[src/trunk]: src/sys/dev/audio audio(9): Call hw_if->getdev without sc_lock.

[riastradh <riastradh%NetBSD.org@localhost>]

details:   https://anonhg.NetBSD.org/src/rev/84681a244495
branches:  trunk
changeset: 1024111:84681a244495
user:      riastradh <riastradh%NetBSD.org@localhost>
date:      Sun Oct 10 11:20:29 2021 +0000

description:
audio(9): Call hw_if->getdev without sc_lock.

Holding sc_lock is not necessary -- I reviewed all ~70 cases in-tree,
and none of them rely on state protected by sc_lock.  Essentially
everything just copies from static data or data initialized at attach
time.

(Exceptions: tms320av110.c issues a bus_space_read_1, but I don't see
any reason why that needs to be serialized; and uaudio.c reads from
sc_dying, but that's not necessary and also not protected by sc_lock
anyway.)

Holding sc_lock is harmful because at least hdafg(4) can trigger module
autoload that leads to pserialize_perform, which waits for logic to run
at softint serial on all CPUs; at the same time, audio_softintr_rd/wr
run at softint serial and take sc_lock, so this leads to deadlock.

diffstat:

 sys/dev/audio/audio.c |  10 +++-------
 1 files changed, 3 insertions(+), 7 deletions(-)

diffs (47 lines):

diff -r b39fb4dd4d78 -r 84681a244495 sys/dev/audio/audio.c
--- a/sys/dev/audio/audio.c     Sun Oct 10 09:17:24 2021 +0000
+++ b/sys/dev/audio/audio.c     Sun Oct 10 11:20:29 2021 +0000
@@ -1,4 +1,4 @@
-/*     $NetBSD: audio.c,v 1.108 2021/09/26 01:16:08 thorpej Exp $      */
+/*     $NetBSD: audio.c,v 1.109 2021/10/10 11:20:29 riastradh Exp $    */
 
 /*-
  * Copyright (c) 2008 The NetBSD Foundation, Inc.
@@ -114,7 +114,7 @@
  *     halt_output             x       x +
  *     halt_input              x       x +
  *     speaker_ctl             x       x
- *     getdev                  -       x
+ *     getdev                  -       -
  *     set_port                -       x +
  *     get_port                -       x +
  *     query_devinfo           -       x
@@ -138,7 +138,7 @@
  */
 
 #include <sys/cdefs.h>
-__KERNEL_RCSID(0, "$NetBSD: audio.c,v 1.108 2021/09/26 01:16:08 thorpej Exp $");
+__KERNEL_RCSID(0, "$NetBSD: audio.c,v 1.109 2021/10/10 11:20:29 riastradh Exp $");
 
 #ifdef _KERNEL_OPT
 #include "audio.h"
@@ -3113,9 +3113,7 @@
                break;
 
        case AUDIO_GETDEV:
-               mutex_enter(sc->sc_lock);
                error = sc->hw_if->getdev(sc->hw_hdl, (audio_device_t *)addr);
-               mutex_exit(sc->sc_lock);
                break;
 
        case AUDIO_GETENC:
@@ -8291,9 +8289,7 @@
 
        case AUDIO_GETDEV:
                TRACE(2, "AUDIO_GETDEV");
-               mutex_enter(sc->sc_lock);
                error = sc->hw_if->getdev(sc->hw_hdl, (audio_device_t *)addr);
-               mutex_exit(sc->sc_lock);
                break;
 
        case AUDIO_MIXER_DEVINFO: