Help with nested signal context warnings

Colin Parker cvparker at
Sat Jan 30 06:38:43 CET 2021

Hello Genodians,
    I'm hoping someone can help or point me to a good reference for the
error that pops up occasionally saying "attempt to handle the same signal
context twice (nested)". This happens within some Sculpt components
(decorator, depot_query), but also happens in this USB wifi driver I'm
trying to make. Mostly I want to understand it - my long explanation is

My basic understanding is Genode gets upset when waiting for a signal
within another signal handler. However, the "natural" way I would like this
driver to behave is to be signal driven - all code is responding to some
signal or other. However, sometimes one needs to do synchronous IO
operations. If it is truly forbidden ever to wait within a signal handler,
I considered a few options, but found all to be non-ideal.
1) Use stub signal handlers to insert elements into a work queue and drive
everything from an event loop. This seems like reinventing the signal
mechanism, but might be what's required.
2) Build state machines to break up synchronous IO operations with no
waiting at all. This makes simple-looking operations complicated and
increases debugging complexity.
3) Busy-wait for synchronous operations. Probably a big performance
penalty, and obviously wasteful.

So I proceeded, because I do notice that sometimes it is OK to wait within
a handler (i.e. the warning does not occur). But I still get the warning
other times, and I can't quite figure out when or why it happens.
Initially, I thought that if I separated things out, so that, say,
Signal_context A waits, but can only receive signals for Signal_context B
or C, it would be OK. Now, I am not totally sure I've actually achieved
this, but I think that I have, and I've become suspicious that I don't
really understand the "rules." So, is there a way to understand when one
can wait safely wait within a signal handler? Is it really as simple as
"Signal A cannot be generated while waiting if a handler for A is on the
stack?" Does the App vs IO signal distinction come into play (I have both).
Also, is there a way to get a Genode app to output a stack trace - I can
patch that warning mesage to output a stack trace and at least see what
actually happened?

Thanks as always to the Genode community for sharing your knowledge with me.

Best regards,
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