Re: [PATCH v2] ALSA: pcm: Unlink substreams before releasing them

[Cen Zhang]

Hi Takashi,

Takashi Iwai <tiwai@xxxxxxx> 于2026年10月15日周四 15:18写道：
>
> The patch I submitted yesterday is such an attempt:
>   https://lore.kernel.org/20261014141212.316223-1-tiwai@xxxxxxx
>
> Unfortunately it couldn't be reviewed by Sashiko, as it's based on my
> latest tree.  And now I noticed that there is still a potential race,
> and fixed now.  Will resubmit v2 patch.
>
>
> thanks,
>
> Takashi

After read the v2 patch. I also have a minor concern: is there still a
lifetime window between snd_refcount_put() and snd_refcount_sync()?


Suppose drain(A) has pinned B's runtime, and close(B) has completed
unlink and drop. Could the following happen on two CPUs?

  drain(A)                         close(B)
  --------                         --------
  finish_wait(&rt->sleep, &wait);
  snd_refcount_put(&rt->refcnt):
    decrement count to zero
                                   snd_refcount_sync(&rt->refcnt);
                                     /* sees zero and returns */
                                   ...
                                   snd_pcm_detach_substream(B);
                                     kfree(rt);
    wake_up(&rt->refcnt.waiter);

Here rt is B's runtime. Since wait_event() can return on its initial
condition check, could the final wake_up() access the embedded
waitqueue after rt has been freed? A holds its own stream lock at
this point, while B has already been unlinked.

Best regards,
Cen Zhang