[AD] [patch] race condition in mixer.c

[Sam Hocevar <sam@xxxxxxxxxx>]

   There is a race condition in mixer.c that causes crashes on SMP
machines. _mix_some_samples() checks for mixer_voice[i].playing but
nothing guarantees that this value won't be changed before the samples
have been mixed.

   I could not find a way to fix this in an elegant way because it
would need a plugin API change, so I just fixed it for my platform
(pthreads). Patch enclosed so that everyone is aware of the problem.

Regards,
-- 
Sam.

--- allegro4-4.0.3.orig/src/mixer.c
+++ allegro4-4.0.3/src/mixer.c
@@ -27,6 +27,12 @@
 #include "allegro.h"
 #include "allegro/internal/aintern.h"
 
+#ifdef HAVE_LIBPTHREAD
+
+#include <pthread.h>
+static pthread_mutex_t voice_mutex;
+
+#endif
 
 
 typedef struct MIXER_VOICE
@@ -258,6 +264,10 @@
 
    mixer_lock_mem();
 
+#ifdef HAVE_LIBPTHREAD
+   pthread_mutex_init(&voice_mutex, NULL);
+#endif
+
    return 0;
 }
 
@@ -268,6 +278,10 @@
  */
 void _mixer_exit(void)
 {
+#ifdef HAVE_LIBPTHREAD
+   pthread_mutex_destroy(&voice_mutex);
+#endif
+
    if (mix_buffer) {
       free(mix_buffer);
       mix_buffer = NULL;
@@ -1065,6 +1079,10 @@
    for (i=0; i<mix_size/2; i++)
       *(l++) = 0x80008000;
 
+#ifdef HAVE_LIBPTHREAD
+   pthread_mutex_lock(&voice_mutex);
+#endif
+
    if (_sound_hq >= 2) {
       /* top quality interpolated 16 bit mixing */
       for (i=0; i<mix_voices; i++) {
@@ -1166,6 +1184,10 @@
       }
    }
 
+#ifdef HAVE_LIBPTHREAD
+   pthread_mutex_unlock(&voice_mutex);
+#endif
+
    _farsetsel(seg);
 
    /* transfer to conventional memory buffer using a clip table */
@@ -1232,9 +1254,15 @@
  */
 void _mixer_release_voice(int voice)
 {
+#ifdef HAVE_LIBPTHREAD
+   pthread_mutex_lock(&voice_mutex);
+#endif
    mixer_voice[voice].playing = FALSE;
    mixer_voice[voice].data8 = NULL;
    mixer_voice[voice].data16 = NULL;
+#ifdef HAVE_LIBPTHREAD
+   pthread_mutex_unlock(&voice_mutex);
+#endif
 }
 
 END_OF_FUNCTION(_mixer_release_voice);