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([2605:8d80:324:220f:2a58:15b7:a6c0:c426]) by smtp.gmail.com with ESMTPSA id d2e1a72fcca58-83f1966471dsm19428314b3a.6.2026.05.19.15.56.25 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Tue, 19 May 2026 15:56:31 -0700 (PDT) Message-ID: <782833db5da77e4aa9761fc410827e7abe8583c8.camel@gmail.com> Subject: Re: [PATCH RESEND bpf-next v10 2/8] bpf: clear list node owner and unlink before drop From: Eduard Zingerman To: Kaitao Cheng Cc: bpf@vger.kernel.org, Alexei Starovoitov , linux-kernel@vger.kernel.org, linux-doc@vger.kernel.org, ast@kernel.org, memxor@gmail.com, corbet@lwn.net, martin.lau@linux.dev, daniel@iogearbox.net, andrii@kernel.org, song@kernel.org, yonghong.song@linux.dev, john.fastabend@gmail.com, kpsingh@kernel.org, sdf@fomichev.me, haoluo@google.com, jolsa@kernel.org, shuah@kernel.org, chengkaitao@kylinos.cn, skhan@linuxfoundation.org, vmalik@redhat.com, linux-kselftest@vger.kernel.org, martin.lau@kernel.org, clm@meta.com, ihor.solodrai@linux.dev, bot+bpf-ci@kernel.org Date: Tue, 19 May 2026 15:56:23 -0700 In-Reply-To: References: <20260512055919.95716-3-kaitao.cheng@linux.dev> <0fb2d99b-b122-44fa-a8bc-9befe6e350bc@linux.dev> <7fa6794161a8bd4fdbc21dad68e86e9770c873cc.camel@gmail.com> <0171629c-bdd3-4661-a4e6-2698dd623c3a@linux.dev> Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable User-Agent: Evolution 3.60.1 (3.60.1-1.fc44) Precedence: bulk X-Mailing-List: linux-doc@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 On Mon, 2026-05-18 at 11:02 +0800, Kaitao Cheng wrote: [...] > > > > The patch does have a bug, however. To fix the issues we are seeing= now, > > > > I propose the additional changes below and would appreciate feedbac= k. > > > >=20 > > > > --- a/kernel/bpf/helpers.c > > > > +++ b/kernel/bpf/helpers.c > > > > @@ -2263,8 +2263,10 @@ void bpf_list_head_free(const struct btf_fie= ld *field, void *list_head, > > > > if (!head->next || list_empty(head)) > > > > goto unlock; > > > > list_for_each_safe(pos, n, head) { > > > > - WRITE_ONCE(container_of(pos, > > > > - struct bpf_list_node_kern, list_head)->owne= r, NULL); > > > > + struct bpf_list_node_kern *node; > > > > + > > > > + node =3D container_of(pos, struct bpf_list_node_ker= n, list_head); > > > > + WRITE_ONCE(node->owner, BPF_PTR_POISON); > > > > list_move_tail(pos, &drain); > > > > } > > > > unlock: > > > > @@ -2272,8 +2274,12 @@ void bpf_list_head_free(const struct btf_fie= ld *field, void *list_head, > > > > __bpf_spin_unlock_irqrestore(spin_lock); > > > >=20 > > > > while (!list_empty(&drain)) { > > > > + struct bpf_list_node_kern *node; > > > > + > > > > pos =3D drain.next; > > > > + node =3D container_of(pos, struct bpf_list_node_ker= n, list_head); > > > > list_del_init(pos); > > > > + WRITE_ONCE(node->owner, NULL); Is CPU allowed to reorder the stores in list_del_init() and WRITE_ONCE()? If it is, I think there is a race here. Thread #1: enter bpf_list_head_free() acquire H1 lock list_move_tail(pos, &drain); // reordered <-- ip here --> WRITE_ONCE(node->owner, BPF_PTR_POISON); // reordered Thread #2: acquire H1 lock n =3D bpf_refcount_acquire() release H1 lock acquire H2 lock enter __bpf_list_add() <-- ip here --> cmpxchg(&node->owner, NULL, BPF_PTR_POISON) [...]