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