linux-kernel - [PATCH] selftests/futex: fix the failed futex

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Message-ID: <tencent_3F58110777CBF8A0ABD3B96DDEDCD75F9706@qq.com>
Date: Fri, 23 Jan 2026 13:41:32 +0800
From: Yuwen Chen <ywen.chen@...mail.com>
To: akpm@...ux-foundation.org
Cc: licayy@...mail.com,
	andrealmeid@...lia.com,
	bigeasy@...utronix.de,
	colin.i.king@...il.com,
	dave@...olabs.net,
	dvhart@...radead.org,
	justinstitt@...gle.com,
	kernel-team@...roid.com,
	linux-kernel@...r.kernel.org,
	linux-kselftest@...r.kernel.org,
	luto@....edu,
	mingo@...hat.com,
	morbo@...gle.com,
	nathan@...nel.org,
	ndesaulniers@...gle.com,
	peterz@...radead.org,
	shuah@...nel.org,
	tglx@...utronix.de,
	usama.anjum@...labora.com,
	ywen.chen@...mail.com,
	Edward Liaw <edliaw@...gle.com>
Subject: [PATCH] selftests/futex: fix the failed futex_requeue test issue

This test item has extremely high requirements for timing and can only
pass the test under specific conditions. The following situations will
lead to test failure:

    MainThread                  Thread1
        |
   pthread_create-------------------
        |                          |
 futex_cmp_requeue                 |
        |                      futex_wait
        |                          |

If the child thread is not waiting in the futex_wait function when the
main thread reaches the futex_cmp_requeue function, the test will fail.

This patch avoids this problem by checking whether the child thread is
in a sleeping state in the main thread.

Fixes: 7cb5dd8e2c8c ("selftests: futex: Add futex compare requeue test")
Signed-off-by: Yuwen Chen <ywen.chen@...mail.com>
Co-developed-by: Edward Liaw <edliaw@...gle.com>
Signed-off-by: Edward Liaw <edliaw@...gle.com>
---
 .../futex/functional/futex_requeue.c          | 65 ++++++++++++++++---
 1 file changed, 55 insertions(+), 10 deletions(-)

diff --git a/tools/testing/selftests/futex/functional/futex_requeue.c b/tools/testing/selftests/futex/functional/futex_requeue.c
index 35d4be23db5da..f17debd756303 100644
--- a/tools/testing/selftests/futex/functional/futex_requeue.c
+++ b/tools/testing/selftests/futex/functional/futex_requeue.c
@@ -7,21 +7,27 @@
 
 #include <pthread.h>
 #include <limits.h>
+#include <stdatomic.h>
 
 #include "futextest.h"
 #include "kselftest_harness.h"
 
-#define timeout_ns  30000000
+#define timeout_s  3 /* 3s */
 #define WAKE_WAIT_US 10000
 
 volatile futex_t *f1;
+static pthread_barrier_t barrier;
 
 void *waiterfn(void *arg)
 {
 	struct timespec to;
+	atomic_int *tid = (atomic_int *)arg;
 
-	to.tv_sec = 0;
-	to.tv_nsec = timeout_ns;
+	to.tv_sec = timeout_s;
+	to.tv_nsec = 0;
+
+	atomic_store(tid, gettid());
+	pthread_barrier_wait(&barrier);
 
 	if (futex_wait(f1, *f1, &to, 0))
 		printf("waiter failed errno %d\n", errno);
@@ -29,22 +35,50 @@ void *waiterfn(void *arg)
 	return NULL;
 }
 
+static int get_thread_state(pid_t pid)
+{
+	FILE *fp;
+	char buf[80], tag[80];
+	char val = 0;
+
+	snprintf(buf, sizeof(buf), "/proc/%d/status", pid);
+	fp = fopen(buf, "r");
+	if (!fp)
+		return -1;
+
+	while (fgets(buf, sizeof(buf), fp))
+		if (fscanf(fp, "%s %c\n", tag, &val) == 2 && !strcmp(tag, "State:"))
+			break;
+
+	fclose(fp);
+	return val;
+}
+
 TEST(requeue_single)
 {
 	volatile futex_t _f1 = 0;
 	volatile futex_t f2 = 0;
 	pthread_t waiter[10];
-	int res;
+	atomic_int tid = 0;
+	int res, state;
 
 	f1 = &_f1;
+	pthread_barrier_init(&barrier, NULL, 2);
 
 	/*
 	 * Requeue a waiter from f1 to f2, and wake f2.
 	 */
-	if (pthread_create(&waiter[0], NULL, waiterfn, NULL))
+	if (pthread_create(&waiter[0], NULL, waiterfn, &tid))
 		ksft_exit_fail_msg("pthread_create failed\n");
 
-	usleep(WAKE_WAIT_US);
+	pthread_barrier_wait(&barrier);
+	pthread_barrier_destroy(&barrier);
+	while ((state = get_thread_state(atomic_load(&tid))) != 'S') {
+		usleep(WAKE_WAIT_US);
+
+		if (state < 0)
+			break;
+	}
 
 	ksft_print_dbg_msg("Requeuing 1 futex from f1 to f2\n");
 	res = futex_cmp_requeue(f1, 0, &f2, 0, 1, 0);
@@ -69,7 +103,8 @@ TEST(requeue_multiple)
 	volatile futex_t _f1 = 0;
 	volatile futex_t f2 = 0;
 	pthread_t waiter[10];
-	int res, i;
+	atomic_int tids[10] = {0};
+	int res, i, state;
 
 	f1 = &_f1;
 
@@ -78,11 +113,21 @@ TEST(requeue_multiple)
 	 * At futex_wake, wake INT_MAX (should be exactly 7).
 	 */
 	for (i = 0; i < 10; i++) {
-		if (pthread_create(&waiter[i], NULL, waiterfn, NULL))
+		pthread_barrier_init(&barrier, NULL, 2);
+
+		if (pthread_create(&waiter[i], NULL, waiterfn, &tids[i]))
 			ksft_exit_fail_msg("pthread_create failed\n");
-	}
 
-	usleep(WAKE_WAIT_US);
+		pthread_barrier_wait(&barrier);
+		pthread_barrier_destroy(&barrier);
+
+		while ((state = get_thread_state(atomic_load(&tids[i]))) != 'S') {
+			usleep(WAKE_WAIT_US);
+
+			if (state < 0)
+				break;
+		}
+	}
 
 	ksft_print_dbg_msg("Waking 3 futexes at f1 and requeuing 7 futexes from f1 to f2\n");
 	res = futex_cmp_requeue(f1, 0, &f2, 3, 7, 0);
-- 
2.34.1