From mboxrd@z Thu Jan 1 00:00:00 1970 From: Philippe Gerum In-Reply-To: <4BD6AE1E.6050704@domain.hid> 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> Content-Type: text/plain; charset="UTF-8" Date: Tue, 27 Apr 2010 11:32:20 +0200 Message-ID: <1272360740.28983.382.camel@domain.hid> Mime-Version: 1.0 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: Jan Kiszka Cc: xenomai-core 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. > > Jan > -- Philippe.