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

From: [email-masked] (Will Deacon)
Date: Thu, 15 Oct 2015 18:26:01 +0100

On Thu, Oct 15, 2015 at 01:21:54PM +0000, Jungseok Lee wrote:
> 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 would be good to have an example backtrace before and after this patch
is applied, to show what it fixes.

> 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 <[email-masked]>
> Cc: James Morse <[email-masked]>
> Cc: Mark Rutland <[email-masked]>
> Signed-off-by: Jungseok Lee <[email-masked]>
> ---
>  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);

AFAICT, the original code is all based on unwind_backtrace in
arch/arm/kernel/unwind.c. Does that need updating too (as a separate patch)?

Will