[PATCH 1/3] ALSA: 6fire: take stream_mutex in the Digital Thru control

["Piotr Gliźniewicz" <p.glizniewicz@gmail.com>]

usb6fire_control_digital_thru_put() calls
usb6fire_control_streaming_update(), which on an idle card with Digital
Thru on selects altsetting 2 and writes the clock register for 96 kHz
through usb6fire_control_set_rate(). usb6fire_pcm_prepare() calls both
functions through usb6fire_pcm_set_rate() under rt->stream_mutex. The
put does not take stream_mutex, so the two can run at the same time.

500 runs of a 0.2 s aplay at 48 kHz while a second loop toggled
"Digital Thru Playback Route" with amixer (12693 on/off cycles, kernel
7.2.9): in 216 cases the two usb_set_interface() calls ran at the same
time and the kernel logged "sysfs: cannot create duplicate filename
'.../3-2:1.1/ep_82'" with a stack dump. 27 of the aplay runs failed,
and for each of them the kernel logged "could not start pcm stream".

Take stream_mutex in the put. The pcm runtime is created before the
controls and freed with the card, so chip->pcm is valid whenever the
put runs. The put waits for a running prepare, which makes synchronous
USB transfers under stream_mutex; on a card that responds late the wait
can reach about 12 s, or 24 s with Digital Thru on, and during the wait
the other controls of the card can be neither read nor written, because
snd_ctl_elem_write() holds controls_rwsem around the put. Before this
patch the put already made transfers of the same kind under
controls_rwsem. No code that holds stream_mutex takes controls_rwsem.

The same test on kernel 7.2.10 with this patch (12863 cycles) gave no
failed aplay run and no "duplicate filename" message.

Fixes: 2475b0d40761 ("ALSA: 6fire - Add support of digital-thru mixer")
Assisted-by: LLM
Signed-off-by: Piotr Gliźniewicz <p.glizniewicz@gmail.com>
---
 sound/usb/6fire/control.c | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/sound/usb/6fire/control.c b/sound/usb/6fire/control.c
index 10104cb42..020e9676d 100644
--- a/sound/usb/6fire/control.c
+++ b/sound/usb/6fire/control.c
@@ -20,6 +20,7 @@
 #include "control.h"
 #include "comm.h"
 #include "chip.h"
+#include "pcm.h"
 
 static const char * const opt_coax_texts[2] = { "Optical", "Coax" };
 static const char * const line_phono_texts[2] = { "Line", "Phono" };
@@ -383,6 +384,12 @@ static int usb6fire_control_digital_thru_put(struct snd_kcontrol *kcontrol,
 	struct control_runtime *rt = snd_kcontrol_chip(kcontrol);
 	int changed = 0;
 
+	/*
+	 * with Digital Thru on, streaming_update() selects the altsetting and
+	 * writes the clock register of an idle card; prepare does the same
+	 * under stream_mutex
+	 */
+	guard(mutex)(&rt->chip->pcm->stream_mutex);
 	if (rt->digital_thru_switch != ucontrol->value.integer.value[0]) {
 		rt->digital_thru_switch = ucontrol->value.integer.value[0];
 		usb6fire_control_streaming_update(rt);
-- 
2.56.0