The OSS emulation in synth-emux helper has a potential AB/BA deadlock
at the simultaneous closing and opening:
close ->
snd_seq_release() ->
sne_seq_free_client() ->
snd_seq_delete_all_ports(): takes client->ports_mutex ->
port_delete() ->
snd_emux_unuse(): takes emux->register_mutex
open ->
snd_seq_oss_open() ->
snd_emux_open_seq_oss(): takes emux->register_mutex ->
snd_seq_event_port_attach() ->
snd_seq_create_port(): takes client->ports_mutex
This patch addresses the deadlock by reducing the rance taking
emux->register_mutex in snd_emux_open_seq_oss(). The lock is needed
for the refcount handling, so move it locally. The calls in
emux_seq.c are already with the mutex, thus they are replaced with the
version without mutex lock/unlock.
Cc: <stable@vger.kernel.org>
Signed-off-by: Takashi Iwai <tiwai@suse.de>
1c94e65c66