LoongArch: Get frame info in unwind_start() when regs is not available

At unwind_start(), it is better to get its frame info here rather than
get them outside, even we don't have 'regs'. In this way we can simply
use unwind_{start, next_frame, done} outside.
Signed-off-by: Jinyang He <hejinyang@loongson.cn>
Signed-off-by: Huacai Chen <chenhuacai@loongson.cn>

arch/loongarch/kernel/process.c

@@ -191,20 +191,14 @@ int copy_thread(struct task_struct *p, const struct kernel_clone_args *args)
 
 unsigned long __get_wchan(struct task_struct *task)
 {
-	unsigned long pc;
+	unsigned long pc = 0;
 	struct unwind_state state;
 
 	if (!try_get_task_stack(task))
 		return 0;
 
-	unwind_start(&state, task, NULL);
-	state.sp = thread_saved_fp(task);
-	get_stack_info(state.sp, state.task, &state.stack_info);
-	state.pc = thread_saved_ra(task);
-#ifdef CONFIG_UNWINDER_PROLOGUE
-	state.type = UNWINDER_PROLOGUE;
-#endif
-	for (; !unwind_done(&state); unwind_next_frame(&state)) {
+	for (unwind_start(&state, task, NULL);
+	     !unwind_done(&state); unwind_next_frame(&state)) {
 		pc = unwind_get_return_address(&state);
 		if (!pc)
 			break;

arch/loongarch/kernel/unwind_guess.c

@@ -26,6 +26,12 @@ void unwind_start(struct unwind_state *state, struct task_struct *task,
 	if (regs) {
 		state->sp = regs->regs[3];
 		state->pc = regs->csr_era;
+	} else if (task && task != current) {
+		state->sp = thread_saved_fp(task);
+		state->pc = thread_saved_ra(task);
+	} else {
+		state->sp = (unsigned long)__builtin_frame_address(0);
+		state->pc = (unsigned long)__builtin_return_address(0);
 	}
 
 	state->task = task;

arch/loongarch/kernel/unwind_prologue.c

@@ -146,12 +146,22 @@ void unwind_start(struct unwind_state *state, struct task_struct *task,
 		    struct pt_regs *regs)
 {
 	memset(state, 0, sizeof(*state));
+	state->type = UNWINDER_PROLOGUE;
 
-	if (regs &&  __kernel_text_address(regs->csr_era)) {
-		state->pc = regs->csr_era;
+	if (regs) {
 		state->sp = regs->regs[3];
+		state->pc = regs->csr_era;
 		state->ra = regs->regs[1];
-		state->type = UNWINDER_PROLOGUE;
+		if (!__kernel_text_address(state->pc))
+			state->type = UNWINDER_GUESS;
+	} else if (task && task != current) {
+		state->sp = thread_saved_fp(task);
+		state->pc = thread_saved_ra(task);
+		state->ra = 0;
+	} else {
+		state->sp = (unsigned long)__builtin_frame_address(0);
+		state->pc = (unsigned long)__builtin_return_address(0);
+		state->ra = 0;
 	}
 
 	state->task = task;