From mboxrd@z Thu Jan 1 00:00:00 1970 Message-ID: <4E1C2B44.5060907@domain.hid> Date: Tue, 12 Jul 2011 13:08:52 +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> <4E1BEC9F.1020404@domain.hid> <4E1BF619.6010609@domain.hid> <4E1C2912.9050605@domain.hid> <4E1C2959.8080004@domain.hid> <4E1C2A2D.9090602@domain.hid> <4E1C2AA5.6060208@domain.hid> In-Reply-To: <4E1C2AA5.6060208@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 07/12/2011 01:06 PM, Jan Kiszka wrote: > On 2011-07-12 13:04, Gilles Chanteperdrix wrote: >> On 07/12/2011 01:00 PM, Jan Kiszka wrote: >>> On 2011-07-12 12:59, Gilles Chanteperdrix wrote: >>>> On 07/12/2011 09:22 AM, Jan Kiszka wrote: >>>>> On 2011-07-12 08:41, Gilles Chanteperdrix wrote: >>>>>> 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)); >>>>> >>>>> That's not reliable, the task might have been migrated by Linux in the >>>>> meantime. We must use the stored gksched. >>>>> >>>>>> + 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; >>>>>> >>>>> >>>>> Again, setting gktarget to NULL and testing for NULL is simply safer, >>>>> and I see no gain in skipping that. But if you prefer the >>>>> micro-optimization, I'll drop it. >>>> >>>> Could not we use an info bit instead of adding a pointer? >>>> >>> >>> "That's not reliable, the task might have been migrated by Linux in the >>> meantime. We must use the stored gksched." >> >> I mean add another info bit to mean that the task is queued for wakeup >> by the gatekeeper. >> >> XNGKQ, or something. > > What additional value does it provide to gksched != NULL? We need that > pointer anyway to identify the gatekeeper that holds a reference. No, the scheduler which holds the reference is xnpod_sched_slot(task_cpu(p)) -- Gilles.