Re: [PATCH bpf-next v1 7/8] selftests/bpf: Add selftests for load-acquire and store-release instructions

[Peilin Ye]

On Mon, Feb 03, 2025 at 04:52:52PM -0800, Eduard Zingerman wrote:
> > > > --- a/tools/testing/selftests/bpf/progs/arena_atomics.c
> > > > +++ b/tools/testing/selftests/bpf/progs/arena_atomics.c
> > > [...]
> > > 
> > > > +SEC("raw_tp/sys_enter")
> > > > +int load_acquire(const void *ctx)
> > > > +{
> > > > +	if (pid != (bpf_get_current_pid_tgid() >> 32))
> > > > +		return 0;
> > > 
> > > Nit: This check is not needed, since bpf_prog_test_run_opts() is used
> > >      to run the tests.
> > 
> > Could you explain a bit more why it's not needed?
> > 
> > I read commit 0f4feacc9155 ("selftests/bpf: Adding pid filtering for
> > atomics test") which added those 'pid' checks to atomics/ tests.  The
> > commit message [1] says the purpose was to "make atomics test able to
> > run in parallel with other tests", which I couldn't understand.
> > 
> > How using bpf_prog_test_run_opts() makes those 'pid' checks unnecessary?
> > 
> > [1] https://lore.kernel.org/bpf/20211006185619.364369-11-fallentree@xxxxxx/#r
> 
> Hi Peilin,
> 
> The entry point for the test looks as follows:
> 
>     void test_arena_atomics(void)
>     {
>     	...
>     	skel = arena_atomics__open();
>     	if (!ASSERT_OK_PTR(skel, "arena atomics skeleton open"))
>     		return;
>     
>     	if (skel->data->skip_tests) { ... }
>     	err = arena_atomics__load(skel);
>     	if (!ASSERT_OK(err, "arena atomics skeleton load"))
>     		return;
>     	skel->bss->pid = getpid();
>     
>     	if (test__start_subtest("add"))
>     		test_add(skel);
>             ...
>     
>     cleanup:
>     	arena_atomics__destroy(skel);
>     }
> 
> Note arena_atomics__{open,load} calls but absence of the
> arena_atomics__attach call. W/o arena_atomics__attach call the
> programs would not be hooked to the designated extension points,
> e.g. "raw_tp/sys_enter".
> 
> The bpf_prog_test_run_opts() invokes BPF_PROG_TEST_RUN command of the
> bpf system call, which does not attach the program either,
> but executes jitted code directly with fake context.
> (See bpf_prog_ops->test_run callback (method?) and
>  bpf_prog_test_run_raw_tp()).
> 
> Same happens in prog{,_tests}/arena.c: no attachment happens after
> commit [2]. Commit [1] is unnecessary after [2].
> 
> [2] commit 04fcb5f9a104 ("selftests/bpf: Migrate from bpf_prog_test_run")

I see.  Thanks for the quick reply as always!

Peilin Ye