| Message ID | 20220311000508.2036640-1-yhs@fb.com (mailing list archive) |
|---|---|
| State | Superseded |
| Delegated to: | BPF |
| Headers | show |
| Series | [bpf-next] selftests/bpf: fix a clang compilation error for send_signal.c | expand |
On Thu, Mar 10, 2022 at 4:05 PM Yonghong Song <yhs@fb.com> wrote: > > 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' 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 <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..05e303119151 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` was there to avoid division by zero. Let's make it `i + j + 1` then. > > buf[0] = sigusr1_received ? '2' : '0'; > ASSERT_EQ(sigusr1_received, 1, "sigusr1_received"); > -- > 2.30.2 >
On 3/10/22 4:17 PM, Andrii Nakryiko wrote: > On Thu, Mar 10, 2022 at 4:05 PM Yonghong Song <yhs@fb.com> wrote: >> >> 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' 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 <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..05e303119151 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` was there to avoid division by zero. Let's make it `i + j + 1` then. Good point. Previously I did this 'i + 1 + j' and run the selftests and it works fine. And for preparing the final patch, I removed "+ 1" to simplify the code but didn't actually run the test which is not great. Will fix in v2. > >> >> buf[0] = sigusr1_received ? '2' : '0'; >> ASSERT_EQ(sigusr1_received, 1, "sigusr1_received"); >> -- >> 2.30.2 >>
diff --git a/tools/testing/selftests/bpf/prog_tests/send_signal.c b/tools/testing/selftests/bpf/prog_tests/send_signal.c index def50f1c5c31..05e303119151 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; buf[0] = sigusr1_received ? '2' : '0'; ASSERT_EQ(sigusr1_received, 1, "sigusr1_received");
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' 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 <yhs@fb.com> --- tools/testing/selftests/bpf/prog_tests/send_signal.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-)