[BUG] ALSA: sb16: possible ABBA deadlock in snd_sb_csp_stop() and snd_sb_csp_load()

[Jia-Ju Bai]

Hello,

I find there is a possible ABBA deadlock in the SB16 driver in Linux 5.10:

In snd_sb_csp_stop():
876:     spin_lock_irqsave(&p->chip->mixer_lock, flags);
882:     spin_lock(&p->chip->reg_lock);

In snd_sb_csp_load():
614:     spin_lock_irqsave(&p->chip->reg_lock, flags);
653:     spin_lock(&p->chip->mixer_lock);

When snd_sb_csp_stop() and snd_sb_csp_load() are concurrently executed, 
the deadlock can occur.

I check the code and find a possible case of such concurrent execution:

#CPU1:
snd_sb16_playback_close
   snd_sb16_csp_playback_close (csp->ops.csp_stop(csp))
     snd_sb_csp_stop

#CPU2:
snd_sb_csp_ioctl
   snd_sb_csp_riff_load
     snd_sb_csp_load_user
       snd_sb_csp_load

I am not quite sure whether this possible deadlock is real and how to 
fix it if it is real.
Any feedback would be appreciated, thanks


Best wishes,
Jia-Ju Bai