Cobalt Preemption of kernel update_fast_timekeeper can cause deadlocks

Lange Norbert norbert.lange at
Thu Dec 20 16:02:36 CET 2018

> -----Original Message-----
> From: Jan Kiszka <jan.kiszka at>
> Sent: Donnerstag, 20. Dezember 2018 14:33
> To: Lange Norbert <norbert.lange at>; Xenomai
> (xenomai at <xenomai at>
> Subject: Re: Cobalt Preemption of kernel update_fast_timekeeper can cause
> deadlocks
> On 20.12.18 13:29, Lange Norbert via Xenomai wrote:
> >> On 19.12.18 19:26, Auel, Kendall via Xenomai wrote:
> >>> Jan,
> >>>
> >>> I'm very much in favor of providing a way to prevent Xenomai modules
> >> from using features which can result in deadlock, if there is a clean
> >> way to detect such a situation.
> >>>
> >>> We used gettimeofday in one of our modules and it mostly worked
> great.
> >> But once in a great while the system would deadlock. Most calls to
> >> gettimeofday are benign and appear to work normally, which is why it
> >> is especially problematic. It would have saved some debug cycles if
> >> there was a kernel log message to warn us of our danger.
> >>>
> >>> Or perhaps we could collect a blacklist of references which will
> >>> produce
> >> warnings when linking a Xenomai module. All of these things are 'nice
> >> to have' but certainly not urgent matters.
> >>
> >> We do have the infrastructure and a small use case for such RT traps
> already:
> >> If you use --mode-check on xeno-config, any usage of malloc and free
> >> from RT contexts will be detected and reported. These calls are evil
> >> as well because they tend no not trigger a syscall in the fast path
> >> and only fail on contention or empty-pool situations of the userspace
> allocator.
> >
> > There is still the issue that the cobald kernel can interrupt the
> > linux kernel while holding a lock.
> > Consider the case that you have a 4 core CPU, several cobalt threads are
> bound to eg. Core 0 (legacy code assuming single core).
> >
> > 1) linux wants to update the timekeeper struct
> > 2) now cobalt preempts the linux kernel while holding the lock on Core
> > 0
> > 3) the cobalt threads run close to each other and thus Core 0 remains in
> cobalt domain for hundreds of ms.
> > 4) finally all cobalt threads (that are bound to core 0) idle and
> > linux can free the lock
> >
> > This means that all Linux threads on *any core* that try to call some
> *gettime functions (possible others) will busywait on the lock.
> >
> You do not need to look at the GTOD lock to construct such delays: every
> Linux spinlock taken on one core that is then interrupted by RT workload for
> a longer period can delay other cores doing Linux stuff that needs that lock.
> That is a generic property of the co-kernel architecture - and the reason you
> should allow Linux to run every few ms, on *every* core.

You are right, I did not realize that.
Userspace usually does not spinlock, so I consider those functions a lot more critical,
clock_gettime is also heavily used (especially for tracing).

Funny enough, the linux x86 vdso handles clock_gettime(CLOCK_MONOTONIC) but not clock_gettime(CLOCK_MONOTONIC_RAW).
Seems the common denominator would be to use rdtsc directly =/
(I know about the pitfalls, but our hardware should have a stable, invariant tsc)

> > That a rt thread (potentially just temporary promoted non-rt thread, or not
> lazily demoted yet) can additionally deadlock the system sits just on top of
> this issue.
> >
> > Regarding to what I am allowed to do:
> > AFAIK a thread started as cobalt thread can freely switch between
> domains, typically around syscalls and the switches are "lazy". What are the
> rules for a thread that needs to collect some data RT (potentially using some
> RT Mutexes with prio inheritance) calling into DSOs that aren’t compiled with
> the "cobalt wrappings" active (say a logging framework that uses libcs
> clock_gettime).
> > Do I manually have to demote the thread somehow before calling DSO
> functions, is it not allowed at all to use DSOs that were compiled with "cobalt
> wrappings"?
> If you are calling into an "unknown" non-RT blob, dropping from RT may
> actually be required. We do not promote explicit mode switches because
> they are not needed if you control (wrap) all your code. This might be  an
> exception.

The non-RT "blob" is the regular linux rootfs in my case, ie. libstdc++ and I plan
to use libnttg-ust and stuff like xml parsers.
I understand this as motivation to actually *have* the POSIX Skin (eases legacy code as well),
as soon as we can muster the time, then anything RT will be explicit and RT only

> >
> >> with posix, you are already
> >> redirected to the RT-safe implementations of those functions.
> >
> > In my case (posix skin, not "native" as I replied earlier), the call
> > came from another DSO which is unaffected by the link-time wrapping.
> > I would likely have to LD_PRELOAD a checker DSO, seems more sane to
> > me, as the calls could originate from implicitly linked DSO aswell
> > (C++ runtime library)
> Is the reason that the other DSOs are not caught at link-time generic or
> specific to your build? The former case should be documented if it exists.

If non-RT libstdc++ calls the function clock_gettime, then it will do so as it totally
ignores what your compiled code does.
Too hook into this, you would need to make sure, that your replacement "clock_gettime" will be in the symbol table before libc is loaded.

> Irrespective of that, I would definitely be interested in a LD_PRELOAD-based
> checker that you can attach to an application easily, without the need to
> switch to link-time wrapping (which is not needed with non-posix skins).

If you don’t know lttng-ust, you could spend a hour or two playing with it,
Eg you can interpose and trace any malloc/free by just preloading the wrapper:
LD_PRELOAD=liblttng-ust-libc-wrapper your_app

This could help with non-posix skins mixing with dangerous other functions aswell.


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