Towards finishing exchange_registers()

- Added mutex_trylock()
- Implemented most of exchange_registers()
- thread_control() now needs a lock for operations that can modify thread context.
- thread_start() does not initialise scheduler flags, now done in thread_create.

TODO:
- Fork/clone'ed threads should retain their context in tcb, not syscall stack.
- exchange_registers() calls in userspace need cleaning up.

include/l4/generic/scheduler.h

@@ -34,10 +34,6 @@ static inline struct ktcb *current_task(void)
 #define SCHED_FL_MASK			(SCHED_FL_SLEEP | SCHED_FL_RESUME \
 					 | SCHED_FL_SUSPEND)
 
-#define __IPC_FL_WAIT			4
-#define IPC_FL_WAIT			(1 << __IPC_FL_WAIT)
-#define IPC_FL_MASK			IPC_FL_WAIT
-
 void sched_runqueue_init(void);
 void sched_start_task(struct ktcb *task);
 void sched_resume_task(struct ktcb *task);

include/l4/generic/tcb.h

@@ -13,6 +13,7 @@
 #include INC_GLUE(memory.h)
 #include INC_GLUE(syscall.h)
 #include INC_GLUE(message.h)
+#include INC_GLUE(context.h)
 #include INC_SUBARCH(mm.h)
 
 enum task_state {
@@ -21,32 +22,6 @@ enum task_state {
 	TASK_RUNNABLE	= 2,
 };
 
-/*
- * This describes the user space register context of each task. Simply set them
- * as regular structure fields, and they'll be copied onto real registers upon
- * a context switch. In the ARM case, they're copied from memory to userspace
- * registers using the LDM instruction with ^, no-pc flavor. See ARMARM.
- */
-typedef struct arm_context {
-	u32 spsr;	/* 0x0 */
-	u32 r0;		/* 0x4 */
-	u32 r1;		/* 0x8 */
-	u32 r2;		/* 0xC */
-	u32 r3;		/* 0x10 */
-	u32 r4;		/* 0x14 */
-	u32 r5;		/* 0x18 */
-	u32 r6; 	/* 0x1C */
-	u32 r7;		/* 0x20 */
-	u32 r8;		/* 0x24 */
-	u32 r9;		/* 0x28 */
-	u32 r10;	/* 0x2C */
-	u32 r11;	/* 0x30 */
-	u32 r12;	/* 0x34 */
-	u32 sp;		/* 0x38 */
-	u32 lr;		/* 0x3C */
-	u32 pc;		/* 0x40 */
-} __attribute__((__packed__)) task_context_t;
-
 #define TASK_ID_INVALID			-1
 struct task_ids {
 	l4id_t tid;
@@ -75,7 +50,9 @@ struct ktcb {
 
 	/* Flags to hint scheduler on future task state */
 	unsigned int schedfl;
-	unsigned int flags;
+
+	/* Lock for blocking thread state modifications via a syscall */
+	struct mutex thread_control_lock;
 
 	/* Other related threads */
 	l4id_t pagerid;