rwlock: revamp completely (#350)

Implements rwlockTryReadLock and rwlockTryWriteLock.
Also implements rwlockIsWriteLockHeldByCurrentThread and
rwlockIsOwnedByCurrentThread.

Also re-designs RwLock to have semantics identical to
Nintendo's (nn::os::ReaderWriterLock). The upshot is mostly
that the lock is now fully recursive/write-preferring.

nx/include/switch/kernel/rwlock.h

@@ -1,7 +1,7 @@
 /**
  * @file rwlock.h
  * @brief Read/write lock synchronization primitive.
- * @author plutoo
+ * @author plutoo, SciresM
  * @copyright libnx Authors
  */
 #pragma once
@@ -11,10 +11,13 @@
 /// Read/write lock structure.
 typedef struct {
     Mutex mutex;
-    CondVar condvar_readers;
-    CondVar condvar_writer;
-    u32  readers : 31;
-    bool writer : 1;
+    CondVar condvar_reader_wait;
+    CondVar condvar_writer_wait;
+    u32 read_lock_count;
+    u32 read_waiter_count;
+    u32 write_lock_count;
+    u32 write_waiter_count;
+    u32 write_owner_tag;
 } RwLock;
 
 /**
@@ -29,6 +32,13 @@ void rwlockInit(RwLock* r);
  */
 void rwlockReadLock(RwLock* r);
 
+/**
+ * @brief Attempts to lock the read/write lock for reading without waiting.
+ * @param r Read/write lock object.
+ * @return 1 if the mutex has been acquired successfully, and 0 on contention.
+ */
+bool rwlockTryReadLock(RwLock* r);
+
 /**
  * @brief Unlocks the read/write lock for reading.
  * @param r Read/write lock object.
@@ -41,8 +51,30 @@ void rwlockReadUnlock(RwLock* r);
  */
 void rwlockWriteLock(RwLock* r);
 
+/**
+ * @brief Attempts to lock the read/write lock for writing without waiting.
+ * @param r Read/write lock object.
+ * @return 1 if the mutex has been acquired successfully, and 0 on contention.
+ */
+bool rwlockTryWriteLock(RwLock* r);
+
 /**
  * @brief Unlocks the read/write lock for writing.
  * @param r Read/write lock object.
  */
 void rwlockWriteUnlock(RwLock* r);
+
+/**
+ * @brief Checks if the write lock is held by the current thread.
+ * @param r Read/write lock object.
+ * @return 1 if the current hold holds the write lock, and 0 if it does not.
+ */
+bool rwlockIsWriteLockHeldByCurrentThread(RwLock* r);
+
+/**
+ * @brief Checks if the read/write lock is owned by the current thread.
+ * @param r Read/write lock object.
+ * @return 1 if the current hold holds the write lock or if it holds read locks acquired
+ *         while it held the write lock, and 0 if it does not.
+ */
+bool rwlockIsOwnedByCurrentThread(RwLock* r);

nx/source/kernel/rwlock.c

@@ -1,59 +1,170 @@
 // Copyright 2018 plutoo
 #include "kernel/mutex.h"
 #include "kernel/rwlock.h"
+#include "../internal.h"
+
+NX_INLINE u32 _GetCurrentThreadTag(void) {
+    return getThreadVars()->handle;
+}
 
 void rwlockInit(RwLock* r) {
     mutexInit(&r->mutex);
-    condvarInit(&r->condvar_readers);
-    condvarInit(&r->condvar_writer);
+    condvarInit(&r->condvar_reader_wait);
+    condvarInit(&r->condvar_writer_wait);
 
-    r->readers = 0;
-    r->writer = false;
+    r->read_lock_count = 0;
+    r->read_waiter_count = 0;
+    r->write_lock_count = 0;
+    r->write_waiter_count = 0;
+    r->write_owner_tag = 0;
 }
 
 void rwlockReadLock(RwLock* r) {
-    mutexLock(&r->mutex);
+    const u32 cur_tag = _GetCurrentThreadTag();
 
-    while (r->writer) {
-        condvarWait(&r->condvar_writer, &r->mutex);
+    if (r->write_owner_tag == cur_tag) {
+        r->read_lock_count++;
+        return;
     }
 
