[Qemu-devel] Re: SDL audio and AIO hogging each other's signals

[malc <av1474@comtv.ru>]

On Wed, 4 Apr 2007, andrzej zaborowski wrote:

> Hi, thanks for quick response!
>
> On 03/04/07, malc <av1474@comtv.ru> wrote:
>> On Tue, 3 Apr 2007, andrzej zaborowski wrote:

[..snip..]

> It should be using ALSA.
>
>> 
>> Here's my theory: signal will be delivered to the arbitrary thread
>> that happens to not block it, for whatever reason, the thread SDL
>> created to do audio processing is picked up, which leads to some
>> system call being interrupted(eventually) and -1 (errno == EINTR), SDL
>> happily continues calling stuff.

Actually the signal can be just handled (by whatever signal handler
QEMU installed with sigaction) in a SDL created thread, so things can,
and mostlikely will, be silently wrong, i.e. EINTR while possible will
not necessarily happen.

>
> Yes, reading the PTHREAD_SIGNAL(3) note, this sounds very likely.
>
>> 
>> One solution would be to explicitly block everything upon entering
>> sdl_callback for the first time. This is ugly and can have any
>> consequences one cares to imagine, but that's SDL for you (any
>> particular reason why you are using it?)
>
> Not really - just had only OSS and SDL compiled into qemu at this moment.
>
> Yes, the suggested solution works. Unfortunately it's neither pretty
> nor correct, because at the time sdl_callback runs, the signal which
> was supposed to wake up sigwait() is already lost and I can't find any
> way to prevent that - we can add a kill(0, SIGUSR2) but this is even
> uglier and again we don't know when sdl_callback is called as a result
> of this signal and when legally. (That's also why I didn't put a
> "return" after pthread_sigmask().)

[..snip..]

> What POSIX needs is a way to set the default signal mask for new threads :-/

http://www.opengroup.org/onlinepubs/007908799/xsh/pthread_create.html
in part reads
<quote>
The signal state of the new thread is initialised as follows:

     * The signal mask is inherited from the creating thread.
</quote>

Hence sequence of:

sigfillset (newset)
pthread_sigmask (SIG_BLOCK, newset, oldset)
SDL_OpenAudio (...)
pthread_sigmask (SGI_SETMASK, oldset, NULL)

Will probably achieve the desired effect.

-- 
vale