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Tue, 7 Jul 2020 15:25:11 +0000 References: <643ef0859371d3bf6280f5014d668fb18b3b6d85.1593209494.git.petrm@mellanox.com> User-agent: mu4e 1.3.3; emacs 26.3 From: Petr Machata To: Cong Wang Cc: Linux Kernel Network Developers , David Miller , Jakub Kicinski , Eric Dumazet , Jiri Pirko , Ido Schimmel Subject: Re: [PATCH net-next v1 1/5] net: sched: Pass root lock to Qdisc_ops.enqueue In-reply-to: Date: Tue, 07 Jul 2020 17:25:08 +0200 Message-ID: <878sfvibzf.fsf@mellanox.com> Content-Type: text/plain X-ClientProxiedBy: AM0PR05CA0093.eurprd05.prod.outlook.com (2603:10a6:208:136::33) To HE1PR05MB4746.eurprd05.prod.outlook.com (2603:10a6:7:a3::22) MIME-Version: 1.0 X-MS-Exchange-MessageSentRepresentingType: 1 Received: from yaviefel (213.220.234.169) by AM0PR05CA0093.eurprd05.prod.outlook.com (2603:10a6:208:136::33) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id 15.20.3153.23 via Frontend Transport; Tue, 7 Jul 2020 15:25:10 +0000 X-Originating-IP: [213.220.234.169] X-MS-PublicTrafficType: Email X-MS-Office365-Filtering-HT: Tenant X-MS-Office365-Filtering-Correlation-Id: 40e0b913-1bdf-4057-0d69-08d82289f020 X-MS-TrafficTypeDiagnostic: HE1PR0501MB2602: X-MS-Exchange-Transport-Forked: True X-Microsoft-Antispam-PRVS: X-MS-Oob-TLC-OOBClassifiers: OLM:8882; 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Should this processing >> lead to a situation where a new packet is to be enqueued on the same port, >> holding the root lock would lead to deadlocks. To solve the issue, qevent >> handler needs to unlock and relock the root lock when necessary. >> >> To that end, add the root lock argument to the qdisc op enqueue, and >> propagate throughout. > > Hmm, but why do you pass root lock down to each ->enqueue()? > > You can find root lock with sch_tree_lock() (or qdisc_lock() if you don't > care about hierarchy), and you already have qdisc as a parameter of > tcf_qevent_handle(). I know, I wanted to make it clear that the lock may end up being used, instead of doing it "stealthily". If you find this inelegant I can push a follow-up that converts tcf_qevent_handle() to sch_tree_unlock().