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

[Jia-Ju Bai <baijiaju1990@gmail.com>]

On 2020/1/12 16:20, Takashi Iwai wrote:
> 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.

Thanks for the reply :)

I am not sure to understand your words.

Do you mean that this code should be also protected by the spinlock?
     if (cm->pcm) {
         if ((status & CM_CHINT0) && cm->channel[0].running)
             snd_pcm_period_elapsed(cm->channel[0].substream);
         if ((status & CM_CHINT1) && cm->channel[1].running)
             snd_pcm_period_elapsed(cm->channel[1].substream);
     }


Best wishes,
Jia-Ju Bai