ALSA: seq: oss: Fix data-race for max_midi_devs access

stable inclusion
from stable-v5.10.142
commit f19a209f6156f1477a9980332c3b08896d62413f
category: bugfix
bugzilla: https://gitee.com/openeuler/kernel/issues/I6CSFH

Reference: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=f19a209f6156f1477a9980332c3b08896d62413f



--------------------------------

commit 22dec134 upstream.

ALSA OSS sequencer refers to a global variable max_midi_devs at
creating a new port, storing it to its own field.  Meanwhile this
variable may be changed by other sequencer events at
snd_seq_oss_midi_check_exit_port() in parallel, which may cause a data
race.

OTOH, this data race itself is almost harmless, as the access to the
MIDI device is done via get_mdev() and it's protected with a refcount,
hence its presence is guaranteed.

Though, it's sill better to address the data-race from the code sanity
POV, and this patch adds the proper spinlock for the protection.

Reported-by: Abhishek Shah <abhishek.shah@columbia.edu>
Cc: <stable@vger.kernel.org>
Link: https://lore.kernel.org/r/CAEHB2493pZRXs863w58QWnUTtv3HHfg85aYhLn5HJHCwxqtHQg@mail.gmail.com
Link: https://lore.kernel.org/r/20220823072717.1706-1-tiwai@suse.de


Signed-off-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Jialin Zhang <zhangjialin11@huawei.com>
Reviewed-by: Zheng Zengkai <zhengzengkai@huawei.com>