Fatal exceptions on context-switching for more than two isolated threads

[Utkarsh Rai]

On Fri, Oct 23, 2020 at 9:57 PM Gedare Bloom <gedare at rtems.org> wrote:

> On Fri, Oct 23, 2020 at 9:37 AM Utkarsh Rai <utkarsh.rai60 at gmail.com>
> wrote:
> >
> >
> >
> > On Thu, Oct 22, 2020 at 11:23 PM Sebastian Huber <
> sebastian.huber at embedded-brains.de> wrote:
> >>
> >> On 22/10/2020 02:40, Utkarsh Rai wrote:
> >>
> >> > Hello, this thread has gone a bit cold over the last few weeks, due to
> >> > my engagement in the university tests.  I have provided a debug trace
> >> > for the issue.  The reason for fatal exceptions is the fact that while
> >> > iterating over the chain of user extensions we access a node whose
> >> > memory location has been set to NO-ACCESS during the context switch.
> >> > Is there any work-around to this?
> >>
> >> The option is to move the User_extensions_Iterator storage out of the
> >> stack to Thread_Control and Per_CPU_Control.
> >>
> >
> > Does this mean that I should add User_extensions_Iterator field in the
> Thread_Control structure for the case
> > when we enable thread stack protection?
> >
>
> You need to dig in a little bit more. From what I understand, there is
> the last_user_extensions_iterator field of the TCB. That is probably
> where you are running into some trouble. See
> score/src/userextiterate.c :193 where this field gets assigned to a
> stack-local variable.


I get an exception just before this when _Chain_Iterator_initialize() is
called, the reason though is the same, accessing
a stack-local variable of a switched out thread. From what I could
understand,  the 'iter' variable(corresponding to the
User_extensions_Iterator type)
is the only stack-local variable of a switched out thread that is accessed
from a different thread.


> Then, you can't walk this chain when the thread
> is out of context.
>
> >>
> >> --
> >> Sebastian Huber, embedded brains GmbH
> >>
> >> Address : Dornierstr. 4, D-82178 Puchheim, Germany
> >> Phone   : +49 89 189 47 41-16
> >> Fax     : +49 89 189 47 41-09
> >> E-Mail  : sebastian.huber at embedded-brains.de
> >> PGP     : Public key available on request.
> >>
> >> Diese Nachricht ist keine geschäftliche Mitteilung im Sinne des EHUG.
> >>
>
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