6.15-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Puranjay Mohan <puranjay@kernel.org>

[ Upstream commit 0769857a07b4451a1dc1c3ad1f1c86a6f4ce136a ]

As BPF doesn't include any barrier instructions, smp_mb() is implemented
by doing a dummy value returning atomic operation. Such an operation
acts a full barrier as enforced by LKMM and also by the work in progress
BPF memory model.

If the returned value is not used, clang[1] can optimize the value
returning atomic instruction in to a normal atomic instruction which
provides no ordering guarantees.

Mark the variable as volatile so the above optimization is never
performed and smp_mb() works as expected.

[1] https://godbolt.org/z/qzze7bG6z

Fixes: 88d706ba7cc5 ("selftests/bpf: Introduce arena spin lock")
Signed-off-by: Puranjay Mohan <puranjay@kernel.org>
Link: https://lore.kernel.org/r/20250710175434.18829-2-puranjay@kernel.org (local)
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
 tools/testing/selftests/bpf/bpf_atomic.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/tools/testing/selftests/bpf/bpf_atomic.h b/tools/testing/selftests/bpf/bpf_atomic.h
index a9674e544322..c550e5711967 100644
--- a/tools/testing/selftests/bpf/bpf_atomic.h
+++ b/tools/testing/selftests/bpf/bpf_atomic.h

@@ -61,7 +61,7 @@ extern bool CONFIG_X86_64 __kconfig __weak;
 
 #define smp_mb()                                 \
 	({                                       \
-		unsigned long __val;             \
+		volatile unsigned long __val;    \
 		__sync_fetch_and_add(&__val, 0); \
 	})
 

-- 
2.39.5