riscv: stacktrace: fix the riscv stacktrace when CONFIG_FRAME_POINTER enabled

As [1] and [2] said, the arch_stack_walk should not to trace itself, or it will
leave the trace unexpectedly when called. The example is when we do "cat
/sys/kernel/debug/page_owner", all pages' stack is the same.

arch_stack_walk+0x18/0x20
stack_trace_save+0x40/0x60
register_dummy_stack+0x24/0x5e
init_page_owner+0x2e

So we use __builtin_frame_address(1) as the first frame to be walked. And mark
the arch_stack_walk() noinline.

We found that pr_cont will affact pages' stack whose task state is RUNNING when
testing "echo t > /proc/sysrq-trigger". So move the place of pr_cont and mark
the function dump_backtrace() noinline.

Also we move the case when task == NULL into else branch, and test for it in
"echo c > /proc/sysrq-trigger".

[1] https://lore.kernel.org/lkml/20210319184106.5688-1-mark.rutland@arm.com/
[2] https://lore.kernel.org/lkml/20210317142050.57712-1-chenjun102@huawei.com/Signed-off-by: Chen Huang <chenhuang5@huawei.com>
Fixes: 5d8544e2 ("RISC-V: Generic library routines and assembly")
Cc: stable@vger.kernel.org
Signed-off-by: Palmer Dabbelt <palmerdabbelt@google.com>

arch/riscv/kernel/stacktrace.c

@@ -27,10 +27,10 @@ void notrace walk_stackframe(struct task_struct *task, struct pt_regs *regs,
 		fp = frame_pointer(regs);
 		sp = user_stack_pointer(regs);
 		pc = instruction_pointer(regs);
-	} else if (task == NULL || task == current) {
-		fp = (unsigned long)__builtin_frame_address(0);
-		sp = sp_in_global;
-		pc = (unsigned long)walk_stackframe;
+	} else if (task == current) {
+		fp = (unsigned long)__builtin_frame_address(1);
+		sp = (unsigned long)__builtin_frame_address(0);
+		pc = (unsigned long)__builtin_return_address(0);
 	} else {
 		/* task blocked in __switch_to */
 		fp = task->thread.s[0];
@@ -106,15 +106,15 @@ static bool print_trace_address(void *arg, unsigned long pc)
 	return true;
 }
 
-void dump_backtrace(struct pt_regs *regs, struct task_struct *task,
+noinline void dump_backtrace(struct pt_regs *regs, struct task_struct *task,
 		    const char *loglvl)
 {
-	pr_cont("%sCall Trace:\n", loglvl);
 	walk_stackframe(task, regs, print_trace_address, (void *)loglvl);
 }
 
 void show_stack(struct task_struct *task, unsigned long *sp, const char *loglvl)
 {
+	pr_cont("%sCall Trace:\n", loglvl);
 	dump_backtrace(NULL, task, loglvl);
 }
 
@@ -139,7 +139,7 @@ unsigned long get_wchan(struct task_struct *task)
 
 #ifdef CONFIG_STACKTRACE
 
-void arch_stack_walk(stack_trace_consume_fn consume_entry, void *cookie,
+noinline void arch_stack_walk(stack_trace_consume_fn consume_entry, void *cookie,
 		     struct task_struct *task, struct pt_regs *regs)
 {
 	walk_stackframe(task, regs, consume_entry, cookie);