[PATCH v4] ACPI: processor_idle: Fix invalid comparison with insertion sort for latency

Kuan-Wei Chiu
2024-07-01T20:56:47Z

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@vger.kernel.org
Reported-by: Julian Sikorski <belegdol@gmail.com>
Closes: https://lore.kernel.org/lkml/70674dc7-5586-4183-8953-8095567e73df@gmail.com/
Signed-off-by: Kuan-Wei Chiu <visitorckw@gmail.com>
---
v3 -> v4:
- Rename the parameter 'arr' to 'states'.
- Add empty lines to enhance readability.

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 | 37 +++++++++++++++--------------------
1 file changed, 16 insertions(+), 21 deletions(-)

diff --git a/drivers/acpi/processor_idle.c b/drivers/acpi/processor_idle.c
index bd6a7857ce05..831fa4a12159 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 +385,24 @@ 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 *states, size_t length)
{
- 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;
+ for (i = 1; i < length; i++) {
+ if (!states[i].valid)
+ continue;

- if (!(x->valid && y->valid))
- return;
- swap(x->latency, y->latency);
+ for (j = i - 1, k = i; j >= 0; j--) {
+ if (!states[j].valid)
+ continue;
+
+ if (states[j].latency > states[k].latency)
+ swap(states[j].latency, states[k].latency);
+
+ k = j;
+ }
+ }
}

static int acpi_processor_power_verify(struct acpi_processor *pr)
@@ -449,10 +447,7 @@ 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);
--
2.34.1