About Genode::Signal reception

Frank Kaiser frank.kaiser at ...24...
Tue Jul 6 10:10:36 CEST 2010

Hello, Norman


Thanks for your reply.

I don't think that a race condition contributes to the problem, because
the first signal reception takes place after the call tx.submit().
Anyway, I made the suggested change (remove the thread start from the
Receiver's constructor and do it explicitly before the instantiation of
Signal_transmitter), but again the signal was received twice.

I checked the properties of the signal, and indeed, as you supposed, on
the first reception the parameter num has the value 0, while on the
second reception the value is 1. This would allow to ignore the unwanted
signal as a temporary workaround, but the observed behaviour casts some
doubt on the reliability of the signalling mechanism. There might by
other situations where the signal is not at all received although it was
sent once.



> -----Original Message-----

> From: Norman Feske [mailto:norman.feske at ...1...]

> Sent: Monday, July 05, 2010 2:30 PM

> To: Genode OS Framework Mailing List

> Subject: Re: About Genode::Signal reception


> Hi Frank,


> > to fire a signal at the receiver for one time. The unexpected
outcome is

> > that it looks as if the receiver gets the signal twice.

> >

> > This is the implementation in /main()/:

> >     Signal_receiver s_rcvr;

> >     Signal_context ct;

> >     Receiver *ser_rx = new (env()->heap()) Receiver(s_rcvr,

> >     Signal_context_capability send_cap = s_rcvr.manage(&ct);

> >     Signal_transmitter tx(send_cap);

> >     tx.submit();


> I assume that you start executing the receiver thread by calling

> 'Thread_base::start()' in the receiver's constructor? If this is the

> case, you have a race condition. The receiver thread is executed right

> before you register the signal context at the signal receiver

> You can fix this by starting the receiver thread after calling

> 's_rcvr.manage':


> In the 'Receiver' class:


>   class Receiver : Thread<STACK_SIZE>

>   {

>     ...

> +   using Thread_base::start;

>     ...

>   };


> In your main function:


>   Receiver *ser_rx = new (env()->heap()) Receiver(s_rcvr,

>   Signal_context_capability send_cap = s_rcvr.manage(&ct);

> + ser_rx->start();

>   Signal_transmitter tx(send_cap);


> BTW, if your code relies on the correct number of signals (that is,

> just using signals as a wake-up mechanism), you need to take the value

> 'signal.num()' into account. If signals are batched, 'num' may be

> than one. In you current error case, the 'num' value returned by the

> first call of 'wait_for_signal' was indeed zero, indicating that is is

> no valid signal.


>   Signal signal = _receiver.wait_for_signal();

> + /* evaluate signal.num() */

> + ...


> Regards

> Norman


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