From patchwork Wed Sep 2 23:53:40 2020 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Patchwork-Submitter: Yonghong Song X-Patchwork-Id: 261598 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-13.1 required=3.0 tests=BAYES_00,DKIMWL_WL_HIGH, DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS, INCLUDES_PATCH, MAILING_LIST_MULTI, SIGNED_OFF_BY, SPF_HELO_NONE, SPF_PASS, USER_AGENT_GIT autolearn=unavailable autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id DB14CC433E2 for ; Wed, 2 Sep 2020 23:53:55 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id AA8C4206F8 for ; Wed, 2 Sep 2020 23:53:55 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (1024-bit key) header.d=fb.com header.i=@fb.com header.b="gkyA+sF/" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726586AbgIBXxy (ORCPT ); Wed, 2 Sep 2020 19:53:54 -0400 Received: from mx0b-00082601.pphosted.com ([67.231.153.30]:23252 "EHLO mx0b-00082601.pphosted.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726594AbgIBXxo (ORCPT ); Wed, 2 Sep 2020 19:53:44 -0400 Received: from pps.filterd (m0148460.ppops.net [127.0.0.1]) by mx0a-00082601.pphosted.com (8.16.0.42/8.16.0.42) with SMTP id 082NdbVI010361 for ; Wed, 2 Sep 2020 16:53:42 -0700 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=fb.com; h=from : to : cc : subject : date : message-id : in-reply-to : references : mime-version : content-transfer-encoding : content-type; s=facebook; bh=F3JhPxINOUTmiCbYAerWNL3PZOiQD4M4hPm8GnCfEZc=; b=gkyA+sF/RoVKlsn853CIuYcPWD+BtKDq5l/odRULTyYdVEqr96pds2jpmns5QCmoW6NK oKoEEcHn/eFSLKN8vbbWoBhfn7dMLvoKjljIIzLygGaWy2qcDlgPSyfE9P7/UhsA/OyT AEh1QT8PhoO8j4H9mwzWz30VOEROpu+IA/U= Received: from maileast.thefacebook.com ([163.114.130.16]) by mx0a-00082601.pphosted.com with ESMTP id 33ae5ujpxb-2 (version=TLSv1.2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128 verify=NOT) for ; Wed, 02 Sep 2020 16:53:42 -0700 Received: from intmgw004.03.ash8.facebook.com (2620:10d:c0a8:1b::d) by mail.thefacebook.com (2620:10d:c0a8:83::6) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256) id 15.1.1979.3; Wed, 2 Sep 2020 16:53:41 -0700 Received: by devbig003.ftw2.facebook.com (Postfix, from userid 128203) id 7C6003704F6E; Wed, 2 Sep 2020 16:53:40 -0700 (PDT) From: Yonghong Song To: , Lorenz Bauer , Martin KaFai Lau , CC: Alexei Starovoitov , Daniel Borkmann , Subject: [PATCH bpf 1/2] bpf: do not use bucket_lock for hashmap iterator Date: Wed, 2 Sep 2020 16:53:40 -0700 Message-ID: <20200902235340.2001375-1-yhs@fb.com> X-Mailer: git-send-email 2.24.1 In-Reply-To: <20200902235340.2001300-1-yhs@fb.com> References: <20200902235340.2001300-1-yhs@fb.com> MIME-Version: 1.0 X-FB-Internal: Safe X-Proofpoint-Virus-Version: vendor=fsecure engine=2.50.10434:6.0.235, 18.0.687 definitions=2020-09-02_17:2020-09-02,2020-09-02 signatures=0 X-Proofpoint-Spam-Details: rule=fb_default_notspam policy=fb_default score=0 mlxscore=0 bulkscore=0 spamscore=0 phishscore=0 malwarescore=0 adultscore=0 priorityscore=1501 suspectscore=8 impostorscore=0 mlxlogscore=566 clxscore=1015 lowpriorityscore=0 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.12.0-2006250000 definitions=main-2009020222 X-FB-Internal: deliver Sender: netdev-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: netdev@vger.kernel.org Currently, for hashmap, the bpf iterator will grab a bucket lock, a spinlock, before traversing the elements in the bucket. This can ensure all bpf visted elements are valid. But this mechanism may cause deadlock if update/deletion happens to the same bucket of the visited map in the program. For example, if we added bpf_map_update_elem() call to the same visited element in selftests bpf_iter_bpf_hash_map.c, we will have the following deadlock: ============================================ WARNING: possible recursive locking detected 5.9.0-rc1+ #841 Not tainted -------------------------------------------- test_progs/1750 is trying to acquire lock: ffff9a5bb73c5e70 (&htab->buckets[i].raw_lock){....}-{2:2}, at: htab_map_update_elem+0x1cf/0x410 but task is already holding lock: ffff9a5bb73c5e20 (&htab->buckets[i].raw_lock){....}-{2:2}, at: bpf_hash_map_seq_find_next+0x94/0x120 other info that might help us debug this: Possible unsafe locking scenario: CPU0 ---- lock(&htab->buckets[i].raw_lock); lock(&htab->buckets[i].raw_lock); *** DEADLOCK *** ... Call Trace: dump_stack+0x78/0xa0 __lock_acquire.cold.74+0x209/0x2e3 lock_acquire+0xba/0x380 ? htab_map_update_elem+0x1cf/0x410 ? __lock_acquire+0x639/0x20c0 _raw_spin_lock_irqsave+0x3b/0x80 ? htab_map_update_elem+0x1cf/0x410 htab_map_update_elem+0x1cf/0x410 ? lock_acquire+0xba/0x380 bpf_prog_ad6dab10433b135d_dump_bpf_hash_map+0x88/0xa9c ? find_held_lock+0x34/0xa0 bpf_iter_run_prog+0x81/0x16e __bpf_hash_map_seq_show+0x145/0x180 bpf_seq_read+0xff/0x3d0 vfs_read+0xad/0x1c0 ksys_read+0x5f/0xe0 do_syscall_64+0x33/0x40 entry_SYSCALL_64_after_hwframe+0x44/0xa9 ... The bucket_lock first grabbed in seq_ops->next() called by bpf_seq_read(), and then grabbed again in htab_map_update_elem() in the bpf program, causing deadlocks. Actually, we do not need bucket_lock here, we can just use rcu_read_lock() similar to netlink iterator where the rcu_read_{lock,unlock} likes below: seq_ops->start(): rcu_read_lock(); seq_ops->next(): rcu_read_unlock(); /* next element */ rcu_read_lock(); seq_ops->stop(); rcu_read_unlock(); Compared to old bucket_lock mechanism, if concurrent updata/delete happens, we may visit stale elements, miss some elements, or repeat some elements. I think this is a reasonable compromise. For users wanting to avoid stale, missing/repeated accesses, bpf_map batch access syscall interface can be used. Signed-off-by: Yonghong Song --- kernel/bpf/hashtab.c | 15 ++++----------- 1 file changed, 4 insertions(+), 11 deletions(-) diff --git a/kernel/bpf/hashtab.c b/kernel/bpf/hashtab.c index 78dfff6a501b..7df28a45c66b 100644 --- a/kernel/bpf/hashtab.c +++ b/kernel/bpf/hashtab.c @@ -1622,7 +1622,6 @@ struct bpf_iter_seq_hash_map_info { struct bpf_map *map; struct bpf_htab *htab; void *percpu_value_buf; // non-zero means percpu hash - unsigned long flags; u32 bucket_id; u32 skip_elems; }; @@ -1632,7 +1631,6 @@ bpf_hash_map_seq_find_next(struct bpf_iter_seq_hash_map_info *info, struct htab_elem *prev_elem) { const struct bpf_htab *htab = info->htab; - unsigned long flags = info->flags; u32 skip_elems = info->skip_elems; u32 bucket_id = info->bucket_id; struct hlist_nulls_head *head; @@ -1656,19 +1654,18 @@ bpf_hash_map_seq_find_next(struct bpf_iter_seq_hash_map_info *info, /* not found, unlock and go to the next bucket */ b = &htab->buckets[bucket_id++]; - htab_unlock_bucket(htab, b, flags); + rcu_read_unlock(); skip_elems = 0; } for (i = bucket_id; i < htab->n_buckets; i++) { b = &htab->buckets[i]; - flags = htab_lock_bucket(htab, b); + rcu_read_lock(); count = 0; head = &b->head; hlist_nulls_for_each_entry_rcu(elem, n, head, hash_node) { if (count >= skip_elems) { - info->flags = flags; info->bucket_id = i; info->skip_elems = count; return elem; @@ -1676,7 +1673,7 @@ bpf_hash_map_seq_find_next(struct bpf_iter_seq_hash_map_info *info, count++; } - htab_unlock_bucket(htab, b, flags); + rcu_read_unlock(); skip_elems = 0; } @@ -1754,14 +1751,10 @@ static int bpf_hash_map_seq_show(struct seq_file *seq, void *v) static void bpf_hash_map_seq_stop(struct seq_file *seq, void *v) { - struct bpf_iter_seq_hash_map_info *info = seq->private; - if (!v) (void)__bpf_hash_map_seq_show(seq, NULL); else - htab_unlock_bucket(info->htab, - &info->htab->buckets[info->bucket_id], - info->flags); + rcu_read_unlock(); } static int bpf_iter_init_hash_map(void *priv_data,