-    r->readers++;
+    mutexLock(&r->mutex);
+
+    while (r->write_waiter_count > 0) {
+        r->read_waiter_count++;
+        condvarWait(&r->condvar_reader_wait, &r->mutex);
+        r->read_waiter_count--;
+    }
+
+    r->read_lock_count++;
 
     mutexUnlock(&r->mutex);
 }
 
+bool rwlockTryReadLock(RwLock* r) {
+    const u32 cur_tag = _GetCurrentThreadTag();
+
+    if (r->write_owner_tag == cur_tag) {
+        r->read_lock_count++;
+        return true;
+    }
+
+    if (!mutexTryLock(&r->mutex)) {
+        return false;
+    }
+
+    const bool got_lock = r->write_waiter_count == 0;
+    if (got_lock) {
+        r->read_lock_count++;
+    }
+
+    mutexUnlock(&r->mutex);
+    return got_lock;
+}
+
 void rwlockReadUnlock(RwLock* r) {
-    mutexLock(&r->mutex);
+    const u32 cur_tag = _GetCurrentThreadTag();
 
-    if (--r->readers == 0) {
-        condvarWakeAll(&r->condvar_readers);
+    if (r->write_owner_tag == cur_tag) {
+        // Write lock is owned by this thread.
+        r->read_lock_count--;
+        if (r->read_lock_count == 0 && r->write_lock_count == 0) {
+            // Relinquish control of the lock.
+            r->write_owner_tag = 0;
+
+            if (r->write_waiter_count > 0) {
+                condvarWakeOne(&r->condvar_writer_wait);
+            } else if (r->read_waiter_count > 0) {
+                condvarWakeAll(&r->condvar_reader_wait);
             }
 
+            // Corresponding mutexLock was called in WriteLock/WriteTryLock.
             mutexUnlock(&r->mutex);
+        }
+    } else {
+        // Write lock isn't owned by this thread.
+        mutexLock(&r->mutex);
+        r->read_lock_count--;
+        if (r->read_lock_count == 0 && r->write_waiter_count > 0) {
+            condvarWakeOne(&r->condvar_writer_wait);
+        }
+        mutexUnlock(&r->mutex);
+    }
 }
 
 void rwlockWriteLock(RwLock* r) {
+    const u32 cur_tag = _GetCurrentThreadTag();
+
+    if (r->write_owner_tag == cur_tag) {
+        r->write_lock_count++;
+        return;
+    }
+
     mutexLock(&r->mutex);
 
-    while (r->writer) {
-        condvarWait(&r->condvar_writer, &r->mutex);
+    while (r->read_lock_count > 0) {
+        r->write_waiter_count++;
+        condvarWait(&r->condvar_writer_wait, &r->mutex);
+        r->write_waiter_count--;
     }
 
-    r->writer = true;
+    r->write_lock_count = 1;
+    r->write_owner_tag = cur_tag;
 
-    while (r->readers > 0) {
-        condvarWait(&r->condvar_readers, &r->mutex);
+    // mutexUnlock(&r->mutex) is intentionally not called here.
+    // The mutex will be unlocked by a call to ReadUnlock or WriteUnlock.
+}
+
+bool rwlockTryWriteLock(RwLock* r) {
+    const u32 cur_tag = _GetCurrentThreadTag();
+
+    if (r->write_owner_tag == cur_tag) {
+        r->write_lock_count++;
+        return true;
     }
 
+    if (!mutexTryLock(&r->mutex)) {
+        return false;
+    }
+
+    if (r->read_lock_count > 0) {
         mutexUnlock(&r->mutex);
+        return false;
+    }
+
+    r->write_lock_count = 1;
+    r->write_owner_tag = cur_tag;
+
+    // mutexUnlock(&r->mutex) is intentionally not called here.
+    // The mutex will be unlocked by a call to ReadUnlock or WriteUnlock.
+    return true;
 }
 
 void rwlockWriteUnlock(RwLock* r) {
-    mutexLock(&r->mutex);
+    // This function assumes the write lock is held.
+    // This means that r->mutex is locked, and r->write_owner_tag == cur_tag.
+    // if (r->write_owner_tag == cur_tag)
+    {
+        r->write_lock_count--;
+        if (r->write_lock_count == 0 && r->read_lock_count == 0) {
+            // Relinquish control of the lock.
+            r->write_owner_tag = 0;
 
-    r->writer = false;
-    condvarWakeAll(&r->condvar_writer);
+            if (r->write_waiter_count > 0) {
+                condvarWakeOne(&r->condvar_writer_wait);
+            } else if (r->read_waiter_count > 0) {
+                condvarWakeAll(&r->condvar_reader_wait);
+            }
 
+            // Corresponding mutexLock was called in WriteLock/WriteTryLock.
             mutexUnlock(&r->mutex);
+        }
+    }
+}
+
+bool rwlockIsWriteLockHeldByCurrentThread(RwLock* r) {
+    return r->write_owner_tag == _GetCurrentThreadTag() && r->write_lock_count > 0;
+}
+
+bool rwlockIsOwnedByCurrentThread(RwLock* r) {
+    return r->write_owner_tag == _GetCurrentThreadTag();
 }