Re: [alsa-devel] [PATCH] ALSA: cmipci: Fix possible a data race in snd_cmipci_interrupt()

[Takashi Iwai <tiwai@suse.de>]

On Sat, 11 Jan 2020 17:30:27 +0100,
Jia-Ju Bai wrote:
> 
> The functions snd_cmipci_interrupt() and snd_cmipci_capture_trigger()
> may be concurrently executed.
> 
> The function snd_cmipci_capture_trigger() calls
> snd_cmipci_pcm_trigger(). In snd_cmipci_pcm_trigger(), the variable
> rec->running is written with holding a spinlock cm->reg_lock. But in
> snd_cmipci_interrupt(), the identical variable cm->channel[0].running
> or cm->channel[1].running is read without holding this spinlock. Thus,
> a possible data race may occur.
> 
> To fix this data race, in snd_cmipci_interrupt(), the variables
> cm->channel[0].running and cm->channel[1].running are read with holding 
> the spinlock cm->reg_lock.
> 
> This data race is found by the runtime testing of our tool DILP-2.
> 
> Signed-off-by: Jia-Ju Bai <baijiaju1990@gmail.com>

Thanks for the patch.

That's indeed a kind of race, but this change won't fix anything in
practice, though.  The inconsistent running flag between those places,
there are two cases:

- running became 0 to 1; this cannot happen, as the irq isn't issued
  before the stream gets started

- running became 1 to 0; this means that the stream gets stopped
  between two points, and it's not better to call
  snd_pcm_period_elapsed() for an already stopped stream.


Takashi

> ---
>  sound/pci/cmipci.c | 8 +++++---
>  1 file changed, 5 insertions(+), 3 deletions(-)
> 
> diff --git a/sound/pci/cmipci.c b/sound/pci/cmipci.c
> index dd9d62e2b633..f791152ec48f 100644
> --- a/sound/pci/cmipci.c
> +++ b/sound/pci/cmipci.c
> @@ -1430,7 +1430,7 @@ static int snd_cmipci_capture_spdif_hw_free(struct snd_pcm_substream *subs)
>  static irqreturn_t snd_cmipci_interrupt(int irq, void *dev_id)
>  {
>  	struct cmipci *cm = dev_id;
> -	unsigned int status, mask = 0;
> +	unsigned int run_flag0, run_flag1, status, mask = 0;
>  	
>  	/* fastpath out, to ease interrupt sharing */
>  	status = snd_cmipci_read(cm, CM_REG_INT_STATUS);
> @@ -1445,15 +1445,17 @@ static irqreturn_t snd_cmipci_interrupt(int irq, void *dev_id)
>  		mask |= CM_CH1_INT_EN;
>  	snd_cmipci_clear_bit(cm, CM_REG_INT_HLDCLR, mask);
>  	snd_cmipci_set_bit(cm, CM_REG_INT_HLDCLR, mask);
> +	run_flag0 = cm->channel[0].running;
> +	run_flag1 = cm->channel[1].running;
>  	spin_unlock(&cm->reg_lock);
>  
>  	if (cm->rmidi && (status & CM_UARTINT))
>  		snd_mpu401_uart_interrupt(irq, cm->rmidi->private_data);
>  
>  	if (cm->pcm) {
> -		if ((status & CM_CHINT0) && cm->channel[0].running)
> +		if ((status & CM_CHINT0) && run_flag0)
>  			snd_pcm_period_elapsed(cm->channel[0].substream);
> -		if ((status & CM_CHINT1) && cm->channel[1].running)
> +		if ((status & CM_CHINT1) && run_flag1)
>  			snd_pcm_period_elapsed(cm->channel[1].substream);
>  	}
>  	return IRQ_HANDLED;
> -- 
> 2.17.1
> 
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