[src/trunk]: src/sys/dev/audio audio(4): Membar audit.

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

details:   https://anonhg.NetBSD.org/src/rev/0addb514621d
branches:  trunk
changeset: 363509:0addb514621d
user:      riastradh <riastradh%NetBSD.org@localhost>
date:      Mon Mar 14 11:47:33 2022 +0000

description:
audio(4): Membar audit.

Won't affect anything on x86 because atomic r/m/w operations are
always full sequential consistency barriers, but might potentially
fix problems on, e.g., arm.

Note 1: I'm not clear on why the track lock is a bespoke mutex made
out of an atomic -- why not just mutex(9)?

Note 2: I'm not convinced the audio_mlog_flush synchronization is
correct; what happens if the softint runs on two CPUs at the same
time and swaps mlog_wpage simultaneously?

Note 3: Should maybe use atomic_load/store_relaxed for mlog_full and
mlog_drop, and atomic_inc/dec for mlog_refs.

diffstat:

 sys/dev/audio/audio.c |  23 ++++++++++++++++-------
 1 files changed, 16 insertions(+), 7 deletions(-)

diffs (94 lines):

diff -r 5c7ef6605319 -r 0addb514621d sys/dev/audio/audio.c
--- a/sys/dev/audio/audio.c     Mon Mar 14 07:05:31 2022 +0000
+++ b/sys/dev/audio/audio.c     Mon Mar 14 11:47:33 2022 +0000
@@ -1,4 +1,4 @@
-/*     $NetBSD: audio.c,v 1.113 2021/12/12 13:05:13 andvar Exp $       */
+/*     $NetBSD: audio.c,v 1.114 2022/03/14 11:47:33 riastradh Exp $    */
 
 /*-
  * Copyright (c) 2008 The NetBSD Foundation, Inc.
@@ -138,7 +138,7 @@
  */
 
 #include <sys/cdefs.h>
-__KERNEL_RCSID(0, "$NetBSD: audio.c,v 1.113 2021/12/12 13:05:13 andvar Exp $");
+__KERNEL_RCSID(0, "$NetBSD: audio.c,v 1.114 2022/03/14 11:47:33 riastradh Exp $");
 
 #ifdef _KERNEL_OPT
 #include "audio.h"
@@ -162,6 +162,7 @@
 #include <sys/kauth.h>
 #include <sys/kernel.h>
 #include <sys/kmem.h>
+#include <sys/lock.h>
 #include <sys/malloc.h>
 #include <sys/mman.h>
 #include <sys/module.h>
@@ -272,13 +273,14 @@
        /* Nothing to do if already in use ? */
        if (atomic_swap_32(&mlog_inuse, 1) == 1)
                return;
+       membar_enter();
 
        int rpage = mlog_wpage;
        mlog_wpage ^= 1;
        mlog_buf[mlog_wpage][0] = '\0';
        mlog_used = 0;
 
-       atomic_swap_32(&mlog_inuse, 0);
+       atomic_store_reease(&mlog_inuse, 0);
 
        if (mlog_buf[rpage][0] != '\0') {
                printf("%s", mlog_buf[rpage]);
@@ -308,6 +310,7 @@
                mlog_drop++;
                return;
        }
+       membar_enter();
 
        va_start(ap, fmt);
        len = vsnprintf(
@@ -321,7 +324,7 @@
                mlog_full++;
        }
 
-       atomic_swap_32(&mlog_inuse, 0);
+       atomic_store_release(&mlog_inuse, 0);
 
        if (mlog_sih)
                softint_schedule(mlog_sih);
@@ -1652,7 +1655,11 @@
 static __inline bool
 audio_track_lock_tryenter(audio_track_t *track)
 {
-       return (atomic_cas_uint(&track->lock, 0, 1) == 0);
+
+       if (atomic_swap_uint(&track->lock, 1) != 0)
+               return false;
+       membar_enter();
+       return true;
 }
 
 /*
@@ -1661,9 +1668,10 @@
 static __inline void
 audio_track_lock_enter(audio_track_t *track)
 {
+
        /* Don't sleep here. */
        while (audio_track_lock_tryenter(track) == false)
-               ;
+               SPINLOCK_BACKOFF_HOOK;
 }
 
 /*
@@ -1672,7 +1680,8 @@
 static __inline void
 audio_track_lock_exit(audio_track_t *track)
 {
-       atomic_swap_uint(&track->lock, 0);
+
+       atomic_store_release(&track->lock, 0);
 }