[PATCH v11 1/5] arm64: Call stack_backtrace() only from within walk_stackframe()

[madvenka@linux.microsoft.com]

From: "Madhavan T. Venkataraman" <madvenka@linux.microsoft.com>

Currently, arch_stack_walk() calls start_backtrace() and walk_stackframe()
separately. There is no need to do that. Instead, call start_backtrace()
from within walk_stackframe(). In other words, walk_stackframe() is the only
unwind function a consumer needs to call.

Currently, the only consumer is arch_stack_walk(). In the future,
arch_stack_walk_reliable() will be another consumer.

Currently, there is a check for a NULL task in unwind_frame(). It is not
needed since all current consumers pass a non-NULL task.

Use struct stackframe only within the unwind functions.

Signed-off-by: Madhavan T. Venkataraman <madvenka@linux.microsoft.com>
---
 arch/arm64/kernel/stacktrace.c | 41 ++++++++++++++++++----------------
 1 file changed, 22 insertions(+), 19 deletions(-)

diff --git a/arch/arm64/kernel/stacktrace.c b/arch/arm64/kernel/stacktrace.c
index 0fb58fed54cb..7217c4f63ef7 100644
--- a/arch/arm64/kernel/stacktrace.c
+++ b/arch/arm64/kernel/stacktrace.c
@@ -69,9 +69,6 @@ static int notrace unwind_frame(struct task_struct *tsk,
 	unsigned long fp = frame->fp;
 	struct stack_info info;
 
-	if (!tsk)
-		tsk = current;
-
 	/* Final frame; nothing to unwind */
 	if (fp == (unsigned long)task_pt_regs(tsk)->stackframe)
 		return -ENOENT;
@@ -143,15 +140,19 @@ static int notrace unwind_frame(struct task_struct *tsk,
 NOKPROBE_SYMBOL(unwind_frame);
 
 static void notrace walk_stackframe(struct task_struct *tsk,
-				    struct stackframe *frame,
+				    unsigned long fp, unsigned long pc,
 				    bool (*fn)(void *, unsigned long), void *data)
 {
+	struct stackframe frame;
+
+	start_backtrace(&frame, fp, pc);
+
 	while (1) {
 		int ret;
 
-		if (!fn(data, frame->pc))
+		if (!fn(data, frame.pc))
 			break;
-		ret = unwind_frame(tsk, frame);
+		ret = unwind_frame(tsk, &frame);
 		if (ret < 0)
 			break;
 	}
@@ -195,17 +196,19 @@ noinline notrace void arch_stack_walk(stack_trace_consume_fn consume_entry,
 			      void *cookie, struct task_struct *task,
 			      struct pt_regs *regs)
 {
-	struct stackframe frame;
-
-	if (regs)
-		start_backtrace(&frame, regs->regs[29], regs->pc);
-	else if (task == current)
-		start_backtrace(&frame,
-				(unsigned long)__builtin_frame_address(1),
-				(unsigned long)__builtin_return_address(0));
-	else
-		start_backtrace(&frame, thread_saved_fp(task),
-				thread_saved_pc(task));
-
-	walk_stackframe(task, &frame, consume_entry, cookie);
+	unsigned long fp, pc;
+
+	if (regs) {
+		fp = regs->regs[29];
+		pc = regs->pc;
+	} else if (task == current) {
+		/* Skip arch_stack_walk() in the stack trace. */
+		fp = (unsigned long)__builtin_frame_address(1);
+		pc = (unsigned long)__builtin_return_address(0);
+	} else {
+		/* Caller guarantees that the task is not running. */
+		fp = thread_saved_fp(task);
+		pc = thread_saved_pc(task);
+	}
+	walk_stackframe(task, fp, pc, consume_entry, cookie);
 }
-- 
2.25.1