From mboxrd@z Thu Jan 1 00:00:00 1970 Message-ID: <4BD6AFC2.1090300@domain.hid> Date: Tue, 27 Apr 2010 11:34:58 +0200 From: Jan Kiszka MIME-Version: 1.0 References: <4BD5987A.2050804@domain.hid> <4BD59A48.5070002@domain.hid> <4BD5BA03.5000101@domain.hid> <1272331158.28983.287.camel@domain.hid> <4BD68843.4030806@domain.hid> <1272356029.28983.333.camel@domain.hid> <4BD69F7D.9060006@domain.hid> <1272359559.28983.380.camel@domain.hid> <4BD6AE1E.6050704@domain.hid> <1272360740.28983.382.camel@domain.hid> In-Reply-To: <1272360740.28983.382.camel@domain.hid> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit Subject: Re: [Xenomai-core] [PATCH] nucleus: Plug race between rpi_clear_remote and rpi_next List-Id: Xenomai life and development List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Philippe Gerum Cc: xenomai-core Philippe Gerum wrote: > On Tue, 2010-04-27 at 11:27 +0200, Jan Kiszka wrote: >> Philippe Gerum wrote: >>> On Tue, 2010-04-27 at 10:25 +0200, Jan Kiszka wrote: >>>> Philippe Gerum wrote: >>>>> On Tue, 2010-04-27 at 08:46 +0200, Jan Kiszka wrote: >>>>>> Philippe Gerum wrote: >>>>>>> On Mon, 2010-04-26 at 18:06 +0200, Jan Kiszka wrote: >>>>>>>> Jan Kiszka wrote: >>>>>>>>> Jan Kiszka wrote: >>>>>>>>>> Hi, >>>>>>>>>> >>>>>>>>>> I'm meditating over an oops in rpi_clear_remote. NULL pointer deref, it >>>>>>>>>> /seems/ like thread->rpi is invalid. Looking at the code, I wonder if >>>>>>>>>> this could explain the bug: >>>>>>>>>> >>>>>>>>>> >>>>>>>>>> static void rpi_clear_remote(struct xnthread *thread) >>>>>>>>>> { >>>>>>>>>> ... >>>>>>>>>> rpi = thread->rpi; >>>>>>>>>> if (unlikely(rpi == NULL)) >>>>>>>>>> return; >>>>>>>>>> >>>>>>>>>> xnlock_get_irqsave(&rpi->rpilock, s); >>>>>>>>>> >>>>>>>>>> /* >>>>>>>>>> * The RPI slot - if present - is always valid, and won't >>>>>>>>>> * change since the thread is resuming on this CPU and cannot >>>>>>>>>> * migrate under our feet. We may grab the remote slot lock >>>>>>>>>> * now. >>>>>>>>>> */ >>>>>>>>>> xnsched_pop_rpi(thread); >>>>>>>>>> thread->rpi = NULL; >>>>>>>>>> >>>>>>>>>> ... >>>>>>>>>> >>>>>>>>>> So we deref (xnsched_pop_rpi) and clear thread->rpi under rpilock, but >>>>>>>>>> we check for it without any protection? Sounds racy. I think 'thread' is >>>>>>>>>> not only pointing to the current thread but could refer to a foreign one >>>>>>>>>> as well, right? Don't we need this: >>>>>>>>>> >>>>>>>>>> diff --git a/ksrc/nucleus/shadow.c b/ksrc/nucleus/shadow.c >>>>>>>>>> index 872c37f..1f995d6 100644 >>>>>>>>>> --- a/ksrc/nucleus/shadow.c >>>>>>>>>> +++ b/ksrc/nucleus/shadow.c >>>>>>>>>> @@ -331,6 +331,12 @@ static void rpi_clear_remote(struct xnthread *thread) >>>>>>>>>> >>>>>>>>>> xnlock_get_irqsave(&rpi->rpilock, s); >>>>>>>>>> >>>>>>>>>> + /* Re-check under lock, someone may have cleared rpi by now. */ >>>>>>>>>> + if (unlikely(thread->rpi == NULL)) { >>>>>>>>>> + xnlock_put_irqrestore(&rpi->rpilock, s); >>>>>>>>>> + return; >>>>>>>>>> + } >>>>>>>>>> + >>>>>>>>>> /* >>>>>>>>>> * The RPI slot - if present - is always valid, and won't >>>>>>>>>> * change since the thread is resuming on this CPU and cannot >>>>>>>>> Another worry: Can thread->rpi become != rpi without being NULL? Or can >>>>>>>>> we really only race for clearance here? >>>>>>>>> >>>>>>>> I think so now, therefore I'm proposing this: >>>>>>>> >>>>>>>> -----------> >>>>>>>> >>>>>>>> Most RPI services work on the current task or the one to be scheduled in >>>>>>>> next, thus are naturally serialized. But rpi_next is not as it can walk >>>>>>>> the chain of RPI requests for