Context switching through an ISR in RTEMS

[Utkarsh Rai]

On Thu, Aug 13, 2020 at 5:10 AM Utkarsh Rai <utkarsh.rai60 at gmail.com> wrote:

> Thanks, I'll check them out.
>
> On Thu, Aug 13, 2020 at 12:56 AM Gedare Bloom <gedare at rtems.org> wrote:
>
>> On Wed, Aug 12, 2020 at 11:33 AM Utkarsh Rai <utkarsh.rai60 at gmail.com>
>> wrote:
>> >
>> > Hello,
>> > I have been testing my code for thread stack isolation against various
>> tests( Some written by me, and remaining already present). One of the
>> limitations that I have found is that I encounter fatal errors whenever a
>> context switch takes place through an ISR. Can you please explain how the
>> context switching procedure works when an interrupt occurs. When I use gdb
>> for stepping through the code it asynchronously moves to context switching
>> code from the executing thread( for example psx16 test).
>> > For thread stack protection,  the part that deals with context
>> switching simply 'sets 'the memory entries of the heir stack and 'unsets'
>> that of the executing stack.
>>
>> There are two issues to start: interrupt stacks and dispatching from an
>> ISR.
>>
>> I think you can start by reading some of the documentation:
>>
>> https://docs.rtems.org/branches/master/c-user/interrupt_manager.html#processing-an-interrupt
>>
>>
>> https://docs.rtems.org/branches/master/c-user/scheduling_concepts.html#dispatching-tasks
>>
>>
>> https://docs.rtems.org/branches/master/c-user/config/general.html#configure-interrupt-stack-size
>>
>>
>> https://docs.rtems.org/branches/master/cpu-supplement/port.html#interrupt-processing
>>
>> You can also find some material in rtems-docs.git/porting -- I don't
>> know where that gets generated.
>>
>> Continue to ask questions, and writing blog posts.
>>
>
So after going through the materials, I was able to understand how an ISR
is registered, ISR stack initialization. What is still not clear to me is
what are the differences between dispatching a task in ISR different and  a
normal context-switch?

For example the psxsignal06 test, we wait for a signal here
<https://git.rtems.org/rtems/tree/testsuites/psxtests/psxsignal06/init.c#n66>,
on setting the breakpoint at the context switch code (cpu_asm.S), after
this line,  I find that the heir context stack is the ISR stack. The next
thread is dispatched from this ISR but as soon as I unset the memory
attributes of the ISR stack I get a fatal error. One possible reason is
that the ISR stack is not page aligned and unsettling its attributes unsets
nearby memory regions. Is there something else that I am missing?
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.rtems.org/pipermail/devel/attachments/20200815/660dad58/attachment.html>