[PATCH] uml: thread creation tidying

fork on UML has always somewhat subtle.  The underlying cause has been the
need to initialize a stack for the new process.  The only portable way to
initialize a new stack is to set it as the alternate signal stack and take a
signal.  The signal handler does whatever initialization is needed and jumps
back to the original stack, where the fork processing is finished.  The basic
context switching mechanism is a jmp_buf for each process.  You switch to a
new process by longjmping to its jmp_buf.

Now that UML has its own implementation of setjmp and longjmp, and I can poke
around inside a jmp_buf without fear that libc will change the structure, a
much simpler mechanism is possible.  The jmpbuf can simply be initialized by
hand.

This eliminates -
	the need to set up and remove the alternate signal stack
	sending and handling a signal
	the signal blocking needed around the stack switching, since
there is no stack switching
	setting up the jmp_buf needed to jump back to the original
stack after the new one is set up

In addition, since jmp_buf is now defined by UML, and not by libc, it can be
embedded in the thread struct.  This makes it unnecessary to have it exist on
the stack, where it used to be.  It also simplifies interfaces, since the
switch jmp_buf used to be a void * inside the thread struct, and functions
which took it as an argument needed to define a jmp_buf variable and assign it
from the void *.

Signed-off-by: Jeff Dike <jdike@addtoit.com>
Cc: Paolo 'Blaisorblade' Giarrusso <blaisorblade@yahoo.it>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

arch/um/kernel/skas/process_kern.c

@@ -33,7 +33,7 @@ void switch_to_skas(void *prev, void *next)
 		switch_timers(0);
 
 	switch_threads(&from->thread.mode.skas.switch_buf,
-		       to->thread.mode.skas.switch_buf);
+		       &to->thread.mode.skas.switch_buf);
 
 	arch_switch_to_skas(current->thread.prev_sched, current);
 
@@ -43,21 +43,21 @@ void switch_to_skas(void *prev, void *next)
 
 extern void schedule_tail(struct task_struct *prev);
 
-void new_thread_handler(int sig)
+/* This is called magically, by its address being stuffed in a jmp_buf
+ * and being longjmp-d to.
+ */
+void new_thread_handler(void)
 {
 	int (*fn)(void *), n;
 	void *arg;
 
-	fn = current->thread.request.u.thread.proc;
-	arg = current->thread.request.u.thread.arg;
-	os_usr1_signal(1);
-	thread_wait(&current->thread.mode.skas.switch_buf,
-		    current->thread.mode.skas.fork_buf);
-
 	if(current->thread.prev_sched != NULL)
 		schedule_tail(current->thread.prev_sched);
 	current->thread.prev_sched = NULL;
 
+	fn = current->thread.request.u.thread.proc;
+	arg = current->thread.request.u.thread.arg;
+
 	/* The return value is 1 if the kernel thread execs a process,
 	 * 0 if it just exits
 	 */
@@ -70,22 +70,13 @@ void new_thread_handler(int sig)
 	else do_exit(0);
 }
 
-void new_thread_proc(void *stack, void (*handler)(int sig))
-{
-	init_new_thread_stack(stack, handler);
-	os_usr1_process(os_getpid());
-}
-
 void release_thread_skas(struct task_struct *task)
 {
 }
 
-void fork_handler(int sig)
+/* Called magically, see new_thread_handler above */
+void fork_handler(void)
 {
-	os_usr1_signal(1);
-	thread_wait(&current->thread.mode.skas.switch_buf,
-		    current->thread.mode.skas.fork_buf);
-  	
 	force_flush_all();
 	if(current->thread.prev_sched == NULL)
 		panic("blech");
@@ -109,7 +100,7 @@ int copy_thread_skas(int nr, unsigned long clone_flags, unsigned long sp,
 		     unsigned long stack_top, struct task_struct * p,
 		     struct pt_regs *regs)
 {
-  	void (*handler)(int);
+  	void (*handler)(void);
 
 	if(current->thread.forking){
 	  	memcpy(&p->thread.regs.regs.skas, &regs->regs.skas,
@@ -128,7 +119,7 @@ int copy_thread_skas(int nr, unsigned long clone_flags, unsigned long sp,
 	}
 
 	new_thread(task_stack_page(p), &p->thread.mode.skas.switch_buf,
-		   &p->thread.mode.skas.fork_buf, handler);
+		   handler);
 	return(0);
 }
 
@@ -182,8 +173,7 @@ int start_uml_skas(void)
 	init_task.thread.request.u.thread.proc = start_kernel_proc;
 	init_task.thread.request.u.thread.arg = NULL;
 	return(start_idle_thread(task_stack_page(&init_task),
-				 &init_task.thread.mode.skas.switch_buf,
-				 &init_task.thread.mode.skas.fork_buf));
+				 &init_task.thread.mode.skas.switch_buf));
 }
 
 int external_pid_skas(struct task_struct *task)