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Date: Tue, 15 Feb 2022 13:07:56 +0000
From: Mark Rutland <mark.rutland@....com>
To: madvenka@...ux.microsoft.com
Cc: broonie@...nel.org, jpoimboe@...hat.com, ardb@...nel.org,
nobuta.keiya@...itsu.com, sjitindarsingh@...il.com,
catalin.marinas@....com, will@...nel.org, jmorris@...ei.org,
linux-arm-kernel@...ts.infradead.org,
live-patching@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v13 04/11] arm64: Split unwind_init()
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@...ux.microsoft.com wrote:
> From: "Madhavan T. Venkataraman" <madvenka@...ux.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@...ux.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
>
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