From mboxrd@z Thu Jan 1 00:00:00 1970 Message-ID: <4BD6AE1E.6050704@domain.hid> Date: Tue, 27 Apr 2010 11:27:58 +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> In-Reply-To: <1272359559.28983.380.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 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. Jan -- Siemens AG, Corporate Technology, CT T DE IT 1 Corporate Competence Center Embedded Linux