ALSA: als4000: Fix capture trigger chip->mode race

snd_als4000_capture_trigger() updates chip->mode under mixer_lock,
while snd_als4000_set_rate() and snd_als4000_playback_trigger()
serialize the same rate-lock state with reg_lock.

The PCM core serializes callbacks only per acted-on substream, or for an
explicitly linked group, so unlinked playback and capture streams can
run concurrently.

That leaves two races on ALS4000 rate-lock state:
- playback and capture trigger callbacks can concurrently update
  chip->mode and lose one of the SB_RATE_LOCK bits
- snd_als4000_set_rate() can observe chip->mode without the capture
  lock bit set and reprogram the shared sample rate while capture is
  being started

Fix this by taking reg_lock as the outer lock in
snd_als4000_capture_trigger() and nesting mixer_lock only for the CR1E
write. This keeps chip->mode serialized with the rest of the ALS4000
rate-lock users while preserving the existing CR1E programming
sequence.

Signed-off-by: Cássio Gabriel <cassiogabrielcontato@gmail.com>
Link: https://patch.msgid.link/20260417-als4000-capture-trigger-race-v1-1-daeffc2feb67@gmail.com
Signed-off-by: Takashi Iwai <tiwai@suse.de>

diff --git a/sound/pci/als4000.c b/sound/pci/als4000.c
index 33034e07b3d680746524a77bd1cbd2ce02d9bc28..636f309c942407c3b87eea8ba5d7f6518c430d9f 100644 (file)
--- a/sound/pci/als4000.c
+++ b/sound/pci/als4000.c
@@ -421,30 +421,26 @@ static int snd_als4000_capture_trigger(struct snd_pcm_substream *substream, int
 {
        struct snd_sb *chip = snd_pcm_substream_chip(substream);
        int result = 0;
-       
-       /* FIXME race condition in here!!!
-          chip->mode non-atomic update gets consistently protected
-          by reg_lock always, _except_ for this place!!
-          Probably need to take reg_lock as outer (or inner??) lock, too.
-          (or serialize both lock operations? probably not, though... - racy?)
-       */
-       guard(spinlock)(&chip->mixer_lock);
-       switch (cmd) {
-       case SNDRV_PCM_TRIGGER_START:
-       case SNDRV_PCM_TRIGGER_RESUME:
-               chip->mode |= SB_RATE_LOCK_CAPTURE;
-               snd_als4_cr_write(chip, ALS4K_CR1E_FIFO2_CONTROL,
-                                                        capture_cmd(chip));
-               break;
-       case SNDRV_PCM_TRIGGER_STOP:
-       case SNDRV_PCM_TRIGGER_SUSPEND:
-               chip->mode &= ~SB_RATE_LOCK_CAPTURE;
-               snd_als4_cr_write(chip, ALS4K_CR1E_FIFO2_CONTROL,
-                                                        capture_cmd(chip));
-               break;
-       default:
-               result = -EINVAL;
-               break;
+
+       guard(spinlock)(&chip->reg_lock);
+       scoped_guard(spinlock, &chip->mixer_lock) {
+               switch (cmd) {
+               case SNDRV_PCM_TRIGGER_START:
+               case SNDRV_PCM_TRIGGER_RESUME:
+                       chip->mode |= SB_RATE_LOCK_CAPTURE;
+                       snd_als4_cr_write(chip, ALS4K_CR1E_FIFO2_CONTROL,
+                                         capture_cmd(chip));
+                       break;
+               case SNDRV_PCM_TRIGGER_STOP:
+               case SNDRV_PCM_TRIGGER_SUSPEND:
+                       chip->mode &= ~SB_RATE_LOCK_CAPTURE;
+                       snd_als4_cr_write(chip, ALS4K_CR1E_FIFO2_CONTROL,
+                                         capture_cmd(chip));
+                       break;
+               default:
+                       result = -EINVAL;
+                       break;
+               }
        }
        return result;
 }