linux-kernel - Re: [alsa-devel] 2.6.26.[6|7]-rt11, alsa rawmidi, seq hang

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Date:	Fri, 07 Nov 2008 15:12:29 -0800
From:	Fernando Lopez-Lezcano <nando@...ma.Stanford.EDU>
To:	Clemens Ladisch <clemens@...isch.de>
Cc:	Takashi Iwai <tiwai@...e.de>, Ingo Molnar <mingo@...e.hu>,
	alsa-devel@...a-project.org, linux-kernel@...r.kernel.org
Subject: Re: [alsa-devel] 2.6.26.[6|7]-rt11, alsa rawmidi, seq hang

On Fri, 2008-11-07 at 10:12 -0800, Fernando Lopez-Lezcano wrote:
> On Fri, 2008-11-07 at 10:12 +0100, Clemens Ladisch wrote:
> > Fernando Lopez-Lezcano wrote:
> > > I'm seeing a realtime patch related hard hang in the kernel alsa
> > > subsystem (MIDI input/output). In a nutshell:
> > >
> > > - alsa rawmidi works (ie: "rawmidi -v -i hw:0" outputs a stream of
> > > messages when pointed to a midi capable card that has an external
> > > keyboard connected).
> > >
> > > - the alsa sequencer interface works (ie: aplaymidi connected to
> > > aseqdump transfers data just fine).
> > >
> > > - BOTH combined do NOT work (ie: use aconnect to connect the port that
> > > corresponds to the external midi interface to aseqdump: aseqdump hangs
> > > forever after transferring the first message and the only way out is a
> > > reboot).
> > 
> > Please try the snd-virmidi driver, then we'd have a test case that does
> > not require MIDI hardware.
> > 
> > > ... including the output of a "echo t >/proc/sysrq-trigger" that
> > > should show where aseqdump currently hangs (or so I think).
> > 
> > It hangs in tasklet_kill(), which gets called while it tries to close
> > the rawmidi port.
> > 
> > The rawmidi framework uses this tasklet to notify the sequencer that new
> > MIDI data is available.  The handler function is
> > snd_rawmidi_input_event_tasklet() in sound/core/rawmidi.c; the sequencer
> > callback that gets called from there is snd_midi_input_event() in
> > core/seq/seq_midi.c.
> > 
> > You say that the first event gets delivered, so it might be possible
> > that the tasklet never finishes executing.  Please check whether the
> > call to snd_seq_kernel_client_dispatch() in snd_midi_input_event()
> > ever returns.
> 
> I added a printk before and after the call, it looks like it gets called
> and it returns (but only _once_... aseqdump hangs in an unkillable state
> as before).

Just in case, the tasklet keeps getting called as long as midi bytes are
received. So the tasklet runs once, snd_midi_input_event gets called
once and the tasklet keeps running again and again, but nobody runs
snd_midi_input_event. Who should?

-- Fernando


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