linux-kernel - Re: [PATCH bpf-next] selftests/bpf: Fix htab_update/reenter

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Message-ID: <aRSi7MADBZBoLLEP@linux.ibm.com>
Date: Wed, 12 Nov 2025 20:38:28 +0530
From: Saket Kumar Bhaskar <skb99@...ux.ibm.com>
To: Alexei Starovoitov <alexei.starovoitov@...il.com>
Cc: bpf <bpf@...r.kernel.org>,
        "open list:KERNEL SELFTEST FRAMEWORK" <linux-kselftest@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Hari Bathini <hbathini@...ux.ibm.com>, sachinpb@...ux.ibm.com,
        Venkat Rao Bagalkote <venkat88@...ux.ibm.com>,
        Andrii Nakryiko <andrii@...nel.org>, Eduard <eddyz87@...il.com>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Martin KaFai Lau <martin.lau@...ux.dev>, Song Liu <song@...nel.org>,
        Yonghong Song <yonghong.song@...ux.dev>,
        John Fastabend <john.fastabend@...il.com>,
        KP Singh <kpsingh@...nel.org>, Stanislav Fomichev <sdf@...ichev.me>,
        Hao Luo <haoluo@...gle.com>, Jiri Olsa <jolsa@...nel.org>,
        Shuah Khan <shuah@...nel.org>
Subject: Re: [PATCH bpf-next] selftests/bpf: Fix htab_update/reenter_update
 selftest failure

On Tue, Nov 11, 2025 at 10:35:39AM -0800, Alexei Starovoitov wrote:
> On Tue, Nov 11, 2025 at 6:33 AM Saket Kumar Bhaskar <skb99@...ux.ibm.com> wrote:
> >
> > On Thu, Nov 06, 2025 at 09:15:39AM -0800, Alexei Starovoitov wrote:
> > > On Wed, Nov 5, 2025 at 9:26 PM Saket Kumar Bhaskar <skb99@...ux.ibm.com> wrote:
> > > >
> > > > Since commit 31158ad02ddb ("rqspinlock: Add deadlock detection and recovery")
> > > > the updated path on re-entrancy now reports deadlock via
> > > > -EDEADLK instead of the previous -EBUSY.
> > > >
> > > > The selftest is updated to align with expected errno
> > > > with the kernel’s current behavior.
> > > >
> > > > Signed-off-by: Saket Kumar Bhaskar <skb99@...ux.ibm.com>
> > > > ---
> > > >  tools/testing/selftests/bpf/prog_tests/htab_update.c | 2 +-
> > > >  1 file changed, 1 insertion(+), 1 deletion(-)
> > > >
> > > > diff --git a/tools/testing/selftests/bpf/prog_tests/htab_update.c b/tools/testing/selftests/bpf/prog_tests/htab_update.c
> > > > index 2bc85f4814f4..98d52bb1446f 100644
> > > > --- a/tools/testing/selftests/bpf/prog_tests/htab_update.c
> > > > +++ b/tools/testing/selftests/bpf/prog_tests/htab_update.c
> > > > @@ -40,7 +40,7 @@ static void test_reenter_update(void)
> > > >         if (!ASSERT_OK(err, "add element"))
> > > >                 goto out;
> > > >
> > > > -       ASSERT_EQ(skel->bss->update_err, -EBUSY, "no reentrancy");
> > > > +       ASSERT_EQ(skel->bss->update_err, -EDEADLK, "no reentrancy");
> > >
> > > Makes sense, but looks like the test was broken for quite some time.
> > > It fails with
> > >         /* lookup_elem_raw() may be inlined and find_kernel_btf_id()
> > > will return -ESRCH */
> > >         bpf_program__set_autoload(skel->progs.lookup_elem_raw, true);
> > >         err = htab_update__load(skel);
> > >         if (!ASSERT_TRUE(!err || err == -ESRCH, "htab_update__load") || err)
> > >
> > > before reaching deadlk check.
> > > Pls make it more robust.
> > > __pcpu_freelist_pop() might be better alternative then lookup_elem_raw().
> > >
> > > pw-bot: cr
> >
> > Hi Alexei,
> >
> > I tried for __pcpu_freelist_pop, looks like it is not good candidate to
> > attach fentry for, as it is non traceable:
> >
> > trace_kprobe: Could not probe notrace function __pcpu_freelist_pop
> >
> > I wasn't able to find any other function for this.
> 
> alloc_htab_elem() is not inlined for me.
> bpf_obj_free_fields() would be another option.
Since alloc_htab_elem() is a static function, wouldn’t its
inlining behavior be compiler-dependent?

static struct htab_elem *alloc_htab_elem(struct bpf_htab *htab, void *key,
                                         void *value, u32 key_size, u32 hash,
                                         bool percpu, bool onallcpus,
                                         struct htab_elem *old_elem)

When the fentry program is instead attached to bpf_obj_free_fields(),
the bpf_map_update_elem() call returns 0 rather than -EDEADLK, 
because bpf_obj_free_fields() is not invoked in the bpf_map_update_elem() 
re-entrancy path:

./test_progs -t htab_update/reenter_update -v
bpf_testmod.ko is already unloaded.
Loading bpf_testmod.ko...
Successfully loaded bpf_testmod.ko.
test_reenter_update:PASS:htab_update__open 0 nsec
test_reenter_update:PASS:htab_update__load 0 nsec
test_reenter_update:PASS:htab_update__attach 0 nsec
test_reenter_update:PASS:add element 0 nsec
test_reenter_update:FAIL:no reentrancy unexpected no reentrancy: actual 0 != expected -35
#143/1   htab_update/reenter_update:FAIL
#143     htab_update:FAIL
Summary: 0/0 PASSED, 0 SKIPPED, 1 FAILED
Successfully unloaded bpf_testmod.ko.