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McKenney" , Uladzislau Rezki , Zqiang , rcu Subject: Re: [PATCH 2/3] rcu/exp: Remove confusing needless full barrier on task unblock Message-ID: <20250225215908.GA1812344@joelnvbox> References: <20250213232559.34163-1-frederic@kernel.org> <20250213232559.34163-3-frederic@kernel.org> Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20250213232559.34163-3-frederic@kernel.org> X-ClientProxiedBy: BL1P221CA0023.NAMP221.PROD.OUTLOOK.COM (2603:10b6:208:2c5::10) To SN7PR12MB8059.namprd12.prod.outlook.com (2603:10b6:806:32b::7) Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 X-MS-PublicTrafficType: Email X-MS-TrafficTypeDiagnostic: SN7PR12MB8059:EE_|SA0PR12MB4351:EE_ X-MS-Office365-Filtering-Correlation-Id: 61157bfd-1946-4986-ed23-08dd55e7a22b X-MS-Exchange-SenderADCheck: 1 X-MS-Exchange-AntiSpam-Relay: 0 X-Microsoft-Antispam: BCL:0;ARA:13230040|376014|366016|1800799024|7053199007; X-Microsoft-Antispam-Message-Info: =?us-ascii?Q?4ckJP6DtDWnDoerSSr4QLnl5a46hYUWGyVs27Bg26lUxXF1Rgz0xOlBVnCab?= =?us-ascii?Q?3IRTGxUC8kHezFfKTSO1JS7otehIKR4XzoQobMD8OdO1Ua/ATq4kPfWpylBq?= =?us-ascii?Q?YcsMqw6t11J3dh4dTtuPisMZM24w4nBSB2J28UXhkqPveAnL8g1RxwTUGoqM?= =?us-ascii?Q?bkBzJsXMbbEX6v6UlzwrmGPrmy3C3K8DqUFs+DTqCRKao2Cu8TCGIJKSijzX?= =?us-ascii?Q?yPddPKDkBWzmbQCmt689BMfR53vsMoIw4cs+vmV3u1otTfwh6/VPUTQxX2D9?= =?us-ascii?Q?zCbRx33tU4Pg+U57v02GTvwUym6JDiErPpzbhV6OwCGiAmfPTRBhwaUAfeiv?= =?us-ascii?Q?oZ9zBSaYveCIeqpwAL5kHCLUDUxOctHNBbKUHc+RRMzuuwtlUftLJBF9QRCB?= =?us-ascii?Q?tNQ2d5dF52BuEyva2Gv/4Rlg+KaN3Rfh8ZoezTy97P0CGXdtpXOMIBmc5VtN?= =?us-ascii?Q?oHvThXibsN/BUq/W+vkCqzJe4+KNGjxI5aXTFnTa49qgvj94ve2AL2ivzznb?= =?us-ascii?Q?1erEJ5ISUhWBd0PqlUs/2bhK/8QZJmCv/AxmkuXC1f2DOKQpR3HZZUE60iiz?= =?us-ascii?Q?U3qhRwiTV85m43Jq7snUDBxzPUz47leDPd5JFINB4padnNSemK54bYRBVFvT?= =?us-ascii?Q?f132fZBVuKxMrgkmNylR6kqka5flHdJRRBZSaaaOwVD1amaae9r2GtctkjqE?= =?us-ascii?Q?ILNiYLJetu4FiYVQBHqtYTDi6y1+yku0lBpQ9bl1VZlAERHfOEVoUF7PtWa+?= =?us-ascii?Q?1q86cAmDYX9tisxl66LWWZNog0iODDTgvNdSjGU7BhtJz4l3+0g2dtbTgMW7?= =?us-ascii?Q?CTnwvfwMwrch6cPT1fE25NZ0O5wiYYBjHm56feZgmjWGMzHXiKfDR0BDdACG?= =?us-ascii?Q?knIXkka5D+bfj96xDQyi+w2XWgmZNtCfjDp5ms+I+zdSOc+IeVczw7T8HLQf?= =?us-ascii?Q?mm+Io4879hEyCZG9wDgbOqqTEWtZkq7opVp7kByqY7Yno98t5GFF1moj6PCp?= =?us-ascii?Q?BeWPC9SxYIPdv/q5hJnw0ru/i6qq0eLxvtYX1lj+iCCDeK10xJ+FfcnRZbvB?= =?us-ascii?Q?OwmWaSXpdF1SB9OFQ32Bk22WPhMVRypfBMBOpsJcFSrnEgToUvDRz2Cn5Klm?= =?us-ascii?Q?l2HpgSLANzLzqmm52x/oPtD2KkT6idqCYTCok14Fbtm5QHEE0rP7g2bqIHS+?