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SubjectRe: [PATCH] ALSA: cmipci: Fix possible a data race in snd_cmipci_interrupt()
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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