Added mutex_control syscall for userspace mutexes.

- Compiles and Codezero runs as normal without touching mutex implementation
- Mutex implementation needs testing.

The mutex control syscall allows userspace programs to declare any virtual
address as a mutex lock and ask for help from the kernel syscall
for resolving locking contentions.

tasks/libl4/src/arm/mutex.S

@@ -0,0 +1,66 @@
+/*
+ * Copyright (C) 2009 Bahadir Balban
+ */
+
+#include <l4lib/arch/asm.h>
+#include <l4lib/mutex.h>
+
+/*
+ * NOTES:
+ *
+ * Recap on swp:
+ *
+ * swp rx, ry, [rz]
+ *
+ * In one instruction:
+ *
+ * 1) Stores the value in ry into location pointed by rz.
+ * 2) Loads the value in the location of rz into rx.
+ * By doing so, in one instruction one can attempt to lock
+ * a word, and discover whether it was already locked.
+ *
+ * Why use tid of thread to lock mutex instead of
+ * a single lock value?
+ *
+ * Because in one atomic instruction, not only the locking attempt
+ * should be able to indicate whether it is locked, but also
+ * the contentions. A unified lock value would not be sufficient.
+ * The only way to indicate a contended lock is to store the
+ * unique TID of the locker.
+ */
+
+
+/*
+ * Any non-negative value that is a potential TID
+ * (including 0) means mutex is locked.
+ */
+
+/*
+ * @r0 = address of mutex word
+ * @r1 = unique tid of current thread
+ */
+BEGIN_PROC(__l4_mutex_lock)
+	swp	r2, r1, [r0]
+	cmp	r2, #L4_MUTEX_UNLOCKED	@ Was the lock available?
+	movne	r0, #L4_MUTEX_CONTENDED	@ Indicate failure
+	moveq	r0, #L4_MUTEX_SUCCESS	@ Indicate success
+	mov	pc, lr
+END_PROC(__l4_mutex_lock)
+
+/*
+ * @r0 = address of mutex word
+ * @r1 = unique tid of current thread
+ */
+BEGIN_PROC(__l4_mutex_unlock)
+	mov	r3, #L4_MUTEX_UNLOCKED
+	swp	r2, r3, [r0]
+	cmp	r2, r1			@ Check lock had original tid value
+	movne	r0, #L4_MUTEX_CONTENDED	@ Indicate contention
+	movne	r0, #L4_MUTEX_SUCCESS	@ Indicate no contention
+	cmp	r2, #L4_MUTEX_UNLOCKED	@ Or - was it already unlocked?
+1:
+	beq	1b			@ If so busy-spin to indicate bug.
+	mov	pc, lr
+END_PROC(__l4_mutex_unlock)
+
+