= =?us-ascii?Q?QuPlYnmAg+4hTDmgpSweOhiy+jcK4rDJ3vAxJ6gs/PHqPNM5htrLI6n7O7uJ?= =?us-ascii?Q?R2M+vUEtAzTzPWcY7Ah9RHq3dB4iKkO18+Nx4zlWZDVG0Cxcyaeuu4JxFRIG?= =?us-ascii?Q?qQvlE/C8uFncrVZq1fs8VqggF98ylwXEO4d2HdFSG7nhqtwApU7+ifcJp0D7?= =?us-ascii?Q?i1h2cHqfWbVgeDT8LuyBEK0lR+hT?= X-Forefront-Antispam-Report: CIP:255.255.255.255;CTRY:;LANG:en;SCL:1;SRV:;IPV:NLI;SFV:NSPM;H:SN7PR12MB8059.namprd12.prod.outlook.com;PTR:;CAT:NONE;SFS:(13230040)(376014)(366016)(1800799024)(7053199007);DIR:OUT;SFP:1101; X-MS-Exchange-AntiSpam-MessageData-ChunkCount: 1 X-MS-Exchange-AntiSpam-MessageData-0: =?us-ascii?Q?j3DsJtjPoOtmpcBAwgUSjEmb70dE3r2Dvggmk0VvRgA8acVpSDA9woKwQ4FD?= =?us-ascii?Q?ggQMgN0P4NcTxc/GGOpCtu/lcZybze6PUGqulZzI7DooqN1iqtOxmBX/yIc/?= =?us-ascii?Q?rRxAUH927YY3jth8gafWpw07bbUM8TN5t7jm0T990qAbb6BhYNNvxjAXUnVU?= =?us-ascii?Q?VQmHI/S01JCcYIaCwpoaaNolqaqlz4UVrAVEuy88Oz96sZDUIRbHySVyz94J?= =?us-ascii?Q?BfiVwJYgKsy91DQK81OPpe8YtBZYCXpLoRzCgAnZGq4RuF29CRt3mS28cRJ7?= =?us-ascii?Q?bMQPKPnEYYN/feJJHzk+LXYqGX0oes0yk9hgfP899jgdc0yyF+GupwhLo93/?= =?us-ascii?Q?T5k9tVJl2ro2hxRtH9odT6MgOCbJjiNqCyrOoQrmMu7edPNDIS8Pj7ioMhIm?= =?us-ascii?Q?kW7EhY+y3GYZdft8miro5SE3n0PBe1ZLlS5Ezikgg01j8aQOJRHGtHFqLpzY?= =?us-ascii?Q?MeiRmZspxgqVUk9rWLxoYVsHxF+MCj9fmGH06wviQs/ZNP6Jsto5y56kMZlY?= =?us-ascii?Q?X4EYn0cbxoLvj/4vI90e/Ti8UUxDeP5Vbhos75bf1l3mf8knCO88S6uSLGS0?= =?us-ascii?Q?yNrHICkIe+lRMe9R7LkwimzRjl4USHW/Od36TP3j97XzGjpv7PA8foMpLoxL?= =?us-ascii?Q?jIEEGPtZ+Xg0+FcQEZzXeLCyuzUecDiov6zr0NFRZjarPdww3UYsdGHptua4?= =?us-ascii?Q?IspbwEeujOSYK0oyp168DRuZ+k+k+u2Ae71JqnrUKmMBqBuIYoHhmRpXx8+d?= =?us-ascii?Q?633hpmrnT3TXPre4/uZzHE3PYMeeX+biYSR+uI33tUbLyAGFO/YyFy2+z1AT?= =?us-ascii?Q?kmER16iHm2WcOv6k9Jd+n4+AlUDQh5Qu004e2i7uiAKIYyJ53obWJq8/QqxC?= =?us-ascii?Q?bS2HBFYxfn2pTifH5TsWkyyrEiIEpZXSJCjxc9VfOUCkFQ103+jeI15VJmkr?= =?us-ascii?Q?T+E3CFq9tGn9PVgCuAYCQ6UAHLmaRd4Vwn++uFrHTgsmAnhyiBxkur3tArvG?= =?us-ascii?Q?QlB+IyAKVlb0HE5sVEfRr+MFCSiBVweaj+HffyAYCpYWB0KqqOdKlkMLBkBn?= =?us-ascii?Q?5xUgo9odkGcE8XxJpMmrqLP03UqYb4w5/zJTa1RXz5mT05qE8IaBCcNTODcP?= =?us-ascii?Q?LpS3L1L2nSvhdtvOx4EItEP1nF8j2F1mb0s/aq7NP0eZIlq7/vVEPYuxicmM?= =?us-ascii?Q?s96uj0Qsb4g8tVkOS0Jsg7ALeIKLus4RgY+COpku9FOk8cctqF4kAOyXyZJQ?= =?us-ascii?Q?0BmXFny9WplMCECmSMPkxY5f/cQ92KpOVIYnYA9EGsE7B/A1VCqRL5aAJDHQ?= =?us-ascii?Q?4ZZLS1B+pBL5MEEfcXAJRy3Kn/W2C6ij6SHj1XutjacQbgLnlHxctTytKBUh?= =?us-ascii?Q?0nwDm0V3IsXxOsF0xhSTt254JY8BKLq6Knf/nv3BswOP+0yjYrGNpyNvukWO?