linux-kernel - [PATCH] selftests/futex: Order calls in futex

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Date: Wed, 19 Jun 2024 00:22:03 +0000
From: Edward Liaw <edliaw@...gle.com>
To: shuah@...nel.org, Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...hat.com>, 
	Peter Zijlstra <peterz@...radead.org>, Darren Hart <dvhart@...radead.org>, 
	Davidlohr Bueso <dave@...olabs.net>, 
	"André Almeida" <andrealmeid@...lia.com>
Cc: linux-kernel@...r.kernel.org, linux-kselftest@...r.kernel.org, 
	kernel-team@...roid.com, Edward Liaw <edliaw@...gle.com>
Subject: [PATCH] selftests/futex: Order calls in futex_requeue

Like fbf4dec70277 ("selftests/futex: Order calls to futex_lock_pi"),
which fixed a flake in futex_lock_pi due to racing between the parent
and child threads.

The same issue can occur in the futex_requeue test, because it expects
waiterfn to make progress to futex_wait before the parent starts to
requeue. This is mitigated by the parent sleeping for WAKE_WAIT_US, but
it still fails occasionally. This can be reproduced by adding a sleep in
the waiterfn before futex_wait:

TAP version 13
1..2
not ok 1 futex_requeue simple returned: 0
not ok 2 futex_requeue simple returned: 0
not ok 3 futex_requeue many returned: 0
not ok 4 futex_requeue many returned: 0

Instead, replace the sleep with barriers to make the sequencing
explicit.

Fixes: 7cb5dd8e2c8c ("selftests: futex: Add futex compare requeue test")
Signed-off-by: Edward Liaw <edliaw@...gle.com>
---
 .../selftests/futex/functional/futex_requeue.c       | 12 +++++++++---
 1 file changed, 9 insertions(+), 3 deletions(-)

diff --git a/tools/testing/selftests/futex/functional/futex_requeue.c b/tools/testing/selftests/futex/functional/futex_requeue.c
index 51485be6eb2f..8f7d3e8bf32a 100644
--- a/tools/testing/selftests/futex/functional/futex_requeue.c
+++ b/tools/testing/selftests/futex/functional/futex_requeue.c
@@ -12,9 +12,9 @@
 
 #define TEST_NAME "futex-requeue"
 #define timeout_ns  30000000
-#define WAKE_WAIT_US 10000
 
 volatile futex_t *f1;
+static pthread_barrier_t barrier;
 
 void usage(char *prog)
 {
@@ -32,6 +32,8 @@ void *waiterfn(void *arg)
 	to.tv_sec = 0;
 	to.tv_nsec = timeout_ns;
 
+	pthread_barrier_wait(&barrier);
+
 	if (futex_wait(f1, *f1, &to, 0))
 		printf("waiter failed errno %d\n", errno);
 
@@ -70,13 +72,15 @@ int main(int argc, char *argv[])
 	ksft_print_msg("%s: Test futex_requeue\n",
 		       basename(argv[0]));
 
+	pthread_barrier_init(&barrier, NULL, 2);
 	/*
 	 * Requeue a waiter from f1 to f2, and wake f2.
 	 */
 	if (pthread_create(&waiter[0], NULL, waiterfn, NULL))
 		error("pthread_create failed\n", errno);
 
-	usleep(WAKE_WAIT_US);
+	pthread_barrier_wait(&barrier);
+	pthread_barrier_destroy(&barrier);
 
 	info("Requeuing 1 futex from f1 to f2\n");
 	res = futex_cmp_requeue(f1, 0, &f2, 0, 1, 0);
@@ -99,6 +103,7 @@ int main(int argc, char *argv[])
 		ksft_test_result_pass("futex_requeue simple succeeds\n");
 	}
 
+	pthread_barrier_init(&barrier, NULL, 11);
 
 	/*
 	 * Create 10 waiters at f1. At futex_requeue, wake 3 and requeue 7.
@@ -109,7 +114,8 @@ int main(int argc, char *argv[])
 			error("pthread_create failed\n", errno);
 	}
 
-	usleep(WAKE_WAIT_US);
+	pthread_barrier_wait(&barrier);
+	pthread_barrier_destroy(&barrier);
 
 	info("Waking 3 futexes at f1 and requeuing 7 futexes from f1 to f2\n");
 	res = futex_cmp_requeue(f1, 0, &f2, 3, 7, 0);
-- 
2.45.2.627.g7a2c4fd464-goog