linux-kernel - Re: [PATCH v3] ACPI: processor_idle: Fix invalid comparison with insertion sort for latency

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Message-ID: <CAJZ5v0iY=S+WKWvDAAWxLcOwvpOG5Cck1gQv4p+FfW1Nca0Yqw@mail.gmail.com>
Date: Mon, 1 Jul 2024 19:36:40 +0200
From: "Rafael J. Wysocki" <rafael@...nel.org>
To: Kuan-Wei Chiu <visitorckw@...il.com>
Cc: rafael@...nel.org, lenb@...nel.org, mario.limonciello@....com, 
	akpm@...ux-foundation.org, jserv@...s.ncku.edu.tw, alexdeucher@...il.com, 
	belegdol@...il.com, regressions@...mhuis.info, linux-acpi@...r.kernel.org, 
	regressions@...ts.linux.dev, linux-kernel@...r.kernel.org, 
	stable@...r.kernel.org
Subject: Re: [PATCH v3] ACPI: processor_idle: Fix invalid comparison with
 insertion sort for latency

On Mon, Jul 1, 2024 at 6:10 PM Kuan-Wei Chiu <visitorckw@...il.com> wrote:
>
> The acpi_cst_latency_cmp comparison function currently used for sorting
> C-state latencies does not satisfy transitivity, causing incorrect
> sorting results. Specifically, if there are two valid acpi_processor_cx
> elements A and B and one invalid element C, it may occur that A < B,
> A = C, and B = C. Sorting algorithms assume that if A < B and A = C,
> then C < B, leading to incorrect ordering.
>
> Given the small size of the array (<=8), we replace the library sort
> function with a simple insertion sort that properly ignores invalid
> elements and sorts valid ones based on latency. This change ensures
> correct ordering of the C-state latencies.
>
> Fixes: 65ea8f2c6e23 ("ACPI: processor idle: Fix up C-state latency if not ordered")
> Cc: stable@...r.kernel.org
> Reported-by: Julian Sikorski <belegdol@...il.com>
> Closes: https://lore.kernel.org/lkml/70674dc7-5586-4183-8953-8095567e73df@gmail.com/
> Signed-off-by: Kuan-Wei Chiu <visitorckw@...il.com>
> ---
> v2 -> v3:
> - Remove #include <linux/sort.h>
> - Cc @stable
>
> Note: I only performed a build test and a simple unit test to ensure
>       the latency of valid elements is correctly sorted in the randomly
>           generated data.
>
>  drivers/acpi/processor_idle.c | 36 ++++++++++++++---------------------
>  1 file changed, 14 insertions(+), 22 deletions(-)
>
> diff --git a/drivers/acpi/processor_idle.c b/drivers/acpi/processor_idle.c
> index bd6a7857ce05..17cc81340b4b 100644
> --- a/drivers/acpi/processor_idle.c
> +++ b/drivers/acpi/processor_idle.c
> @@ -16,7 +16,6 @@
>  #include <linux/acpi.h>
>  #include <linux/dmi.h>
>  #include <linux/sched.h>       /* need_resched() */
> -#include <linux/sort.h>
>  #include <linux/tick.h>
>  #include <linux/cpuidle.h>
>  #include <linux/cpu.h>
> @@ -386,25 +385,21 @@ static void acpi_processor_power_verify_c3(struct acpi_processor *pr,
>         acpi_write_bit_register(ACPI_BITREG_BUS_MASTER_RLD, 1);
>  }
>
> -static int acpi_cst_latency_cmp(const void *a, const void *b)
> +static void acpi_cst_latency_sort(struct acpi_processor_cx *arr, size_t length)

s/arr/states/ please.

>  {
> -       const struct acpi_processor_cx *x = a, *y = b;
> +       int i, j, k;
>
> -       if (!(x->valid && y->valid))
> -               return 0;
> -       if (x->latency > y->latency)
> -               return 1;
> -       if (x->latency < y->latency)
> -               return -1;
> -       return 0;
> -}
> -static void acpi_cst_latency_swap(void *a, void *b, int n)
> -{
> -       struct acpi_processor_cx *x = a, *y = b;
> -
> -       if (!(x->valid && y->valid))
> -               return;
> -       swap(x->latency, y->latency);
> +       for (i = 1; i < length; i++) {
> +               if (!arr[i].valid)
> +                       continue;

Please add an empty line here (and analogously below).

> +               for (j = i - 1, k = i; j >= 0; j--) {
> +                       if (!arr[j].valid)
> +                               continue;
> +                       if (arr[j].latency > arr[k].latency)
> +                               swap(arr[j].latency, arr[k].latency);

And here.

> +                       k = j;
> +               }
> +       }
>  }
>
>  static int acpi_processor_power_verify(struct acpi_processor *pr)
> @@ -449,10 +444,7 @@ static int acpi_processor_power_verify(struct acpi_processor *pr)
>
>         if (buggy_latency) {
>                 pr_notice("FW issue: working around C-state latencies out of order\n");
> -               sort(&pr->power.states[1], max_cstate,
> -                    sizeof(struct acpi_processor_cx),
> -                    acpi_cst_latency_cmp,
> -                    acpi_cst_latency_swap);
> +               acpi_cst_latency_sort(&pr->power.states[1], max_cstate);
>         }
>
>         lapic_timer_propagate_broadcast(pr);
> --