threads: use thread ID for mutex owner

include/vlc_threads.h

@@ -228,13 +228,13 @@ typedef struct
         struct {
             atomic_uint value;
             atomic_uint recursion;
-            _Atomic (const void *) owner;
+            atomic_ulong owner;
         };
 #endif
         struct {
             unsigned int value;
             unsigned int recursion;
-            const void *owner;
+            unsigned long owner;
         } dummy;
     };
 } vlc_mutex_t;
@@ -248,7 +248,7 @@ typedef struct
 #define VLC_STATIC_MUTEX { \
     .value = ATOMIC_VAR_INIT(0), \
     .recursion = ATOMIC_VAR_INIT(0), \
-    .owner = ATOMIC_VAR_INIT(NULL), \
+    .owner = ATOMIC_VAR_INIT(0), \
 }
 
 /**
@@ -803,8 +803,8 @@ VLC_API void vlc_control_cancel(vlc_cleanup_t *);
 /**
  * Thread identifier.
  *
- * This function returns an unique identifier of the calling thread. The
- * identifier cannot change for the entire lifetime of the thread, and two
+ * This function returns a non-zero unique identifier of the calling thread.
+ * The identifier cannot change for the entire lifetime of the thread, and two
  * concurrent threads cannot have the same identifier.
  *
  * The thread identifier has no defined semantics other than uniqueness,

src/misc/threads.c

@@ -117,7 +117,7 @@ static void vlc_mutex_init_common(vlc_mutex_t *mtx, bool recursive)
 {
     atomic_init(&mtx->value, 0);
     atomic_init(&mtx->recursion, recursive);
-    atomic_init(&mtx->owner, NULL);
+    atomic_init(&mtx->owner, 0);
 }
 
 void vlc_mutex_init(vlc_mutex_t *mtx)
@@ -130,9 +130,6 @@ void vlc_mutex_init_recursive(vlc_mutex_t *mtx)
     vlc_mutex_init_common(mtx, true);
 }
 
-static _Thread_local char thread_self[1];
-#define THREAD_SELF ((const void *)thread_self)
-
 bool vlc_mutex_held(const vlc_mutex_t *mtx)
 {
 #if defined(__clang__) && !defined(__apple_build_version__) && (__clang_major__ < 8)
@@ -151,8 +148,8 @@ bool vlc_mutex_held(const vlc_mutex_t *mtx)
      * Even though other threads may modify the owner field at any time,
      * they will never make it compare equal to the calling thread.
      */
-    return THREAD_SELF == atomic_load_explicit(&tmp_mtx->owner,
-                                               memory_order_relaxed);
+    return vlc_thread_id() == atomic_load_explicit(&tmp_mtx->owner,
+                                                   memory_order_relaxed);
 }
 
 void vlc_mutex_lock(vlc_mutex_t *mtx)
@@ -172,7 +169,7 @@ void vlc_mutex_lock(vlc_mutex_t *mtx)
         vlc_atomic_wait(&mtx->value, 2);
 
     vlc_restorecancel(canc);
-    atomic_store_explicit(&mtx->owner, THREAD_SELF, memory_order_relaxed);
+    atomic_store_explicit(&mtx->owner, vlc_thread_id(), memory_order_relaxed);
 }
 
 int vlc_mutex_trylock(vlc_mutex_t *mtx)
@@ -199,7 +196,8 @@ int vlc_mutex_trylock(vlc_mutex_t *mtx)
     if (atomic_compare_exchange_strong_explicit(&mtx->value, &value, 1,
                                                 memory_order_acquire,
                                                 memory_order_relaxed)) {
-        atomic_store_explicit(&mtx->owner, THREAD_SELF, memory_order_relaxed);
+        atomic_store_explicit(&mtx->owner, vlc_thread_id(),
+                              memory_order_relaxed);
         return 0;
     }
 
@@ -219,7 +217,7 @@ void vlc_mutex_unlock(vlc_mutex_t *mtx)
         return;
     }
 
-    atomic_store_explicit(&mtx->owner, NULL, memory_order_relaxed);
+    atomic_store_explicit(&mtx->owner, 0, memory_order_relaxed);
 
     switch (atomic_exchange_explicit(&mtx->value, 0, memory_order_release)) {
         case 2: