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([2620:10d:c092:500::5:c044]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-48a77449ce5sm30956335e9.0.2026.04.28.06.20.51 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Tue, 28 Apr 2026 06:20:51 -0700 (PDT) Message-ID: Date: Tue, 28 Apr 2026 14:20:51 +0100 Precedence: bulk X-Mailing-List: bpf@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH bpf-next v3 03/10] bpf: Implement get_next_key() resizable hashtab To: Herbert Xu Cc: bpf@vger.kernel.org, ast@kernel.org, andrii@kernel.org, daniel@iogearbox.net, kafai@meta.com, kernel-team@meta.com, eddyz87@gmail.com, memxor@gmail.com, Mykyta Yatsenko , Emil Tsalapatis References: <20260424-rhash-v3-0-d0fa0ce4379b@meta.com> <20260424-rhash-v3-3-d0fa0ce4379b@meta.com> Content-Language: en-US From: Mykyta Yatsenko In-Reply-To: Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit On 4/28/26 11:33 AM, Herbert Xu wrote: > On Fri, Apr 24, 2026 at 12:50:45PM -0700, Mykyta Yatsenko wrote: >> >> static int rhtab_map_get_next_key(struct bpf_map *map, void *key, void *next_key) >> { >> - return -EOPNOTSUPP; >> + struct bpf_rhtab *rhtab = container_of(map, struct bpf_rhtab, map); >> + struct rhashtable_iter iter; >> + struct rhtab_elem *elem; >> + int ret = -ENOENT; >> + >> + /* >> + * Hold RCU across enter_from + walk_start to prevent the >> + * element cached by enter_from from being freed before >> + * walk_start re-acquires RCU. >> + */ >> + guard(rcu)(); >> + /* If key is not found, places iterator at the beginning */ >> + rhashtable_walk_enter_from(&rhtab->ht, &iter, key, rhtab->params); >> + rhashtable_walk_start(&iter); >> + >> + elem = rhtab_iter_next(&iter); >> + if (elem) { >> + memcpy(next_key, elem->data, map->key_size); >> + ret = 0; >> + } >> + >> + rhashtable_walk_stop(&iter); >> + rhashtable_walk_exit(&iter); >> + >> + return ret; >> } > > The same issue from v2 still exists here. If a rehash occurs > in between calls to rhtab_map_get_next_key you could be iterating > over the table forever. > > Cheers, Yes, I agree that it is possible. For this function kernel does not iterate the entire hashtable, each get_next_key() call comes from userspace via syscall, maybe the risk is acceptable in this situation? Other functions, where kernel does iterate over the rhashtable, it fails and returns -EBUSY on resize.