadow.c index 01f4200..4742c02 100644 --- a/ksrc/nucleus/shadow.c +++ b/ksrc/nucleus/shadow.c @@ -2527,6 +2527,18 @@ static inline void do_taskexit_event(struct task_struct *p) magic = xnthread_get_magic(thread); xnlock_get_irqsave(&nklock, s); + if (xnthread_test_info(thread, XNATOMIC)) { + struct xnsched *gksched = xnpod_sched_slot(task_cpu(p)); + xnlock_put_irqrestore(&nklock, s); + + /* Thread is in flight to primary mode, wait for the + gatekeeper to be done with it. */ + down(&gksched->gksync); + up(&gksched->gksync); + + xnlock_get_irqsave(&nklock, s); + } + /* Prevent wakeup call from xnshadow_unmap(). */ xnshadow_thrptd(p) = NULL; xnthread_archtcb(thread)->user_task = NULL; -- Gilles.