From d3b351f65bf42ccda1f686de3ccb21ea1a0c4f5a Mon Sep 17 00:00:00 2001 From: Yonghong Song Date: Thu, 10 Mar 2022 16:37:21 -0800 Subject: [PATCH] selftests/bpf: Fix a clang compilation error for send_signal.c Building selftests/bpf with latest clang compiler (clang15 built from source), I hit the following compilation error: /.../prog_tests/send_signal.c:43:16: error: variable 'j' set but not used [-Werror,-Wunused-but-set-variable] volatile int j = 0; ^ 1 error generated. The problem also exists with clang13 and clang14. clang12 is okay. In send_signal.c, we have the following code ... volatile int j = 0; [...] for (int i = 0; i < 100000000 && !sigusr1_received; i++) j /= i + 1; ... to burn CPU cycles so bpf_send_signal() helper can be tested in NMI mode. Slightly changing 'j /= i + 1' to 'j /= i + j + 1' or 'j++' can fix the problem. Further investigation indicated this should be a clang bug ([1]). The upstream fix will be proposed later. But it is a good idea to workaround the issue to unblock people who build kernel/selftests with clang. [1] https://discourse.llvm.org/t/strange-clang-unused-but-set-variable-error-with-volatile-variables/60841 Signed-off-by: Yonghong Song Signed-off-by: Daniel Borkmann Link: https://lore.kernel.org/bpf/20220311003721.2177170-1-yhs@fb.com --- tools/testing/selftests/bpf/prog_tests/send_signal.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-) diff --git a/tools/testing/selftests/bpf/prog_tests/send_signal.c b/tools/testing/selftests/bpf/prog_tests/send_signal.c index def50f1c5c31..d71226e34c34 100644 --- a/tools/testing/selftests/bpf/prog_tests/send_signal.c +++ b/tools/testing/selftests/bpf/prog_tests/send_signal.c @@ -65,7 +65,7 @@ static void test_send_signal_common(struct perf_event_attr *attr, /* wait a little for signal handler */ for (int i = 0; i < 100000000 && !sigusr1_received; i++) - j /= i + 1; + j /= i + j + 1; buf[0] = sigusr1_received ? '2' : '0'; ASSERT_EQ(sigusr1_received, 1, "sigusr1_received"); -- 2.11.0