When no stack pointer is supplied, show_stack() starts at thread.esp0.
This is the saved userspace register frame, not where a task was switched
out. For kernel threads it points to the zeroed frame created by
copy_thread(), so sched_show_task() prints no useful backtrace.

Use thread.ksp for non-current tasks. It is saved by switch_to() and is
already used by __get_wchan(). For current, start at the current stack
frame, as is already done when no task is supplied.

Tested on qemu-system-m68k -M virt using SysRq-t. All task dumps contained
resolved frames.

Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
Assisted-by: Claude:claude-opus-4-8
Signed-off-by: Karl Mehltretter <kmehltretter@gmail.com>
---
diff --git a/arch/m68k/kernel/traps.c b/arch/m68k/kernel/traps.c
index d2f25e8895e5..60759ba3cbd4 100644
--- a/arch/m68k/kernel/traps.c
+++ b/arch/m68k/kernel/traps.c
@@ -936,8 +936,8 @@ void show_stack(struct task_struct *task, unsigned long *stack,
 	int i;
 
 	if (!stack) {
-		if (task)
-			stack = (unsigned long *)task->thread.esp0;
+		if (task && task != current)
+			stack = (unsigned long *)task->thread.ksp;
 		else
 			stack = (unsigned long *)&stack;
 	}