Re: [PATCH 3/4] x86/unwind: include __schedule() in stack traces

[Miroslav Benes <mbenes@suse.cz>]

On Mon, 9 Jan 2017, Josh Poimboeuf wrote:

> In the following commit:
> 
>   0100301bfdf5 ("sched/x86: Rewrite the switch_to() code")
> 
> ... the layout of the 'inactive_task_frame' struct was designed to have
> a frame pointer header embedded in it, so that the unwinder could use
> the 'bp' and 'ret_addr' fields to report __schedule() on the stack (or
> ret_from_fork() for newly forked tasks which haven't actually run yet).
> 
> Finish the job by changing get_frame_pointer() to return a pointer to
> inactive_task_frame's 'bp' field rather than 'bp' itself.  This allows
> the unwinder to start one frame higher on the stack, so that it properly
> reports __schedule().
> 
> Reported-by: Miroslav Benes <mbenes@suse.cz>

You can also add my

Tested-by: Miroslav Benes <mbenes@suse.cz>

One ignorant question below.

> Signed-off-by: Josh Poimboeuf <jpoimboe@redhat.com>
> ---
>  arch/x86/include/asm/stacktrace.h |  5 +----
>  arch/x86/include/asm/switch_to.h  | 10 +++++++++-
>  2 files changed, 10 insertions(+), 5 deletions(-)
> 
> diff --git a/arch/x86/include/asm/stacktrace.h b/arch/x86/include/asm/stacktrace.h
> index 20ce3db..2e41c50 100644
> --- a/arch/x86/include/asm/stacktrace.h
> +++ b/arch/x86/include/asm/stacktrace.h
> @@ -52,16 +52,13 @@ static inline bool on_stack(struct stack_info *info, void *addr, size_t len)
>  static inline unsigned long *
>  get_frame_pointer(struct task_struct *task, struct pt_regs *regs)
>  {
> -	struct inactive_task_frame *frame;
> -
>  	if (regs)
>  		return (unsigned long *)regs->bp;
>  
>  	if (task == current)
>  		return __builtin_frame_address(0);
>  
> -	frame = (struct inactive_task_frame *)task->thread.sp;
> -	return (unsigned long *)READ_ONCE_NOCHECK(frame->bp);
> +	return &((struct inactive_task_frame *)task->thread.sp)->bp;

You effectively remove one of the changes from the previous patch - 
READ_ONCE_NOCHECK. Is it intentional?

Regards,
Miroslav