[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>

1 files changed, 16 insertions, 52 deletions

diff --git a/arch/um/os-Linux/skas/process.c b/arch/um/os-Linux/skas/process.c
index 42e3d1ed802c..cb9ab54146cc 100644
--- a/arch/um/os-Linux/skas/process.c
+++ b/arch/um/os-Linux/skas/process.c
@@ -444,56 +444,22 @@ void map_stub_pages(int fd, unsigned long code,
 	}
 }
 
-void new_thread(void *stack, void **switch_buf_ptr, void **fork_buf_ptr,
-		void (*handler)(int))
+void new_thread(void *stack, jmp_buf *buf, void (*handler)(void))
 {
-	unsigned long flags;
-	jmp_buf switch_buf, fork_buf;
-
-	*switch_buf_ptr = &switch_buf;
-	*fork_buf_ptr = &fork_buf;
-
-	/* Somewhat subtle - siglongjmp restores the signal mask before doing
-	 * the longjmp.  This means that when jumping from one stack to another
-	 * when the target stack has interrupts enabled, an interrupt may occur
-	 * on the source stack.  This is bad when starting up a process because
-	 * it's not supposed to get timer ticks until it has been scheduled.
-	 * So, we disable interrupts around the sigsetjmp to ensure that
-	 * they can't happen until we get back here where they are safe.
-	 */
-	flags = get_signals();
-	block_signals();
-	if(UML_SETJMP(&fork_buf) == 0)
-		new_thread_proc(stack, handler);
-
-	remove_sigstack();
-
-	set_signals(flags);
+	(*buf)[0].JB_IP = (unsigned long) handler;
+	(*buf)[0].JB_SP = (unsigned long) stack +
+		(PAGE_SIZE << UML_CONFIG_KERNEL_STACK_ORDER) - sizeof(void *);
 }
 
 #define INIT_JMP_NEW_THREAD 0
-#define INIT_JMP_REMOVE_SIGSTACK 1
-#define INIT_JMP_CALLBACK 2
-#define INIT_JMP_HALT 3
-#define INIT_JMP_REBOOT 4
-
-void thread_wait(void *sw, void *fb)
-{
-	jmp_buf buf, **switch_buf = sw, *fork_buf;
-
-	*switch_buf = &buf;
-	fork_buf = fb;
-	if(UML_SETJMP(&buf) == 0)
-		UML_LONGJMP(fork_buf, INIT_JMP_REMOVE_SIGSTACK);
-}
+#define INIT_JMP_CALLBACK 1
+#define INIT_JMP_HALT 2
+#define INIT_JMP_REBOOT 3
 
-void switch_threads(void *me, void *next)
+void switch_threads(jmp_buf *me, jmp_buf *you)
 {
-	jmp_buf my_buf, **me_ptr = me, *next_buf = next;
-
-	*me_ptr = &my_buf;
-	if(UML_SETJMP(&my_buf) == 0)
-		UML_LONGJMP(next_buf, 1);
+	if(UML_SETJMP(me) == 0)
+		UML_LONGJMP(you, 1);
 }
 
 static jmp_buf initial_jmpbuf;
@@ -503,23 +469,21 @@ static void (*cb_proc)(void *arg);
 static void *cb_arg;
 static jmp_buf *cb_back;
 
-int start_idle_thread(void *stack, void *switch_buf_ptr, void **fork_buf_ptr)
+int start_idle_thread(void *stack, jmp_buf *switch_buf)
 {
-	jmp_buf **switch_buf = switch_buf_ptr;
 	int n;
 
 	set_handler(SIGWINCH, (__sighandler_t) sig_handler,
 		    SA_ONSTACK | SA_RESTART, SIGUSR1, SIGIO, SIGALRM,
 		    SIGVTALRM, -1);
 
-	*fork_buf_ptr = &initial_jmpbuf;
 	n = UML_SETJMP(&initial_jmpbuf);
 	switch(n){
 	case INIT_JMP_NEW_THREAD:
-		new_thread_proc((void *) stack, new_thread_handler);
-		break;
-	case INIT_JMP_REMOVE_SIGSTACK:
-		remove_sigstack();
+		(*switch_buf)[0].JB_IP = (unsigned long) new_thread_handler;
+		(*switch_buf)[0].JB_SP = (unsigned long) stack +
+			(PAGE_SIZE << UML_CONFIG_KERNEL_STACK_ORDER) -
+			sizeof(void *);
 		break;
 	case INIT_JMP_CALLBACK:
 		(*cb_proc)(cb_arg);
@@ -534,7 +498,7 @@ int start_idle_thread(void *stack, void *switch_buf_ptr, void **fork_buf_ptr)
 	default:
 		panic("Bad sigsetjmp return in start_idle_thread - %d\n", n);
 	}
-	UML_LONGJMP(*switch_buf, 1);
+	UML_LONGJMP(switch_buf, 1);
 }
 
 void initial_thread_cb_skas(void (*proc)(void *), void *arg)