Re: [PATCH 2/5] ARM: stacktrace: Avoid duplicate saving of exception PC value

From: Linus Walleij
Date: Mon Jul 18 2022 - 05:01:53 EST

On Tue, Jul 12, 2022 at 4:18 AM Li Huafei <lihuafei1@xxxxxxxxxx> wrote:

> Because an exception stack frame is not created in the exception entry,
> save_trace() does special handling for the exception PC, but this is
> only needed when CONFIG_FRAME_POINTER_UNWIND=y. When
> CONFIG_ARM_UNWIND=y, unwind annotations have been added to the exception
> entry and save_trace() will repeatedly save the exception PC:
>
> [0x7f000090] hrtimer_hander+0x8/0x10 [hrtimer]
> [0x8019ec50] __hrtimer_run_queues+0x18c/0x394
> [0x8019f760] hrtimer_run_queues+0xbc/0xd0
> [0x8019def0] update_process_times+0x34/0x80
> [0x801ad2a4] tick_periodic+0x48/0xd0
> [0x801ad3dc] tick_handle_periodic+0x1c/0x7c
> [0x8010f2e0] twd_handler+0x30/0x40
> [0x80177620] handle_percpu_devid_irq+0xa0/0x23c
> [0x801718d0] generic_handle_domain_irq+0x24/0x34
> [0x80502d28] gic_handle_irq+0x74/0x88
> [0x8085817c] generic_handle_arch_irq+0x58/0x78
> [0x80100ba8] __irq_svc+0x88/0xc8
> [0x80108114] arch_cpu_idle+0x38/0x3c
> [0x80108114] arch_cpu_idle+0x38/0x3c <==== duplicate saved exception PC
> [0x80861bf8] default_idle_call+0x38/0x130
> [0x8015d5cc] do_idle+0x150/0x214
> [0x8015d978] cpu_startup_entry+0x18/0x1c
> [0x808589c0] rest_init+0xd8/0xdc
> [0x80c00a44] arch_post_acpi_subsys_init+0x0/0x8
>
> We can move the special handling of the exception PC in save_trace() to
> the unwind_frame() of the frame pointer unwinder.
>
> Signed-off-by: Li Huafei <lihuafei1@xxxxxxxxxx>

This is another very nice patch!
Reviewed-by: Linus Waleij <linus.walleij@xxxxxxxxxx>

Nitpick:

> + if ((unsigned long)&regs[1] > ALIGN(frame->sp, THREAD_SIZE))
> + return -EINVAL;

It'd be nice to add a comment saying what is in regs[1] at this point
so it is easier to understand the code. Not your fault as it is just
moved code, but if you have time please add a small comment.

Yours,
Linus Walleij