Re: [Xenomai-help] Deadlock with interrupt shield activated / Avoid mode switches at compile time

[Philippe Gerum <rpm@xenomai.org>]

On Wed, 2010-07-28 at 10:41 +0200, Markus Seemann wrote:
> Hello,
> 
> a more theoretical question:
> 
> Is it possible to provocate a deadlock while switching a RTTask from primary mode to secondary mode when the interrupt shield is activated?
> Perhaps with this kind of scenario:
> - linux process takes semaphore
> - RTTask switches to secondary mode and also tries to take this semaphore
> - The semaphore would be released in ISR context of the linux kernel. But the interrupt shield prevents the execution of this isr.
> 
> How realistic is this scenario?

The I-shield would be disengaged as soon as a non-Xenomai thread is
scheduled in, resuming the linux ISR flow. So unless the CPU remains
100% busy to process Xenomai threads either in primary or secondary mode
- which would be the first problem to solve - this can't happen (the RT
task for one would block, pending on the semaphore, creating an
opportunity for non-RT threads to resume).

This is not to say that the I-shield might not bring other tricky
issue(s) in, but this one is not possible by design.

> 
> 
> Another question:
> Is it possible to avoid mode switches for realtime tasks at compile time for a specific task?
> 

Avoiding a priori would mean to deny compilation when a code path which
may lead to a linux syscall is detected; not quite simple to implement
for userland apps given otherwise valid dependencies on the glibc
Xenomai may have (e.g. for creating threads).

> Thanks in advance,
> Markus
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-- 
Philippe.