On Fri, Apr 17, 2015 at 08:26:32PM +0200, Jan Kiszka wrote:
> On 2015-04-17 20:24, Gilles Chanteperdrix wrote:
> > On Fri, Apr 17, 2015 at 08:17:26PM +0200, Jan Kiszka wrote:
> >> On 2015-04-17 20:12, Gilles Chanteperdrix wrote:
> >>> On Fri, Apr 17, 2015 at 08:10:40PM +0200, Jan Kiszka wrote:
> >>>> On 2015-04-17 20:08, Gilles Chanteperdrix wrote:
> >>>>> On Fri, Apr 17, 2015 at 08:05:57PM +0200, Jan Kiszka wrote:
> >>>>>> On 2015-04-17 19:50, Gilles Chanteperdrix wrote:
> >>>>>>> On Fri, Apr 17, 2015 at 07:34:30PM +0200, Jan Kiszka wrote:
> >>>>>>>> Hi,
> >>>>>>>>
> >>>>>>>> analyzing page faults of an application that prefers to set its own
> >>>>>>>> stacks, I noticed a problem in Xenomai (2 and 3), at least from the
> >>>>>>>> usability POV: We document the minimum stack stack as
> >>>>>>>> PTHREAD_STACK_MIN
> >>>>>>>> + 1 page, at least in Xenomai 3, and we enforce that on thread
> >>>>>>>> creation.
> >>>>>>>> However, enforcement is doomed to fail if the stack is preallocated
> >>>>>>>> (and
> >>>>>>>> that too small).
> >>>>>>>>
> >>>>>>>> As we cannot detect if the user set a stack address in
> >>>>>>>> pthread_attr_t, I
> >>>>>>>> would suggest to fail thread creation instead of performing it with
> >>>>>>>> improper parameters. Other suggestions? If not, I would prepare a
> >>>>>>>> patch
> >>>>>>>> for Xenomai 3 (for 2 only if desired).
> >>>>>>>
> >>>>>>> It seems to me we can detect the parameters in the pthread_attr_t
> >>>>>>> using pthread_attr_getstack. So, we can get __wrap_pthread_create to
> >>>>>>> fail if the size is not sufficient.
> >>>>>>
> >>>>>> Nope, unfortunately not:
> >>>>>>
> >>>>>> "If the pthread_attr_getstack() function is called before the stackaddr
> >>>>>> attribute has been set, the behavior is unspecified."
> >>>>>
> >>>>> It is unspecified by POSIX, but Xenomai supports only two
> >>>>> implementations, glibc and uClibc, so, we can look at what these two
> >>>>> libraries do. I would bet they return you a NULL stack pointer or
> >>>>> something.
> >>>>
> >>>> I would have expected that, too, but the results for glibc seem random.
> >>>> Plus there is the risk that something changes, thus we become
> >>>> version-dependent.
> >>>
> >>> Ok then, what about the influence of pthread_attr_setstack() on
> >>> pthread_attr_getstacksize(), maybe more luck there?
> >>
> >> setstack defines the size getstacksize returns. So does setstacksize.
> >>
> >> What happens during setstacksize is apparently that the address is set
> >> to NULL - size. But, again, that is just the current glibc behaviour.
> >
> > Yes, ok, but in order to test the stack size passed by the user, it
> > simply means we can use getstacksize and bail out if not sufficient.
>
> Sure, that would be the best way.
>
> Along that, I would recommend raising the minimum to 2*PTHREAD_STACK_MIN.
What for ? We know that the system crashes if the stack is too
small, but that is the consequence of the user choice, there is not
much we can do about it. What I would concentrate on, and I think we
had that on xenomai 2.x is to have a reasonable default stack size.
Actually, we could wrap pthread_attr_init to define our own default
and be done with it.
--
Gilles.
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