[PATCH] arm64: Synchonise dump_backtrace() with perf callchain

[Jungseok Lee]

dump_backtrace() has its own backtrace logic unlike perf callchain which
relies on walk_stackframe(). They behave differently when a symbol is
recorded. Perf writes it down *before* calling unwind_frame(), but
dump_backtrace() prints it out *after* unwind_frame(). As a result, the
last valid symbol is not added to a list in case of dump_backtrace().

This patch catches up the last symbol as synchronising dump_backtrace()
with perf callchain. However, the patch does not cover a case where MMU
is disabled. That is, a physical address can be stored in stack frame,
but it's not handled. For example, a swapper process falls into this case.
Unlike a swapper from a secondary core, a swapper on a boot cpu, which
starting from __mmap_switched(), can't be tracked down with a simple
conversion, phys_to_virt(), because PC is retrieved from LR - 4, not LR.

It is a big tradeoff to change both head.S and unwind_frame() structure
for a few of symbols in *.S, so this hunk does not take care of the case.

Cc: AKASHI Takahiro <takahiro.akashi at linaro.org>
Cc: James Morse <james.morse at arm.com>
Cc: Mark Rutland <mark.rutland at arm.com>
Signed-off-by: Jungseok Lee <jungseoklee85 at gmail.com>
---
 arch/arm64/kernel/traps.c | 16 +++++++++++-----
 1 file changed, 11 insertions(+), 5 deletions(-)

diff --git a/arch/arm64/kernel/traps.c b/arch/arm64/kernel/traps.c
index f93aae5..4ddb928 100644
--- a/arch/arm64/kernel/traps.c
+++ b/arch/arm64/kernel/traps.c
@@ -103,12 +103,13 @@ static void dump_mem(const char *lvl, const char *str, unsigned long bottom,
 	set_fs(fs);
 }
 
-static void dump_backtrace_entry(unsigned long where, unsigned long stack)
+static void dump_backtrace_entry(unsigned long where)
 {
+	/*
+	 * The highest stack frame of a swapper process stores PC in a form
+	 * of physical address, but this case is not handled.
+	 */
 	print_ip_sym(where);
-	if (in_exception_text(where))
-		dump_mem("", "Exception stack", stack,
-			 stack + sizeof(struct pt_regs), false);
 }
 
 static void dump_instr(const char *lvl, struct pt_regs *regs)
@@ -172,12 +173,17 @@ static void dump_backtrace(struct pt_regs *regs, struct task_struct *tsk)
 	pr_emerg("Call trace:\n");
 	while (1) {
 		unsigned long where = frame.pc;
+		unsigned long stack;
 		int ret;
 
+		dump_backtrace_entry(where);
 		ret = unwind_frame(&frame);
 		if (ret < 0)
 			break;
-		dump_backtrace_entry(where, frame.sp);
+		stack = frame.sp;
+		if (in_exception_text(where))
+			dump_mem("", "Exception stack", stack,
+				 stack + sizeof(struct pt_regs), false);
 	}
 }
 
-- 
2.5.0