a CPU independently. In that case, >>>>>>>> clearing RPI via rpi_clear_remote can race with rpi_next, and if the >>>>>>>> former loses after checking thread->rpi for NULL, we will dereference a >>>>>>>> NULL pointer in xnsched_pop_rpi(). >>>>>>>> >>>>>>>> Signed-off-by: Jan Kiszka >>>>>>>> --- >>>>>>>> ksrc/nucleus/shadow.c | 9 +++++++++ >>>>>>>> 1 files changed, 9 insertions(+), 0 deletions(-) >>>>>>>> >>>>>>>> diff --git a/ksrc/nucleus/shadow.c b/ksrc/nucleus/shadow.c >>>>>>>> index 872c37f..cf7c08f 100644 >>>>>>>> --- a/ksrc/nucleus/shadow.c >>>>>>>> +++ b/ksrc/nucleus/shadow.c >>>>>>>> @@ -332,6 +332,15 @@ static void rpi_clear_remote(struct xnthread *thread) >>>>>>>> xnlock_get_irqsave(&rpi->rpilock, s); >>>>>>>> >>>>>>>> /* >>>>>>>> + * Re-check under lock. Someone may have invoked rpi_next and cleared >>>>>>>> + * rpi by now. >>>>>>>> + */ >>>>>>>> + if (unlikely(!rpi_p(thread))) { >>>>>>>> + xnlock_put_irqrestore(&rpi->rpilock, s); >>>>>>>> + return; >>>>>>>> + } >>>>>>>> + >>>>>>>> + /* >>>>>>>> * The RPI slot - if present - is always valid, and won't >>>>>>>> * change since the thread is resuming on this CPU and cannot >>>>>>>> * migrate under our feet. We may grab the remote slot lock >>>>>>>> >>>>>>> The suggested patch papers over the actual issue, which is that >>>>>>> rpi_clear_remote() may not invoke rpi_next(), because it may only affect >>>>>> I don't think that in our case rpi_clear_remote called rpi_next and >>>>>> therefore crashed. It should rather have been the scenario of both >>>>>> running in parallel on different CPUs, the former on behalf of a >>>>>> migrated shadow that wants to clear its remainders on the remote CPU, >>>>>> the latter on that CPU, picking a new top RPI after some other shadow >>>>>> was removed from the queue. Is this a possible scenario, and would your >>>>>> patch cover it? >>>>>> >>>>>>> the RPI state of the argument thread which must be a local one, and not >>>>>>> that of any random thread that happens to be linked to the remote RPI >>>>>>> queue. >>>>>>> >>>>>>> By calling rpi_next(), rpi_clear_remote() shoots itself in the foot, >>>>>>> allowing a concurrent invocation of itself on a remote CPU, to fiddle >>>>>>> with the rpi backlink of a thread which is not active on the >>>>>>> local/per-cpu Xenomai scheduler instance, which is the point where >>>>>>> things start to hit the crapper. >>>>>>> >>>>>>> Now, unless I can't even synchronize the couple of neurons I have left >>>>>>> at this hour, the following patch should better fix the issue, because >>>>>>> it restores the two basic rules that apply to the whole RPI machinery, >>>>>>> namely: >>>>>>> >>>>>>> - rpi_* calls may only alter the contents of the local scheduler's RPI >>>>>>> queue, with the notable exception of rpi_clear_remote() which may remove >>>>>>> the given _local_ thread only, from a remote RPI slot. >>>>>>> >>>>>>> - rpi_* calls may only change the RPI state of threads which are >>>>>>> controlled by the local Xenomai scheduler instance, except rpi_push() >>>>>>> when called for setting up the RPI state of an emerging thread, in which >>>>>>> case this is a no-conflict zone. >>>>>>> >>>>>>> That breakage was introduced in the early 2.5.1 timeframe, so 2.4.x >>>>>>> should be immune from this particular bug. >>>>>>> >>>>>>> diff --git a/ksrc/nucleus/shadow.c b/ksrc/nucleus/shadow.c >>>>>>> index 872c37f..1397ed1 100644 >>>>>>> --- a/ksrc/nucleus/shadow.c >>>>>>> +++ b/ksrc/nucleus/shadow.c >>>>>>> @@ -340,7 +340,12 @@ static void rpi_clear_remote(struct xnthread *thread) >>>>>>> xnsched_pop_rpi(thread); >>>>>>> thread->rpi = NULL; >>>>>>> >>>>>>> - if (rpi_next(rpi, s) == NULL) >>>>>>> + /* >>>>>>> + * If the remote RPI queue was emptied, prepare for kicking >>>>>>> + * xnshadow_rpi_check() on the relevant CPU to demote the root >>>>>>> + * thread priority there. >>>>>>> + */ >>>>>>> + if (xnsched_peek_rpi(rpi) == NULL) /* No rpi_next() here. */ >>>>>>> rcpu = xnsched_cpu(rpi); >>>>>>> >>>>>>> xnlock_put_irqrestore(&rpi->rpilock, s); >>>>>>> >>>>>>> >>>>>> I have to confess, I do not understand how the patch may relate to our >>>>>> crash. But that's because I still only have a semi-understanding of this >>>>>> frightening complex RPI code. However, the fact that thread->rpi is now >>>>>> again only checked outside its protecting lock leaves me with a very bad >>>>>> feeling... >>>>>> >>>>> My point is that we should not have to protect a section of code which >>>>> may never conflict in any case, by design; we will likely agree that >>>>> sprinkling locks everywhere to get a warm and fuzzy feeling is no >>>>> solution, it's actually a significant source of regression. >>>>> >>>>> The idea, behind keeping most rpi_* operations applicable to locally >>>>> scheduled threads, is to introduce such a design, even when remote RPI >>>>> slots are involved. thread->sched == xnpod_current_sched() for each >>>>> rpi_*(sched, ...) calls is what is important in this logic. Another >>>>> original assumption was that no RPI updates could be done in interrupt >>>>> context, which is now wrong due to the change in xnshadow_rpi_check(). >>>>> >>>>> In short: we have to make sure that rpi_next() does not break the basic >>>>> assumptions of the RPI core, first. >>>> Please check my scenario again: My concern is that a thread can be >>>> queued for a short while on a remote sched, >>> No, it can't, that is the crux of the matter, well, at least, this >>> should not be possible if the basic assumptions are preserved (have a >>> look at the rpi_clear_remote() callers, the target thread may not >>> migrate or be scheduled in linux-wise, or exit RPI via rpi_pop() during >>> the call -- all places where the rpi backlink may be cleared). Only a >>> caller operating from the local CPU should be allowed to alter the RPI >>> state of threads linked to the RPI slot of that same CPU. >>> >>> rpi_clear_remote() is not even an exception to this, since it alters a >>> remote RPI slot, but for a thread which does run on the local CPU. >>> >>>> and while that is the case, >>>> it can be manipulated (/wrt ->rpi) _concurrently_ (as we do not hold the >>>> remote rpilock all the time). I'm quite sure now that your patch does >>>> not change this. >>> My patch attempts fixing the core issue, not just plugging one of its >>> bad outcomes. >>> >>> Again, the point is not to pretend that your patch is wrong, and it >>> surely "plugs" one issue we have due to rpi_next(). The point is to make >>> sure that all issues are covered, by fixing the usage of rpi_next(), or >>> find another way to fix what rpi_next() was supposed to fix in the first >>> place. >> So, if you are right, we could (in theory) replace rpilock with local >> IRQ disabling? That would be the proof from me that it doesn't matter to >> test thread->rpi outside the lock. > > Noo, you did not get it yet. The rpilock is protecting the per-cpu RPI > queues, NOT thread->rpi. So linking a thread to an RPI queue and setting its ->rpi are not always related? Jan -- Siemens AG, Corporate Technology, CT T DE IT 1 Corporate Competence Center Embedded Linux