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

[Jia-Ju Bai]

On 2021/7/15 18:33, Takashi Iwai wrote:
> On Thu, 15 Jul 2021 12:20:36 +0200,
> Jia-Ju Bai wrote:
>> 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
> The impact must be quite low, as both functions run in different
> state (running or stopped), so those are basically exclusive.
> And, above all, there is no VM supporting this chip, hence it's only
> for the real hardware and it's about very old ISA boards that maybe
> only less than handful people in the world can run now.
>
> About the fix: just split the locks in snb_sb_csp_stop() (also
> snd_sb_csp_start()) like below should suffice.

Thanks for the feedback and explanation :)
The patch looks good to me.

>
> --- a/sound/isa/sb/sb16_csp.c
> +++ b/sound/isa/sb/sb16_csp.c
> @@ -816,6 +816,7 @@ static int snd_sb_csp_start(struct snd_sb_csp * p, int sample_width, int channel
>   	mixR = snd_sbmixer_read(p->chip, SB_DSP4_PCM_DEV + 1);
>   	snd_sbmixer_write(p->chip, SB_DSP4_PCM_DEV, mixL & 0x7);
>   	snd_sbmixer_write(p->chip, SB_DSP4_PCM_DEV + 1, mixR & 0x7);
> +	spin_unlock_irqrestore(&p->chip->mixer_lock, flags);
>   
>   	spin_lock(&p->chip->reg_lock);
>   	set_mode_register(p->chip, 0xc0);	/* c0 = STOP */
> @@ -855,6 +856,7 @@ static int snd_sb_csp_start(struct snd_sb_csp * p, int sample_width, int channel
>   	spin_unlock(&p->chip->reg_lock);
>   
>   	/* restore PCM volume */
> +	spin_lock_irqsave(&p->chip->mixer_lock, flags);
>   	snd_sbmixer_write(p->chip, SB_DSP4_PCM_DEV, mixL);
>   	snd_sbmixer_write(p->chip, SB_DSP4_PCM_DEV + 1, mixR);
>   	spin_unlock_irqrestore(&p->chip->mixer_lock, flags);
> @@ -880,6 +882,7 @@ static int snd_sb_csp_stop(struct snd_sb_csp * p)
>   	mixR = snd_sbmixer_read(p->chip, SB_DSP4_PCM_DEV + 1);
>   	snd_sbmixer_write(p->chip, SB_DSP4_PCM_DEV, mixL & 0x7);
>   	snd_sbmixer_write(p->chip, SB_DSP4_PCM_DEV + 1, mixR & 0x7);
> +	spin_unlock_irqrestore(&p->chip->mixer_lock, flags);
>   
>   	spin_lock(&p->chip->reg_lock);
>   	if (p->running & SNDRV_SB_CSP_ST_QSOUND) {
> @@ -894,6 +897,7 @@ static int snd_sb_csp_stop(struct snd_sb_csp * p)
>   	spin_unlock(&p->chip->reg_lock);
>   
>   	/* restore PCM volume */
> +	spin_lock_irqsave(&p->chip->mixer_lock, flags);
>   	snd_sbmixer_write(p->chip, SB_DSP4_PCM_DEV, mixL);
>   	snd_sbmixer_write(p->chip, SB_DSP4_PCM_DEV + 1, mixR);
>   	spin_unlock_irqrestore(&p->chip->mixer_lock, flags);