Re: [PATCH] selftests/rseq: fix kselftest Clang build warnings

[Justin Stitt]

Ping.

Looking to get this patch and [1] slated for 6.7 which fixes some
kselftest builds on older kernels.

On Wed, Sep 13, 2023 at 6:03 AM Justin Stitt <justinstitt@xxxxxxxxxx> wrote:
>
> When building with Clang, I am getting many warnings from the selftests/rseq tree.
>
> Here's one such example from rseq tree:
> |  param_test.c:1234:10: error: address argument to atomic operation must be a pointer to _Atomic type ('intptr_t *' (aka 'long *') invalid)
> |   1234 |         while (!atomic_load(&args->percpu_list_ptr)) {}
> |        |                 ^           ~~~~~~~~~~~~~~~~~~~~~~
> |  /usr/local/google/home/justinstitt/repos/tc-build/build/llvm/final/lib/clang/18/include/stdatomic.h:140:29: note: expanded from macro 'atomic_load'
> |    140 | #define atomic_load(object) __c11_atomic_load(object, __ATOMIC_SEQ_CST)
> |        |                             ^                 ~~~~~~
>
> Use compiler builtins `__atomic_load_n()` and `__atomic_store_n()` with
> accompanying __ATOMIC_ACQUIRE and __ATOMIC_RELEASE, respectively. This
> will fix the warnings because the compiler builtins do not expect their
> arguments to have _Atomic type. This should also make TSAN happier.
>
> Link: https://github.com/ClangBuiltLinux/linux/issues/1698
> Link: https://github.com/ClangBuiltLinux/continuous-integration2/issues/61
> Suggested-by: Mathieu Desnoyers <mathieu.desnoyers@xxxxxxxxxxxx>
> Signed-off-by: Justin Stitt <justinstitt@xxxxxxxxxx>
> ---
> Note: Previous RFC https://lore.kernel.org/r/20230908-kselftest-param_test-c-v1-1-e35bd9052d61@xxxxxxxxxx
> ---
>  tools/testing/selftests/rseq/param_test.c | 20 ++++++++++----------
>  1 file changed, 10 insertions(+), 10 deletions(-)
>
> diff --git a/tools/testing/selftests/rseq/param_test.c b/tools/testing/selftests/rseq/param_test.c
> index bf951a490bb4..20403d58345c 100644
> --- a/tools/testing/selftests/rseq/param_test.c
> +++ b/tools/testing/selftests/rseq/param_test.c
> @@ -1231,7 +1231,7 @@ void *test_membarrier_worker_thread(void *arg)
>         }
>
>         /* Wait for initialization. */
> -       while (!atomic_load(&args->percpu_list_ptr)) {}
> +       while (!__atomic_load_n(&args->percpu_list_ptr, __ATOMIC_ACQUIRE)) {}
>
>         for (i = 0; i < iters; ++i) {
>                 int ret;
> @@ -1299,22 +1299,22 @@ void *test_membarrier_manager_thread(void *arg)
>         test_membarrier_init_percpu_list(&list_a);
>         test_membarrier_init_percpu_list(&list_b);
>
> -       atomic_store(&args->percpu_list_ptr, (intptr_t)&list_a);
> +       __atomic_store_n(&args->percpu_list_ptr, (intptr_t)&list_a, __ATOMIC_RELEASE);
>
> -       while (!atomic_load(&args->stop)) {
> +       while (!__atomic_load_n(&args->stop, __ATOMIC_ACQUIRE)) {
>                 /* list_a is "active". */
>                 cpu_a = rand() % CPU_SETSIZE;
>                 /*
>                  * As list_b is "inactive", we should never see changes
>                  * to list_b.
>                  */
> -               if (expect_b != atomic_load(&list_b.c[cpu_b].head->data)) {
> +               if (expect_b != __atomic_load_n(&list_b.c[cpu_b].head->data, __ATOMIC_ACQUIRE)) {
>                         fprintf(stderr, "Membarrier test failed\n");
>                         abort();
>                 }
>
>                 /* Make list_b "active". */
> -               atomic_store(&args->percpu_list_ptr, (intptr_t)&list_b);
> +               __atomic_store_n(&args->percpu_list_ptr, (intptr_t)&list_b, __ATOMIC_RELEASE);
>                 if (rseq_membarrier_expedited(cpu_a) &&
>                                 errno != ENXIO /* missing CPU */) {
>                         perror("sys_membarrier");
> @@ -1324,27 +1324,27 @@ void *test_membarrier_manager_thread(void *arg)
>                  * Cpu A should now only modify list_b, so the values
>                  * in list_a should be stable.
>                  */
> -               expect_a = atomic_load(&list_a.c[cpu_a].head->data);
> +               expect_a = __atomic_load_n(&list_a.c[cpu_a].head->data, __ATOMIC_ACQUIRE);
>
>                 cpu_b = rand() % CPU_SETSIZE;
>                 /*
>                  * As list_a is "inactive", we should never see changes
>                  * to list_a.
>                  */
> -               if (expect_a != atomic_load(&list_a.c[cpu_a].head->data)) {
> +               if (expect_a != __atomic_load_n(&list_a.c[cpu_a].head->data, __ATOMIC_ACQUIRE)) {
>                         fprintf(stderr, "Membarrier test failed\n");
>                         abort();
>                 }
>
>                 /* Make list_a "active". */
> -               atomic_store(&args->percpu_list_ptr, (intptr_t)&list_a);
> +               __atomic_store_n(&args->percpu_list_ptr, (intptr_t)&list_a, __ATOMIC_RELEASE);
>                 if (rseq_membarrier_expedited(cpu_b) &&
>                                 errno != ENXIO /* missing CPU*/) {
>                         perror("sys_membarrier");
>                         abort();
>                 }
>                 /* Remember a value from list_b. */
> -               expect_b = atomic_load(&list_b.c[cpu_b].head->data);
> +               expect_b = __atomic_load_n(&list_b.c[cpu_b].head->data, __ATOMIC_ACQUIRE);
>         }
>
>         test_membarrier_free_percpu_list(&list_a);
> @@ -1401,7 +1401,7 @@ void test_membarrier(void)
>                 }
>         }
>
> -       atomic_store(&thread_args.stop, 1);
> +       __atomic_store_n(&thread_args.stop, 1, __ATOMIC_RELEASE);
>         ret = pthread_join(manager_thread, NULL);
>         if (ret) {
>                 errno = ret;
>
> ---
> base-commit: 2dde18cd1d8fac735875f2e4987f11817cc0bc2c
> change-id: 20230908-kselftest-param_test-c-1763b62e762f
>
> Best regards,
> --
> Justin Stitt <justinstitt@xxxxxxxxxx>
>

[1]: https://lore.kernel.org/all/20230908-kselftest-09-08-v2-0-0def978a4c1b@xxxxxxxxxx/

Thanks
Justin