[PATCH 2/2] selftests/membarrier: Skip unpermitted membarrier command test if preregistered by libc

On thread creation, Musl registers the private expedited memory barrier,
see pthread_create [1]. Thus, invoking the barrier command will no
longer be rejected by the kernel with EPERM. The test checking this will
fail. Check if the memory barrier command has been registered and skip
the test in this case.

Link: https://git.musl-libc.org/cgit/musl/tree/src/thread/pthread_create.c#n260 [1]
Signed-off-by: Chris Gellermann <[email protected]>
---
 .../selftests/membarrier/membarrier_test_impl.h        | 10 ++++++++++
 1 file changed, 10 insertions(+)

diff --git a/tools/testing/selftests/membarrier/membarrier_test_impl.h b/tools/testing/selftests/membarrier/membarrier_test_impl.h
index 29aac3bc4988..b4dcbb32538d 100644
--- a/tools/testing/selftests/membarrier/membarrier_test_impl.h
+++ b/tools/testing/selftests/membarrier/membarrier_test_impl.h
@@ -113,6 +113,16 @@ static int test_membarrier_private_expedited_fail(void)
 	int cmd = MEMBARRIER_CMD_PRIVATE_EXPEDITED, flags = 0;
 	const char *test_name = "sys membarrier MEMBARRIER_CMD_PRIVATE_EXPEDITED not registered failure";
 
+	/*
+	 * Some C libraries, like Musl, register the private expedited barrier
+	 * command when creating a thread. Expecting an EPERM on an unregistered
+	 * command will therefore no longer work. Skip the test in this case.
+	 */
+	if (MEMBARRIER_CMD_REGISTER_PRIVATE_EXPEDITED & membarrier_get_registrations()) {
+		ksft_test_result_skip("%s test: Command already registered\n", test_name);
+		return 0;
+	}
+
 	if (sys_membarrier(cmd, flags) != -1) {
 		ksft_exit_fail_msg(
 			"%s test: flags = %d. Should fail, but passed\n",
-- 
2.47.3