ALSA: oss: Fix potential deadlock at unregistration

We took sound_oss_mutex around the calls of unregister_sound_special()
at unregistering OSS devices.  This may, however, lead to a deadlock,
because we manage the card release via the card's device object, and
the release may happen at unregister_sound_special() call -- which
will take sound_oss_mutex again in turn.

Although the deadlock might be fixed by relaxing the rawmidi mutex in
the previous commit, it's safer to move unregister_sound_special()
calls themselves out of the sound_oss_mutex, too.  The call is
race-safe as the function has a spinlock protection by itself.

Link: https://lore.kernel.org/r/CAB7eexJP7w1B0mVgDF0dQ+gWor7UdkiwPczmL7pn91xx8xpzOA@mail.gmail.com
Cc: <stable@vger.kernel.org>
Link: https://lore.kernel.org/r/20221011070147.7611-2-tiwai@suse.de
Signed-off-by: Takashi Iwai <tiwai@suse.de>

diff --git a/sound/core/sound_oss.c b/sound/core/sound_oss.c
index 7ed0a2a..2751bf2 100644 (file)
--- a/sound/core/sound_oss.c
+++ b/sound/core/sound_oss.c
@@ -162,7 +162,6 @@ int snd_unregister_oss_device(int type, struct snd_card *card, int dev)
                mutex_unlock(&sound_oss_mutex);
                return -ENOENT;
        }
-       unregister_sound_special(minor);
        switch (SNDRV_MINOR_OSS_DEVICE(minor)) {
        case SNDRV_MINOR_OSS_PCM:
                track2 = SNDRV_MINOR_OSS(cidx, SNDRV_MINOR_OSS_AUDIO);
@@ -174,12 +173,18 @@ int snd_unregister_oss_device(int type, struct snd_card *card, int dev)
                track2 = SNDRV_MINOR_OSS(cidx, SNDRV_MINOR_OSS_DMMIDI1);
                break;
        }
-       if (track2 >= 0) {
-               unregister_sound_special(track2);
+       if (track2 >= 0)
                snd_oss_minors[track2] = NULL;
-       }
        snd_oss_minors[minor] = NULL;
        mutex_unlock(&sound_oss_mutex);
+
+       /* call unregister_sound_special() outside sound_oss_mutex;
+        * otherwise may deadlock, as it can trigger the release of a card
+        */
+       unregister_sound_special(minor);
+       if (track2 >= 0)
+               unregister_sound_special(track2);
+
        kfree(mptr);
        return 0;
 }