[PATCH bpf-next] libarena: Use compiler load-acquire/store-release in bpf_atomic.h

[Kumar Kartikeya Dwivedi <memxor@gmail.com>]

Teach libarena's BPF atomic primitives to use compiler builtins for
load-acquire and store-release when Clang advertises
__BPF_FEATURE_LOAD_ACQ_STORE_REL. Older compilers continue to use the
existing barrier-based fallback.

Notably, as BPF programs begin running on arm64, it is better to use the
more appropriate variants since we can no longer rely on x86 TSO ordering.

Commit 880442305a39 ("bpf: Introduce load-acquire and store-release instructions")
introduced support, hence kernels from 6.15 onwards are needed when
compiling with compilers supporting these instructions. We have
relatively recent kernel version requirements in libarena anyway, and
have not cut first release, hence declare such a dependency.

Signed-off-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
---
 .../selftests/bpf/libarena/include/bpf_atomic.h  | 16 ++++++++++++++++
 1 file changed, 16 insertions(+)

diff --git a/tools/testing/selftests/bpf/libarena/include/bpf_atomic.h b/tools/testing/selftests/bpf/libarena/include/bpf_atomic.h
index b7b230431929..ec8d606e7ce3 100644
--- a/tools/testing/selftests/bpf/libarena/include/bpf_atomic.h
+++ b/tools/testing/selftests/bpf/libarena/include/bpf_atomic.h
@@ -86,6 +86,21 @@ extern bool CONFIG_X86_64 __kconfig __weak;
 /* Control dependency provides LOAD->STORE, provide LOAD->LOAD */
 #define smp_acquire__after_ctrl_dep() ({ smp_rmb(); })

+#if defined(__BPF_FEATURE_LOAD_ACQ_STORE_REL)
+#define __BPF_HAVE_LOAD_ACQUIRE_STORE_RELEASE 1
+#endif
+
+#ifdef __BPF_HAVE_LOAD_ACQUIRE_STORE_RELEASE
+/*
+ * Clang advertises this feature when it can lower acquire/release atomic
+ * builtins to BPF_LOAD_ACQ/BPF_STORE_REL. Older compilers keep using the
+ * barrier-based fallback below. The generated instructions require kernel
+ * verifier/JIT support added in Linux 6.15; compile for an older BPF CPU to
+ * keep using the fallback when targeting older kernels.
+ */
+#define smp_load_acquire(p) __atomic_load_n((p), __ATOMIC_ACQUIRE)
+#define smp_store_release(p, val) __atomic_store_n((p), (val), __ATOMIC_RELEASE)
+#else
 #define smp_load_acquire(p)                                  \
 	({                                                   \
 		__unqual_typeof(*(p)) __v = READ_ONCE(*(p)); \
@@ -102,6 +117,7 @@ extern bool CONFIG_X86_64 __kconfig __weak;
 		barrier();             \
 		WRITE_ONCE(*(p), val); \
 	})
+#endif

 #define smp_cond_load_relaxed_label(p, cond_expr, label)                \
 	({                                                              \
--
2.53.0