winmm: Allow a FORCE_SHUTDOWN on driver close?

[Maarten Lankhorst]

Peter Beutner schreef:
> AFAIK you get the DRVM_EXIT message only when winmm is unloading.
> Alternatively you could do it in response to the DRV_CLOSE message which winmm
> sends just before it tries to unload the driver.
>
> That said I don't see how your code in ALSA_MixerExit() could deadlock.
> It effectively tries to exit the thread the same way as MIX_Close, doesn't it?
> So the thread should exit safely.
> Imo the question is rather why the thread isn't exiting and you are waiting
> forever. Forcibly calling TerminateThread just covers the problem
The deadlock isn't in the alsa code, it's in the kernel code:
Kernel holds thread lock and calls closing winmm code, closing code
closes mixer driver, mixer closing waits on thread, thread waits till it
can hold thread lock -> deadlock.

Under normal circumstances, MIX_Open will be called as often as
MIX_Close is. If the programmer is lazy and never closes its handle,
MIX_Close will be called when winmm is shutting down, in that case I
would want to pass some kind of signal that it can't wait on thread exit
because it would deadlock. The thread should still be terminated in
MIX_Close then, so unless you pass a flag that application hasn't shut
down the device driver properly, this potentially causes the deadlock to
occur.
The thread termination in ALSA_MixerExit should never happen: it would
indicate a bug in winmm that it doesn't close mixer before releasing it,
but I put it in as a safeguard, that's why it will print an ERR too.

Maarten