unify lock and owner fields of mutex structure

this change is necessary to free up one slot in the mutex structure so
that we can use doubly-linked lists in the implementation of robust
mutexes.

src/internal/pthread_impl.h

@@ -47,7 +47,6 @@ struct pthread {
 #define _m_type __u.__i[0]
 #define _m_lock __u.__i[1]
 #define _m_waiters __u.__i[2]
-#define _m_owner __u.__i[3]
 #define _m_count __u.__i[4]
 #define _c_block __u.__i[0]
 #define _c_clock __u.__i[1]

src/thread/pthread_mutex_lock.c

@@ -4,10 +4,11 @@ int pthread_mutex_lock(pthread_mutex_t *m)
 {
 	int r;
 	while ((r=pthread_mutex_trylock(m)) == EBUSY) {
+		if (!(r=m->_m_lock)) continue;
 		if (m->_m_type == PTHREAD_MUTEX_ERRORCHECK
-		 && m->_m_owner == pthread_self()->tid)
+		 && r == pthread_self()->tid)
 			return EDEADLK;
-		__wait(&m->_m_lock, &m->_m_waiters, 1, 0);
+		__wait(&m->_m_lock, &m->_m_waiters, r, 0);
 	}
 	return r;
 }

src/thread/pthread_mutex_trylock.c

@@ -9,14 +9,13 @@ int pthread_mutex_trylock(pthread_mutex_t *m)
 
 	tid = pthread_self()->tid;
 
-	if (m->_m_owner == tid && m->_m_type == PTHREAD_MUTEX_RECURSIVE) {
+	if (m->_m_lock == tid && m->_m_type == PTHREAD_MUTEX_RECURSIVE) {
 		if ((unsigned)m->_m_count >= INT_MAX) return EAGAIN;
 		m->_m_count++;
 		return 0;
 	}
 
-	if (m->_m_owner || a_xchg(&m->_m_lock, 1)) return EBUSY;
-	m->_m_owner = tid;
+	if (m->_m_lock || a_cas(&m->_m_lock, 0, tid)) return EBUSY;
 	m->_m_count = 1;
 	return 0;
 }

src/thread/pthread_mutex_unlock.c

@@ -3,13 +3,12 @@
 int pthread_mutex_unlock(pthread_mutex_t *m)
 {
 	if (m->_m_type != PTHREAD_MUTEX_NORMAL) {
-		if (m->_m_owner != pthread_self()->tid)
+		if (m->_m_lock != pthread_self()->tid)
 		 	return EPERM;
 		if (m->_m_type == PTHREAD_MUTEX_RECURSIVE && --m->_m_count)
 			return 0;
 	}
 
-	m->_m_owner = 0;
 	m->_m_lock = 0;
 	if (m->_m_waiters) __wake(&m->_m_lock, 1, 0);
 	return 0;