mutex: remove _locked member

It just adds complexity, and doesn't reduce locking overhead, as we must
take the spinlock anyway during unlock.

Remove it, since it makes conversion to a recursive mutex (needed
by fs/vfs/vfs_mount.c) more complicated.

core/mutex.cc

@@ -9,16 +9,13 @@ struct waiter {
 
 extern "C" void mutex_lock(mutex_t *mutex)
 {
-    if (mutex_trylock(mutex)) {
-        return;
-    }
     struct waiter w;
 
     w.thread = sched::thread::current();
 
     spin_lock(&mutex->_wait_lock);
-    if (mutex_trylock(mutex)) {
-        // mutex was unlocked just before we grabbed _wait_lock
+    if (!mutex->_owner) {
+        mutex->_owner = w.thread;
         spin_unlock(&mutex->_wait_lock);
         return;
     }
@@ -44,12 +41,14 @@ extern "C" void mutex_lock(mutex_t *mutex)
 
 extern "C" bool mutex_trylock(mutex_t *mutex)
 {
-    if (!__sync_lock_test_and_set(&mutex->_locked, 1)) {
+    bool ret = false;
+    spin_lock(&mutex->_wait_lock);
+    if (!mutex->_owner) {
         mutex->_owner = sched::thread::current();
-        return true;
-    } else {
-        return false;
+        ret = true;
     }
+    spin_unlock(&mutex->_wait_lock);
+    return ret;
 }
 
 extern "C" void mutex_unlock(mutex_t *mutex)
@@ -60,7 +59,6 @@ extern "C" void mutex_unlock(mutex_t *mutex)
         mutex->_wait_list.first->thread->wake();
     } else {
         mutex->_owner = nullptr;
-        __sync_lock_release(&mutex->_locked, 0);
     }
     spin_unlock(&mutex->_wait_lock);
 }

include/osv/mutex.h

@@ -20,7 +20,6 @@ static inline void spinlock_init(spinlock_t *sl)
 }
 
 struct cmutex {
-    bool _locked;
     spinlock_t _wait_lock;
     void *_owner;
     struct wait_list {
@@ -39,7 +38,7 @@ void mutex_unlock(mutex_t* m);
 
 static __always_inline void mutex_init(mutex_t* m)
 {
-    m->_locked = false;
+    m->_owner = 0;
     m->_wait_list.first = 0;
     m->_wait_list.last = 0;
     spinlock_init(&m->_wait_lock);