[PATCH AUTOSEL 6.17-6.16] selftests/bpf: Fix arena_spin_lock selftest failure

[Sasha Levin <sashal@kernel.org>]

From: Saket Kumar Bhaskar <skb99@linux.ibm.com>

[ Upstream commit a9d4e9f0e871352a48a82da11a50df7196fe567a ]

For systems having CONFIG_NR_CPUS set to > 1024 in kernel config
the selftest fails as arena_spin_lock_irqsave() returns EOPNOTSUPP.
(eg - incase of powerpc default value for CONFIG_NR_CPUS is 8192)

The selftest is skipped incase bpf program returns EOPNOTSUPP,
with a descriptive message logged.

Tested-by: Venkat Rao Bagalkote <venkat88@linux.ibm.com>
Signed-off-by: Saket Kumar Bhaskar <skb99@linux.ibm.com>
Link: https://lore.kernel.org/r/20250913091337.1841916-1-skb99@linux.ibm.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---

LLM Generated explanations, may be completely bogus:

YES
- `arena_spin_lock` returns `-EOPNOTSUPP` once `CONFIG_NR_CPUS` exceeds
  1024 (`tools/testing/selftests/bpf/progs/bpf_arena_spin_lock.h:497`),
  so on platforms like powerpc (default 8192 CPUs) every test run exits
  early and the user space harness currently asserts that the retval
  must be zero
  (`tools/testing/selftests/bpf/prog_tests/arena_spin_lock.c:41-47`
  before this change), causing the subtests to fail outright.
- The patch teaches the harness to recognize that specific failure mode:
  `spin_lock_thread()` now short‑circuits when it sees `-EOPNOTSUPP`
  instead of tripping the ASSERT
  (`tools/testing/selftests/bpf/prog_tests/arena_spin_lock.c:44-50`),
  and the BPF program annotates the condition by setting `test_skip = 3`
  before returning
  (`tools/testing/selftests/bpf/progs/arena_spin_lock.c:40-44`).
- After all worker threads complete, the host test checks that flag,
  prints an explicit skip message, and marks the subtest as skipped
  instead of comparing the counter and failing
  (`tools/testing/selftests/bpf/prog_tests/arena_spin_lock.c:94-101`).
  This lets kselftest succeed on high-NR_CPUS systems while still
  reporting the unsupported configuration.
- The change is entirely confined to selftests, has no runtime or ABI
  impact, and aligns the tests with the documented hardware limitation,
  making it a low-risk fix for a real, reproducible failure on existing
  platforms.

 .../selftests/bpf/prog_tests/arena_spin_lock.c      | 13 +++++++++++++
 tools/testing/selftests/bpf/progs/arena_spin_lock.c |  5 ++++-
 2 files changed, 17 insertions(+), 1 deletion(-)

diff --git a/tools/testing/selftests/bpf/prog_tests/arena_spin_lock.c b/tools/testing/selftests/bpf/prog_tests/arena_spin_lock.c
index 0223fce4db2bc..693fd86fbde62 100644
--- a/tools/testing/selftests/bpf/prog_tests/arena_spin_lock.c
+++ b/tools/testing/selftests/bpf/prog_tests/arena_spin_lock.c
@@ -40,8 +40,13 @@ static void *spin_lock_thread(void *arg)
 
 	err = bpf_prog_test_run_opts(prog_fd, &topts);
 	ASSERT_OK(err, "test_run err");
+
+	if (topts.retval == -EOPNOTSUPP)
+		goto end;
+
 	ASSERT_EQ((int)topts.retval, 0, "test_run retval");
 
+end:
 	pthread_exit(arg);
 }
 
@@ -63,6 +68,7 @@ static void test_arena_spin_lock_size(int size)
 	skel = arena_spin_lock__open_and_load();
 	if (!ASSERT_OK_PTR(skel, "arena_spin_lock__open_and_load"))
 		return;
+
 	if (skel->data->test_skip == 2) {
 		test__skip();
 		goto end;
@@ -86,6 +92,13 @@ static void test_arena_spin_lock_size(int size)
 			goto end_barrier;
 	}
 
+	if (skel->data->test_skip == 3) {
+		printf("%s:SKIP: CONFIG_NR_CPUS exceed the maximum supported by arena spinlock\n",
+		       __func__);
+		test__skip();
+		goto end_barrier;
+	}
+
 	ASSERT_EQ(skel->bss->counter, repeat * nthreads, "check counter value");
 
 end_barrier:
diff --git a/tools/testing/selftests/bpf/progs/arena_spin_lock.c b/tools/testing/selftests/bpf/progs/arena_spin_lock.c
index c4500c37f85e0..086b57a426cf5 100644
--- a/tools/testing/selftests/bpf/progs/arena_spin_lock.c
+++ b/tools/testing/selftests/bpf/progs/arena_spin_lock.c
@@ -37,8 +37,11 @@ int prog(void *ctx)
 #if defined(ENABLE_ATOMICS_TESTS) && defined(__BPF_FEATURE_ADDR_SPACE_CAST)
 	unsigned long flags;
 
-	if ((ret = arena_spin_lock_irqsave(&lock, flags)))
+	if ((ret = arena_spin_lock_irqsave(&lock, flags))) {
+		if (ret == -EOPNOTSUPP)
+			test_skip = 3;
 		return ret;
+	}
 	if (counter != limit)
 		counter++;
 	bpf_repeat(cs_count);
-- 
2.51.0