FROMLIST: selftests/futex: Order calls in futex_requeue

Fixes a race between parent and child threads in futex_requeue.

Similar to fbf4dec702 ("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: 7cb5dd8e2c ("selftests: futex: Add futex compare requeue test")
Signed-off-by: Edward Liaw <edliaw@google.com>
Reviewed-by: Muhammad Usama Anjum <usama.anjum@collabora.com>
Reviewed-by: André Almeida <andrealmeid@igalia.com>

Link: https://lore.kernel.org/linux-kselftest/20240918231102.234253-1-edliaw@google.com/
Bug: 367134942
Bug: 260003172
Test: ./run_test_only.sh -t 'selftests kselftest_futex_requeue'
Change-Id: I8650c173bfd6333e00d0dede381a10310f2f42d1
Signed-off-by: Edward Liaw <edliaw@google.com>

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);