Re: [PATCH v13 04/11] arm64: Split unwind_init()

[Mark Rutland <mark.rutland@arm.com>]

Hi Madhavan,

The diff itself largely looks good, but we need to actually write the comments.
Can you pleaes pick up the wording I've written below for those?

That and renaming `unwind_init_from_current` to `unwind_init_from_caller`.

With those I think this is good, but I'd like to see the updated version before
I provide Acked-by or Reviewed-by tags -- hopefully that's just a formality! :)

On Mon, Jan 17, 2022 at 08:56:01AM -0600, madvenka@linux.microsoft.com wrote:
> From: "Madhavan T. Venkataraman" <madvenka@linux.microsoft.com>
> 
> unwind_init() is currently a single function that initializes all of the
> unwind state. Split it into the following functions and call them
> appropriately:
> 
> 	- unwind_init_from_regs() - initialize from regs passed by caller.
> 
> 	- unwind_init_from_current() - initialize for the current task
> 	  from the caller of arch_stack_walk().
> 
> 	- unwind_init_from_task() - initialize from the saved state of a
> 	  task other than the current task. In this case, the other
> 	  task must not be running.
> 
> This is done for two reasons:
> 
> 	- the different ways of initializing are clear
> 
> 	- specialized code can be added to each initializer in the future.
> 
> Signed-off-by: Madhavan T. Venkataraman <madvenka@linux.microsoft.com>
> ---
>  arch/arm64/kernel/stacktrace.c | 54 +++++++++++++++++++++++++++-------
>  1 file changed, 44 insertions(+), 10 deletions(-)
> 
> diff --git a/arch/arm64/kernel/stacktrace.c b/arch/arm64/kernel/stacktrace.c
> index a1a7ff93b84f..b2b568e5deba 100644
> --- a/arch/arm64/kernel/stacktrace.c
> +++ b/arch/arm64/kernel/stacktrace.c
> @@ -33,11 +33,8 @@
>   */
>  
>  
> -static void unwind_init(struct unwind_state *state, unsigned long fp,
> -			unsigned long pc)
> +static void unwind_init_common(struct unwind_state *state)
>  {
> -	state->fp = fp;
> -	state->pc = pc;
>  #ifdef CONFIG_KRETPROBES
>  	state->kr_cur = NULL;
>  #endif
> @@ -56,6 +53,46 @@ static void unwind_init(struct unwind_state *state, unsigned long fp,
>  	state->prev_type = STACK_TYPE_UNKNOWN;
>  }
>  
> +/*
> + * TODO: document requirements here.
> + */

Please make this:

/*
 * Start an unwind from a pt_regs.
 *
 * The unwind will begin at the PC within the regs.
 *
 * The regs must be on a stack currently owned by the calling task.
 */

> +static inline void unwind_init_from_regs(struct unwind_state *state,
> +					 struct pt_regs *regs)
> +{

In future we could add:

	WARN_ON_ONCE(!on_accessible_stack(current, regs, sizeof(*regs), NULL));

... to validate the requirements, but I'm happy to lave that for a future patch
so this patch can be a pure refactoring.

> +	unwind_init_common(state);
> +
> +	state->fp = regs->regs[29];
> +	state->pc = regs->pc;
> +}
> +
> +/*
> + * TODO: document requirements here.
> + *
> + * Note: this is always inlined, and we expect our caller to be a noinline
> + * function, such that this starts from our caller's caller.
> + */

Please make this:

/*
 * Start an unwind from a caller.
 *
 * The unwind will begin at the caller of whichever function this is inlined
 * into.
 *
 * The function which invokes this must be noinline.
 */

> +static __always_inline void unwind_init_from_current(struct unwind_state *state)

Can we please rename s/current/caller/ here? That way it's clear *where* in
current we're unwinding from, and the fact that it's current is implicit but
obvious.

> +{

Similarly to unwind_init_from_regs(), in a future patch we could add:

	WARN_ON_ONCE(task == current);

... but for now we can omit that so this patch can be a pure refactoring.

> +	unwind_init_common(state);
> +
> +	state->fp = (unsigned long)__builtin_frame_address(1);
> +	state->pc = (unsigned long)__builtin_return_address(0);
> +}
> +
> +/*
> + * TODO: document requirements here.
> + *
> + * The caller guarantees that the task is not running.
> + */

Please make this:

/*
 * Start an unwind from a blocked task.
 *
 * The unwind will begin at the blocked tasks saved PC (i.e. the caller of
 * cpu_switch_to()).
 *
 * The caller should ensure the task is blocked in cpu_switch_to() for the
 * duration of the unwind, or the unwind will be bogus. It is never valid to
 * call this for the current task.
 */

Thanks,
Mark.

> +static inline void unwind_init_from_task(struct unwind_state *state,
> +					 struct task_struct *task)
> +{
> +	unwind_init_common(state);
> +
> +	state->fp = thread_saved_fp(task);
> +	state->pc = thread_saved_pc(task);
> +}
> +
>  /*
>   * Unwind from one frame record (A) to the next frame record (B).
>   *
> @@ -195,14 +232,11 @@ noinline notrace void arch_stack_walk(stack_trace_consume_fn consume_entry,
>  	struct unwind_state state;
>  
>  	if (regs)
> -		unwind_init(&state, regs->regs[29], regs->pc);
> +		unwind_init_from_regs(&state, regs);
>  	else if (task == current)
> -		unwind_init(&state,
> -				(unsigned long)__builtin_frame_address(1),
> -				(unsigned long)__builtin_return_address(0));
> +		unwind_init_from_current(&state);
>  	else
> -		unwind_init(&state, thread_saved_fp(task),
> -				thread_saved_pc(task));
> +		unwind_init_from_task(&state, task);
>  
>  	unwind(task, &state, consume_entry, cookie);
>  }
> -- 
> 2.25.1
>