From: bot+bpf-ci@kernel.org
To: donggeunyoo.kernel@gmail.com,bpf@vger.kernel.org
Cc: ast@kernel.org,daniel@iogearbox.net,andrii@kernel.org,eddyz87@gmail.com,memxor@gmail.com,martin.lau@linux.dev,song@kernel.org,yonghong.song@linux.dev,jolsa@kernel.org,emil@etsalapatis.com,ihor.solodrai@linux.dev,linux-kselftest@vger.kernel.org,linux-kernel@vger.kernel.org,donggeunyoo.kernel@gmail.com,ast@kernel.org,andrii@kernel.org,daniel@iogearbox.net,martin.lau@kernel.org,eddyz87@gmail.com,yonghong.song@linux.dev,mason@kernel.org,ihor.solodrai@linux.dev
Subject: Re: [PATCH bpf 1/2] bpf: Zero-fill other CPUs when BPF_F_CPU creates a per-cpu hash element
Date: Sun, 20 Sep 2026 10:19:00 +0000 (UTC) [thread overview]
Message-ID: <eeaf9112a1ba86d1cbfbd5287cc67bcc13adb07861cced93eb4c51167758b3fe@mail.kernel.org> (raw)
In-Reply-To: <20260920093153.439743-2-donggeunyoo.kernel@gmail.com>
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> diff --git a/kernel/bpf/hashtab.c b/kernel/bpf/hashtab.c
> index 4f495dcbf670c..c4683d0e4c149 100644
> --- a/kernel/bpf/hashtab.c
> +++ b/kernel/bpf/hashtab.c
> @@ -1056,12 +1056,12 @@ static void pcpu_init_value(struct bpf_htab *htab, void __percpu *pptr,
> * known initial values for cpus other than current one
> * (onallcpus=false always when coming from bpf prog).
> */
> - if (!onallcpus) {
> - int current_cpu = raw_smp_processor_id();
> + if (!onallcpus || (map_flags & BPF_F_CPU)) {
> + int init_cpu = onallcpus ? map_flags >> 32 : raw_smp_processor_id();
> int cpu;
>
> for_each_possible_cpu(cpu) {
> - if (cpu == current_cpu)
> + if (cpu == init_cpu)
> copy_map_value(&htab->map, per_cpu_ptr(pptr, cpu), value);
> else /* Since elem is preallocated, we cannot touch special fields */
> zero_map_value(&htab->map, per_cpu_ptr(pptr, cpu));
Does the block comment above this condition need updating? It says the value
goes to the "current one" and explains the whole branch with "onallcpus=false
always when coming from bpf prog."
With the patch, the branch is also taken when onallcpus is true and BPF_F_CPU
is set. In that case:
if (!onallcpus || (map_flags & BPF_F_CPU)) {
int init_cpu = onallcpus ? map_flags >> 32 : raw_smp_processor_id();
the value goes to the CPU named in map_flags >> 32, while the current CPU
might be one of the CPUs that gets zero-filled.
The rationale also seems specific to the BPF program case. For BPF_F_CPU
updates there is no BPF program involved (htab_map_check_update_flags()
returns -EINVAL for !onallcpus && map_flags > BPF_EXIST), so the reason to
zero the other slots is that the per-cpu area is recycled and must not hand
back the previous tenant's values, as the commit message explains.
Could the comment be updated to cover both entry conditions?
---
AI reviewed your patch. Please fix the bug or email reply why it's not a bug.
See: https://github.com/kernel-patches/vmtest/blob/master/ci/claude/README.md
CI run summary: https://github.com/kernel-patches/bpf/actions/runs/35502758763
next prev parent reply other threads:[~2026-09-20 10:19 UTC|newest]
Thread overview: 7+ messages / expand[flat|nested] mbox.gz Atom feed top
2026-09-20 9:31 [PATCH bpf 0/2] bpf: fix per-cpu initialization of a BPF_F_CPU created " Donggeun Yoo
2026-09-20 9:31 ` [PATCH bpf 1/2] bpf: Zero-fill other CPUs when BPF_F_CPU creates a per-cpu " Donggeun Yoo
2026-09-20 10:19 ` bot+bpf-ci [this message]
2026-09-20 10:29 ` Donggeun Yoo
2026-09-20 16:21 ` Alexei Starovoitov
2026-09-20 9:31 ` [PATCH bpf 2/2] selftests/bpf: Test per-cpu initialization of a BPF_F_CPU created element Donggeun Yoo
[not found] ` <20260920095859.635921F000FF@smtp.kernel.org>
2026-09-20 10:21 ` Donggeun Yoo
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