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

[Andrew Morton <akpm@linux-foundation.org>]

On Mon,  3 Aug 2026 14:49:00 +0200 Chris Gellermann <christian.gellermann@codasip.com> wrote:

> 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.
> 

Who the heck maintains sched/membarrier?

Short answer: everybody I've ever met.  All are cheerfully cc'ed.

Perhaps finer-tuned answer: Peter.

> --- 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;
> +	}

AI review
(https://sashiko.dev/#/patchset/20260803124900.3328789-1-christian.gellermann@codasip.com)
thinks that membarrier_get_registrations() can return -1 on old
kernels, so we should check for that before testing for
MEMBARRIER_CMD_REGISTER_PRIVATE_EXPEDITED.

Seems nonsensical to me - membarrier_get_registrations() doesn't do that.

And policy (my policy, at least) is that selftests test the kernel
they're shipped with and anyone who tries to run them against any older
or newer kernel is all out of luck.


That being said, and stepping back a bit, is this test testing
something which we would test?  If pass/fail depends upon deepest
details of the chosen libc flavour then perhaps the whole test was
ill-conceived.  wdyt?