Fixed a dead-lock that could happen if an attribute was changed and you

had some running live queries.
SimpleLock now allows nesting; added some missing atomic operations to
make its implementation as easy as possible.


git-svn-id: file:///srv/svn/repos/haiku/trunk/current@1901 a95241bf-73f2-0310-859d-f6bbb57e9c96

src/add-ons/kernel/file_systems/bfs/Lock.h

@@ -10,11 +10,19 @@
 
 
 #include <KernelExport.h>
+#include "Utility.h"
 
 
-// configure here if and when real benaphores should be used
+// Configure here if and when real benaphores should be used
+// Benaphores doesn't make much sense in the kernel, so they are only
+// enabled for the user-space test application
 #ifdef USER
-#	define USE_BENAPHORE 1
+#	define USE_BENAPHORE
+	// if defined, benaphores are used for the Semaphore class
+//#	define FAST_LOCK
+	// the ReadWriteLock class uses a second Semaphore to
+	// speed up locking - only makes sense if USE_BENAPHORE
+	// is defined, too.
 #endif
 
 
@@ -136,7 +144,6 @@ class Locker {
 // expensive regarding that BeOS only allows for a total of 64k
 // semaphores.
 
-//#define FAST_LOCK
 #ifdef FAST_LOCK
 class ReadWriteLock {
 	public:
@@ -317,35 +324,43 @@ class WriteLocked {
 
 // A simple locking structure that doesn't use a semaphore - it's useful
 // if you have to protect critical parts with a short runtime.
+// It also allows to nest several locks for the same thread.
 
 class SimpleLock {
 	public:
 		SimpleLock()
 			:
-			fLock(0),
+			fHolder(-1),
-			fUnlock(0)
+			fCount(0)
 		{
 		}
 
 		status_t Lock(bigtime_t time = 500)
 		{
-			int32 turn = atomic_add(&fLock,1);
+			int32 thisThread = find_thread(NULL);
-			while (turn != fUnlock)
+			int32 current;
+			while ((current = atomic_test_and_set(&fHolder, thisThread, -1)) != -1) {
+				if (current == thisThread)
+					break;
+
 				snooze(time);
+			}
 
 			// ToDo: the lock cannot fail currently! We may want
 			// to change this
+			atomic_add(&fCount, 1);
 			return B_OK;
 		}
 
 		void Unlock()
 		{
-			atomic_add(&fUnlock,1);
+			if (atomic_add(&fCount, -1) == 1)
+				atomic_set(&fHolder, -1);
 		}
 
 	private:
-		vint32	fLock;
+		vint32	fHolder;
-		vint32	fUnlock;
+		vint32	fCount;
 };
 
 // A convenience class to lock the SimpleLock, note the

src/add-ons/kernel/file_systems/bfs/Utility.h

@@ -107,4 +107,31 @@ template<class Node> struct list {
 };
 
 
+// Some atomic operations that are somehow missing in BeOS:
+//
+//	atomic_test_and_set(value, newValue, testAgainst)
+//		sets "value" to "newValue", if "value" is equal to "testAgainst"
+//	atomic_set(value, newValue)
+//		sets "value" to "newValue"
+
+#if __INTEL__
+	inline int32
+	atomic_test_and_set(volatile int32 *value, int32 newValue, int32 testAgainst)
+	{
+		int32 oldValue;
+		asm volatile("lock; cmpxchg %%ecx, (%%edx)"
+			: "=a" (oldValue) : "a" (testAgainst), "c" (newValue), "d" (value));
+		return oldValue;
+	}
+
+	inline void
+	atomic_set(volatile int32 *value, int32 newValue)
+	{
+		asm volatile("lock; xchg %%eax, (%%edx)"
+			: : "a" (newValue), "d" (value));
+	}
+#else
+#	error The macros atomic_set(), and atomic_test_and_set() are not defined for the target processor
+#endif
+
 #endif	/* UTILITY_H */

src/add-ons/kernel/file_systems/bfs/kernel_interface.cpp

@@ -474,7 +474,7 @@ bfs_suspend_vnode(void *_ns, void *_node)
 static int
 bfs_walk(void *_ns, void *_directory, const char *file, char **_resolvedPath, vnode_id *vnid)
 {
-	FUNCTION_START(("file = %s\n",file));
+	//FUNCTION_START(("file = %s\n",file));
 
 	if (_ns == NULL || _directory == NULL || file == NULL)
 		return B_BAD_VALUE;