On Thu, Mar 2, 2023 at 11:39 AM Alvin Šipraga <alvin@xxxxxxx> wrote:
>
> From: Alvin Šipraga <alsi@xxxxxxxxxxxxxxx>
>
> In the unbind callback for f_uac1 and f_uac2, a call to snd_card_free()
> via g_audio_cleanup() will disconnect the card and then wait for all
> resources to be released, which happens when the refcount falls to zero.
> Since userspace can keep the refcount incremented by not closing the
> relevant file descriptor, the call to unbind may block indefinitely.
> This can cause a deadlock during reboot, as evidenced by the following
> blocked task observed on my machine:
>
> task:reboot state:D stack:0 pid:2827 ppid:569 flags:0x0000000c
> Call trace:
> __switch_to+0xc8/0x140
> __schedule+0x2f0/0x7c0
> schedule+0x60/0xd0
> schedule_timeout+0x180/0x1d4
> wait_for_completion+0x78/0x180
> snd_card_free+0x90/0xa0
> g_audio_cleanup+0x2c/0x64
> afunc_unbind+0x28/0x60
> ...
> kernel_restart+0x4c/0xac
> __do_sys_reboot+0xcc/0x1ec
> __arm64_sys_reboot+0x28/0x30
> invoke_syscall+0x4c/0x110
> ...
>
> The issue can also be observed by opening the card with arecord and
> then stopping the process through the shell before unbinding:
>
> # arecord -D hw:UAC2Gadget -f S32_LE -c 2 -r 48000 /dev/null
> Recording WAVE '/dev/null' : Signed 32 bit Little Endian, Rate 48000 Hz, Stereo
> ^Z[1]+ Stopped arecord -D hw:UAC2Gadget -f S32_LE -c 2 -r 48000 /dev/null
> # echo gadget.0 > /sys/bus/gadget/drivers/configfs-gadget/unbind
> (observe that the unbind command never finishes)
>
> Fix the problem by using snd_card_free_when_closed() instead, which will
> still disconnect the card as desired, but defer the task of freeing the
> resources to the core once userspace closes its file descriptor.
It seems nobody has tested that use-case before. Thank you for fixing it
Reviewed-by: Ruslan Bilovol <ruslan.bilovol@xxxxxxxxx>
>
> Fixes: 132fcb460839 ("usb: gadget: Add Audio Class 2.0 Driver")
> Cc: stable@xxxxxxxxxxxxxxx
> Signed-off-by: Alvin Šipraga <alsi@xxxxxxxxxxxxxxx>
> ---
> drivers/usb/gadget/function/u_audio.c | 2 +-
> 1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/drivers/usb/gadget/function/u_audio.c b/drivers/usb/gadget/function/u_audio.c
> index c1f62e91b012..4a42574b4a7f 100644
> --- a/drivers/usb/gadget/function/u_audio.c
> +++ b/drivers/usb/gadget/function/u_audio.c
> @@ -1422,7 +1422,7 @@ void g_audio_cleanup(struct g_audio *g_audio)
> uac = g_audio->uac;
> card = uac->card;
> if (card)
> - snd_card_free(card);
> + snd_card_free_when_closed(card);
>
> kfree(uac->p_prm.reqs);
> kfree(uac->c_prm.reqs);
> --
> 2.39.1
>
