Re: [PATCH] ALSA: firewire-motu: Protect register DSP event queue positions

[Takashi Sakamoto]

Hi,

On Thu, May 21, 2026 at 08:01:23AM -0300, Cássio Gabriel wrote:
> The register DSP event queue is updated under parser->lock, but
> snd_motu_register_dsp_message_parser_count_event() reads pull_pos and
> push_pos without the lock.
> snd_motu_register_dsp_message_parser_copy_event() also reads both queue
> positions before taking the lock.
> 
> Protect these accesses with parser->lock as well. This keeps the hwdep
> poll/read path consistent with the producer side and with the cached
> meter/parameter accessors.
> 
> Fixes: 634ec0b2906e ("ALSA: firewire-motu: notify event for parameter change in register DSP model")
> Cc: stable@xxxxxxxxxxxxxxx
> Signed-off-by: Cássio Gabriel <cassiogabrielcontato@xxxxxxxxx>
> ---
>  sound/firewire/motu/motu-register-dsp-message-parser.c | 11 +++++++----
>  1 file changed, 7 insertions(+), 4 deletions(-)

Reviewed-by: Takashi Sakamoto <o-takashi@xxxxxxxxxxxxx>

I remember that I was investigating whether to use the
'snd_motu.hwdep_lock' spin lock when working for this code,
especially for 'pull_pos'. The 'snd_hwdep.exclusive' is used to constraint
the number of file descriptors to open it, and the spin lock would be
protect the concurrent access, since the pull_pos is referred and updated
by user process only.  However,
snd_motu_register_dsp_message_parser_copy_event() is called outside of
the spin lock. The suggested change is preferable.


Thanks

Takashi Sakamoto