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

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Date: Tue,  2 Jul 2024 00:10:39 +0800
From: Kuan-Wei Chiu <visitorckw@...il.com>
To: rafael@...nel.org
Cc: 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,
	Kuan-Wei Chiu <visitorckw@...il.com>,
	stable@...r.kernel.org
Subject: [PATCH v3] ACPI: processor_idle: Fix invalid comparison with insertion sort for latency

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)
 {
-	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;
+		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);
+			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);
-- 
2.34.1