From: Yonghong Song <yonghong.song@linux.dev>
To: Puranjay Mohan <puranjay@kernel.org>,
Andrii Nakryiko <andrii@kernel.org>,
Eduard Zingerman <eddyz87@gmail.com>,
Mykola Lysenko <mykolal@fb.com>,
Alexei Starovoitov <ast@kernel.org>,
Daniel Borkmann <daniel@iogearbox.net>,
Martin KaFai Lau <martin.lau@linux.dev>,
Song Liu <song@kernel.org>,
John Fastabend <john.fastabend@gmail.com>,
KP Singh <kpsingh@kernel.org>,
Stanislav Fomichev <sdf@fomichev.me>, Hao Luo <haoluo@google.com>,
Jiri Olsa <jolsa@kernel.org>,
Kumar Kartikeya Dwivedi <memxor@gmail.com>,
"Paul E . McKenney" <paulmck@kernel.org>
Cc: bpf@vger.kernel.org, lkmm@lists.linux.dev
Subject: Re: [PATCH bpf-next 1/1] selftests/bpf: fix implementation of smp_mb()
Date: Thu, 10 Jul 2025 12:28:22 -0700 [thread overview]
Message-ID: <f914d8c1-0e87-43ed-b5af-7dba776d76ee@linux.dev> (raw)
In-Reply-To: <20250710175434.18829-2-puranjay@kernel.org>
On 7/10/25 10:54 AM, Puranjay Mohan wrote:
> 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
You are using llvm19 in the above godbolt run.
But from llvm20, instead of 'lock ...' insn, 'atomic_fetch_or'
will be generated so barrier semantics will be preserved.
Since CI is using llvm20, so we should not have any problem.
But for llvm19 or lower, the patch does fix a problem for arm64 etc.
So in case that maintainer agrees with this patch, my ACK is below:
Acked-by: Yonghong Song <yonghong.song@linux.dev>
>
> Fixes: 88d706ba7cc5 ("selftests/bpf: Introduce arena spin lock")
> Signed-off-by: Puranjay Mohan <puranjay@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); \
> })
>
next prev parent reply other threads:[~2025-07-10 19:28 UTC|newest]
Thread overview: 4+ messages / expand[flat|nested] mbox.gz Atom feed top
2025-07-10 17:54 [PATCH bpf-next 0/1] A tool to verify the BPF memory model Puranjay Mohan
2025-07-10 17:54 ` [PATCH bpf-next 1/1] selftests/bpf: fix implementation of smp_mb() Puranjay Mohan
2025-07-10 19:28 ` Yonghong Song [this message]
2025-07-17 1:50 ` [PATCH bpf-next 0/1] A tool to verify the BPF memory model patchwork-bot+netdevbpf
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