[Openmcl-devel] Asynchronous callback made from real-time thread
letz at rd.grame.fr
Fri May 30 20:33:05 UTC 2003
>On Fri, 30 May 2003, Letz Stephane wrote:
>> We are trying to interface a Midi package (MidiShare :
>> www.grame.fr/MidiShare) with OpenMCL. Using MidiShare allows a Lisp
>> program to access a full Midi API : receive, send Midi events, tasks
>> For real-time handling of incoming Midi events, there is a safe
>> solution that consist in doing polling on the incoming event fifo in
>> a Lisp thread.
>> We tried to implement a more real-time way by calling back the Lisp
>> code directly from the real-time Midi thread using a lisp callback
>> defined with defcallback.
>> This seems to almost work on 0.13.5 version but crash after some
>> time. It crash immediately with the 0.14 alpha version.
>I have a hunch that the "seems to work on 0.13.5" part means "is
>able to run for a while scribbling over some other threads' stacks
>before anything actually notices."
>Do you remember how it crashes in 0.14 ? If it dies with a kernel
>debugger message saying something like "No TCR for thread ...", that's
>actually fairly mundane.
It gives this kind of messages:
Thread 6 Crashed:
#0 0x00122de4 in tsd_get (thread_manager.c:361)
#1 0x00123850 in get_tcr (thread_manager.c:681)
#2 0x00123ac4 in suspend_other_threads (thread_manager.c:745)
#3 0x001215e0 in Bug (lisp-exceptions.c:2442)
#4 0x00123888 in get_tcr (thread_manager.c:688)
#5 0x00123ac4 in suspend_other_threads (thread_manager.c:745)
>If a thread that lisp didn't create tries to
>run a lisp callback, some stacks and other thread-specific data
>structures need to be created and the GC needs to be told about the
>new thread. The code to accomplish that isn't too different from the
>code that creates a lisp thread in the first place. (At least I don't
>think that it should be: that case isn't handled yet, and all that
>happens in that case is a trip to the kernel debugger. There also
>needs to be a little bit of work done on the lisp side in such a case
>to establish a lisp process and to establish some special variable
>bindings and generally make the thread look more like other lisp
>If you're seeing some other type of crash in 0.14, please let me know
>what you can about it. From what you describe, the "No TCR for thread"
>error sounds like a likely result.
>> Is it possible to implement this kind of asynchronous callback made
>> from real-time thread in OpenMCL?
>Calling lisp code from any native thread (other than the initial one)
>can't possibly work in 0.13.5.
>Calling lisp code from native threads is ... what happens all the time
>in 0.14. (If lisp didn't create the thread, there's some lisp-specific
>setup that's supposed to happen on the first callback on such a thread;
>once that happens, the thread should look like any other.)
Could you explain this a little more?
>It is -very- hard to support reentrancy: if the handler can be invoked
>while it's running, that's very hard to deal with at the application
>level and even harder to support at the level of the lisp runtime.
>(The cases where it can be supported at all require access to the
>thread's context - as provided to a signal handler - and require a bit
>of care to ensure that atomic operations like memory allocation happen
I don't think that we have to support reentrancy on the Lisp side.
The real-time C Midi handler already deal with reentrancy.
>If the realtime thread is doing something like:
> (loop ; most of this is probably really C code ...
There is no blocking (wait-for-midi-event) function of the C side.
But it can probably be implemented using a condition variable.
>that shouldn't be a problem (and is very, very similar to what happens
>all the time in 0.14)
>> Is there any special issue to
>> consider : thread synchronization, interaction with the garbage
>Yes there are such issues. Fortunately, you shouldn't have to deal
>with many of them ... (you -do- need to deal with application-specific
>The GC in OpenMCL is fairly quick, but it's not real-time or very
>close to it. It's also not concurrent: it suspends all other threads
>while it's running. I don't know what kind of real-time constraints
>might be involved in MIDI programming, but (if and when things are
>running reliably) you probably want to minimize consing.
Is there a way to disable GC during the realtime callback?
>Note that if the MidiShare package allows you to do a blocking wait
>on the event fifo (as the real-time thread presumably does), that may
>also be a reasonable approach (and one which avoids the "callback on
>non-lisp thread" complexity.)
Having a blocking wait on the event fifo is probably possible, but
i'm interested also to know more about the "complex" solution (:
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