rwlock: Move over to condvar implementation. (#255)

This one is superior to the previous because it is write-preferring.

nx/include/switch/kernel/rwlock.h

@@ -6,12 +6,15 @@
  */
 #pragma once
 #include "../kernel/mutex.h"
+#include "../kernel/condvar.h"
 
 /// Read/write lock structure.
 typedef struct {
-    RMutex r;
-    RMutex g;
-    u64 b;
+    Mutex mutex;
+    CondVar condvar_readers;
+    CondVar condvar_writer;
+    u32  readers : 31;
+    bool writer : 1;
 } RwLock;
 
 /**

nx/source/kernel/rwlock.c

@@ -3,33 +3,57 @@
 #include "kernel/rwlock.h"
 
 void rwlockInit(RwLock* r) {
-    rmutexInit(&r->r);
-    rmutexInit(&r->g);
-    r->b = 0;
+    mutexInit(&r->mutex);
+    condvarInit(&r->condvar_readers);
+    condvarInit(&r->condvar_writer);
+
+    r->readers = 0;
+    r->writer = false;
 }
 
 void rwlockReadLock(RwLock* r) {
-    rmutexLock(&r->r);
+    mutexLock(&r->mutex);
 
-    if (r->b++ == 0)
-        rmutexLock(&r->g);
+    while (r->writer) {
+        condvarWait(&r->condvar_writer, &r->mutex);
+    }
 
-    rmutexUnlock(&r->r);
+    r->readers++;
+
+    mutexUnlock(&r->mutex);
 }
 
 void rwlockReadUnlock(RwLock* r) {
-    rmutexLock(&r->r);
+    mutexLock(&r->mutex);
 
-    if (r->b-- == 1)
-        rmutexUnlock(&r->g);
+    if (--r->readers == 0) {
+        condvarWakeAll(&r->condvar_readers);
+    }
 
-    rmutexUnlock(&r->r);
+    mutexUnlock(&r->mutex);
 }
 
 void rwlockWriteLock(RwLock* r) {
-    rmutexLock(&r->g);
+    mutexLock(&r->mutex);
+
+    while (r->writer) {
+        condvarWait(&r->condvar_writer, &r->mutex);
+    }
+
+    r->writer = true;
+
+    while (r->readers > 0) {
+        condvarWait(&r->condvar_readers, &r->mutex);
+    }
+
+    mutexUnlock(&r->mutex);
 }
 
 void rwlockWriteUnlock(RwLock* r) {
-    rmutexUnlock(&r->g);
+    mutexLock(&r->mutex);
+
+    r->writer = false;
+    condvarWakeAll(&r->condvar_writer);
+
+    mutexUnlock(&r->mutex);
 }