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

[Eduard Zingerman <eddyz87@gmail.com>]

On Tue, 2025-02-04 at 00:30 +0000, Peilin Ye wrote:
> Hi Eduard,
> 
> One more question (for my understanding):
> 
> On Tue, Jan 28, 2025 at 05:06:03PM -0800, Eduard Zingerman wrote:
> > On Sat, 2025-01-25 at 02:19 +0000, Peilin Ye 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@fb.com/#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")