From mboxrd@z Thu Jan 1 00:00:00 1970 Message-ID: <4E1C2A2D.9090602@domain.hid> Date: Tue, 12 Jul 2011 13:04:13 +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> In-Reply-To: <4E1C2959.8080004@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 01:00 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. -- Gilles.