L4Linux threads - interrupt context

Oskar Senft o.senft at sirrix.com
Thu Aug 24 17:01:54 CEST 2006

Hi L4Linux hackers!

When creating threads within the L4Linux kernel and running the command

the thread obviously ;-) is being run in the interrupt context. This means
the thread (let's call it thread X) must not sleep and is restricted in
other ways. This implies, that I cannot run Linux functions that would
cause the thread X to sleep - that's clear.

On top of that, let's create the following scenario: some other task issues
an IPC call to the described L4Linux thread X waiting for IPC. For handling
the call, thread X would need to use Linux functions causing thread X to
sleep. This will cause a kernel panic, deadlock or other severe errors.

Our idea for solving that is to use *another* "normal" *Linux* thread
(let's call it Z) not running in interrupt context. This thread Z would
sleep until it is awakened by the L4Linux thread X and then call the Linux
function that causes thread Z to sleep. But: the L4Linux thread X would
have to send a reply to the IPC call immediately. So, I would need another
L4 thread Y (not known to L4Linux), acting as a notifier thread for the
calling task. This thread would receive the IPC notify call, go into a L4
semaphore waiting to be woken by thread Z.

So, the simple IPC call from another task which should use a Linux function
 requires *three* threads to run ...

If we had the possiblity to send the reply to an incoming IPC from
_another_ thread than the called thread, this at least would eliminate the
notifier thread Y.

If we had the possibility to run L4Linux threads (like thread X) in some
Linux context where it could sleep, this could even eliminate both threads
Y and Z.

Is there a simpler solution? Is it possible to make it easier?

Best regards

Sirrix AG security technologies - http://www.sirrix.com
Oskar Senft  eMail: o.senft at sirrix.com
Tel +49 (681) 936 251 - 119   Fax +49 (681) 936 251 - 519
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