[tip: locking/futex] selftests/futex: Provide thread creation and synchronization helpers

[tip-bot2 for Yuwen Chen]

The following commit has been merged into the locking/futex branch of tip:

Commit-ID:     6c4a1b972643d72bd26f674677df8b5a6a3fe94f
Gitweb:        https://git.kernel.org/tip/6c4a1b972643d72bd26f674677df8b5a6a3fe94f
Author:        Yuwen Chen <ywen.chen@xxxxxxxxxxx>
AuthorDate:    Mon, 18 May 2026 10:16:40 +08:00
Committer:     Thomas Gleixner <tglx@xxxxxxxxxx>
CommitterDate: Mon, 20 Jul 2026 20:58:51 +02:00

selftests/futex: Provide thread creation and synchronization helpers

There are timing issues in the use of threads in some selftests for futexes
as the tests rely on timed waits to ensure that the other test thread[s]
reached the lock wait function in the kernel.

That "works" on halfways idle systems, but fails under load which results
in tests failing.

Provide a set of helper functions to create test threads and to wait for
them to reach the lock wait by monitoring /proc/$PID/wchan.

[ tglx: Fixup coding style, move the timeout into the helper, adapt to test
  	harness changes and massage change log ]

Signed-off-by: Yuwen Chen <ywen.chen@xxxxxxxxxxx>
Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxx>
Link: https://patch.msgid.link/tencent_4C90BE98BBC48B38B9FBC6A95C45CF49F309@xxxxxx
---
 tools/testing/selftests/futex/functional/Makefile    |   3 +-
 tools/testing/selftests/futex/include/futex_thread.h | 117 ++++++++++-
 2 files changed, 119 insertions(+), 1 deletion(-)
 create mode 100644 tools/testing/selftests/futex/include/futex_thread.h

diff --git a/tools/testing/selftests/futex/functional/Makefile b/tools/testing/selftests/futex/functional/Makefile
index d59faf7..a03bd5a 100644
--- a/tools/testing/selftests/futex/functional/Makefile
+++ b/tools/testing/selftests/futex/functional/Makefile
@@ -11,7 +11,8 @@ endif
 
 LOCAL_HDRS := \
 	../include/futextest.h \
-	../include/atomic.h
+	../include/atomic.h \
+	../include/futex_thread.h
 TEST_GEN_PROGS := \
 	futex_wait_timeout \
 	futex_wait_wouldblock \
diff --git a/tools/testing/selftests/futex/include/futex_thread.h b/tools/testing/selftests/futex/include/futex_thread.h
new file mode 100644
index 0000000..a908829
--- /dev/null
+++ b/tools/testing/selftests/futex/include/futex_thread.h
@@ -0,0 +1,117 @@
+/* SPDX-License-Identifier: GPL-2.0-or-later */
+
+#ifndef _FUTEX_THREAD_H
+#define _FUTEX_THREAD_H
+#include <errno.h>
+#include <pthread.h>
+#include <stdio.h>
+#include <string.h>
+#include <unistd.h>
+
+#include "kselftest_harness.h"
+
+#define USEC_PER_SEC		1000000L
+#define WAIT_FOR_THREAD_SECS	1
+#define WAIT_FOR_THREAD_USECS	(WAIT_FOR_THREAD_SECS * USEC_PER_SEC)
+#define WAIT_THREAD_RETRIES	100
+
+struct futex_thread {
+	pthread_t		thread;
+	pthread_barrier_t	barrier;
+	pid_t			tid;
+	int			(*threadfn)(void *arg);
+	void			*arg;
+	int			retval;
+};
+
+static inline int __wait_for_thread(FILE *fp, struct __test_metadata *_metadata)
+{
+	unsigned int sleep_time_us = WAIT_FOR_THREAD_USECS / WAIT_THREAD_RETRIES;
+	char buf[80] = "";
+
+	for (int i = 0; i < WAIT_THREAD_RETRIES; i++) {
+		if (!fgets(buf, sizeof(buf), fp))
+			return EIO;
+		if (!strncmp(buf, "futex", 5))
+			return 0;
+		usleep(sleep_time_us);
+		rewind(fp);
+	}
+
+	TH_LOG("/proc/$PID/wchan contains \"%s\". Trying to continue.", buf);
+	return 0;
+}
+
+static void *__futex_thread_fn(void *arg)
+{
+	struct futex_thread *t = arg;
+
+	t->tid = gettid();
+	pthread_barrier_wait(&t->barrier);
+	t->retval = t->threadfn(t->arg);
+	return NULL;
+}
+
+/**
+ * futex_wait_for_thread - Wait for the child thread to sleep in the futex context
+ * @t:          Thread handle.
+ * @_metadata:	Test metadata for TH_LOG() context
+ */
+static inline int futex_wait_for_thread(struct futex_thread *t, struct __test_metadata *_metadata)
+{
+	char fname[80];
+	FILE *fp;
+	int res;
+
+	snprintf(fname, sizeof(fname), "/proc/%d/wchan", t->tid);
+	fp = fopen(fname, "r");
+	if (!fp) {
+		/* If /proc/... is not available, sleep */
+		if (errno != ENOENT)
+			return errno;
+		TH_LOG("/proc/$PID/wchan not accessible, continue with sleep()");
+		sleep(WAIT_FOR_THREAD_SECS);
+		return 0;
+	}
+
+	res = __wait_for_thread(fp, _metadata);
+	fclose(fp);
+	return res;
+}
+
+/**
+ * futex_thread_create - Create a new thread for testing.
+ * @t:        The handle of the newly created thread.
+ * @threadfn: The new thread starts execution by invoking threadfn
+ * @arg:      The parameters passed to threadfn.
+ */
+static inline int futex_thread_create(struct futex_thread *t, int (*threadfn)(void *), void *arg)
+{
+	pthread_barrier_init(&t->barrier, NULL, 2);
+
+	t->tid = 0;
+	t->threadfn = threadfn;
+	t->arg = arg;
+
+	if (pthread_create(&t->thread, NULL, __futex_thread_fn, t) < 0) {
+		int ret = errno;
+		pthread_barrier_destroy(&t->barrier);
+		return ret;
+	}
+
+	pthread_barrier_wait(&t->barrier);
+	return 0;
+}
+
+/**
+ * futex_thread_destroy - Wait for and reclaim the resources of the thread.
+ * @t:      Thread handle.
+ */
+static inline int futex_thread_destroy(struct futex_thread *t)
+{
+	pthread_join(t->thread, NULL);
+	pthread_barrier_destroy(&t->barrier);
+	return t->retval;
+}
+
+#endif