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McKenney" , Kumar Kartikeya Dwivedi , Sebastian Andrzej Siewior , neeraj.iitr10@gmail.com, urezki@gmail.com, boqun.feng@gmail.com, rcu@vger.kernel.org, Tejun Heo , bpf@vger.kernel.org, Alexei Starovoitov , Daniel Borkmann , John Fastabend , Andrea Righi , Zqiang References: <20260320222916.19987-1-boqun@kernel.org> Content-Language: en-US From: Joel Fernandes In-Reply-To: Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit X-ClientProxiedBy: BN0PR04CA0088.namprd04.prod.outlook.com (2603:10b6:408:ea::33) To DS0PR12MB6486.namprd12.prod.outlook.com (2603:10b6:8:c5::21) Precedence: bulk X-Mailing-List: bpf@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 X-MS-PublicTrafficType: Email X-MS-TrafficTypeDiagnostic: DS0PR12MB6486:EE_|CH3PR12MB7571:EE_ X-MS-Office365-Filtering-Correlation-Id: 89b86d0b-67dc-4bba-8fac-08de89b57b03 X-MS-Exchange-SenderADCheck: 1 X-MS-Exchange-AntiSpam-Relay: 0 X-Microsoft-Antispam: BCL:0;ARA:13230040|1800799024|366016|7416014|376014|18002099003|22082099003|56012099003; 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However as BPF instrument can >> happen basically everywhere (including where a scheduler lock is held), >> call_srcu() now needs to avoid acquiring scheduler lock because >> otherwise it could cause deadlock [1]. Fix this by following what the >> previous RCU Tasks Trace did: using an irq_work to delay the queuing of >> the work to start process_srcu(). >> >> [boqun: Apply Joel's feedback] >> [boqun: Apply Andrea's test feedback] >> >> Reported-by: Andrea Righi >> Closes: https://lore.kernel.org/all/abjzvz_tL_siV17s@gpd4/ >> Fixes: commit c27cea4416a3 ("rcu: Re-implement RCU Tasks Trace in terms of SRCU-fast") >> Link: https://lore.kernel.org/rcu/3c4c5a29-24ea-492d-aeee-e0d9605b4183@nvidia.com/ [1] >> Suggested-by: Zqiang >> Tested-by: Andrea Righi >> Signed-off-by: Boqun Feng > > I have the feeling that this problem should be solved at the BPF > level. Tracepoints can fire at any time, in that sense they are like NMIs, > and NMIs shouldn't acquire locks, let alone call call_rcu_*() > > BPF should arrange for delaying such operations to more appropriate contexts. > > I understand this is a regression trigerred by an RCU change but to me it > rather reveals a hidden design issue rather than an API breakage. Sure, but there is a valid point I think by those who say that "This was working before the RCU change". So in that sense, we ought to have this (possibly short-term fix).