= =?us-ascii?Q?VV9jJTAaZbUzR0hrpBonbAox9Wem4HMs67z54FO+lmdDz9glhOStlOQpRRYn?= =?us-ascii?Q?8yDSCOp2xDr8K7waDSLkyVPoaffoyVAMBPW4DZpAxwTD+09kfw94XbXiJQGY?= =?us-ascii?Q?aw3gWqUpaE3rdf1inoDNJZ33QxRYUceUoNGU+RQA?= X-OriginatorOrg: Nvidia.com X-MS-Exchange-CrossTenant-Network-Message-Id: 61157bfd-1946-4986-ed23-08dd55e7a22b X-MS-Exchange-CrossTenant-AuthSource: SN7PR12MB8059.namprd12.prod.outlook.com X-MS-Exchange-CrossTenant-AuthAs: Internal X-MS-Exchange-CrossTenant-OriginalArrivalTime: 25 Feb 2025 21:59:10.4742 (UTC) X-MS-Exchange-CrossTenant-FromEntityHeader: Hosted X-MS-Exchange-CrossTenant-Id: 43083d15-7273-40c1-b7db-39efd9ccc17a X-MS-Exchange-CrossTenant-MailboxType: HOSTED X-MS-Exchange-CrossTenant-UserPrincipalName: K0JDSgRhzlem0xH7XWq89oUYNsQeRD6p1jlLNRSQKOyKS6oIhvSNzq/WQAY/7jA+zTyho0vcyVIG3y33cgJwsw== X-MS-Exchange-Transport-CrossTenantHeadersStamped: SA0PR12MB4351 On Fri, Feb 14, 2025 at 12:25:58AM +0100, Frederic Weisbecker wrote: > A full memory barrier in the RCU-PREEMPT task unblock path advertizes > to order the context switch (or rather the accesses prior to > rcu_read_unlock()) with the expedited grace period fastpath. > > However the grace period can not complete without the rnp calling into > rcu_report_exp_rnp() with the node locked. This reports the quiescent > state in a fully ordered fashion against updater's accesses thanks to: > > 1) The READ-SIDE smp_mb__after_unlock_lock() barrier accross nodes > locking while propagating QS up to the root. > > 2) The UPDATE-SIDE smp_mb__after_unlock_lock() barrier while holding the > the root rnp to wait/check for the GP completion. > > 3) The (perhaps redundant given step 1) and 2)) smp_mb() in rcu_seq_end() > before the grace period completes. > > This makes the explicit barrier in this place superflous. Therefore > remove it as it is confusing. > > Signed-off-by: Frederic Weisbecker > --- > kernel/rcu/tree_plugin.h | 1 - > 1 file changed, 1 deletion(-) > > diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h > index 3c0bbbbb686f..d51cc7a5dfc7 100644 > --- a/kernel/rcu/tree_plugin.h > +++ b/kernel/rcu/tree_plugin.h > @@ -534,7 +534,6 @@ rcu_preempt_deferred_qs_irqrestore(struct task_struct *t, unsigned long flags) > WARN_ON_ONCE(rnp->completedqs == rnp->gp_seq && > (!empty_norm || rnp->qsmask)); > empty_exp = sync_rcu_exp_done(rnp); > - smp_mb(); /* ensure expedited fastpath sees end of RCU c-s. */ I was wondering though, this is a slow path and the smp_mb() has been there since 2009 or so. Not sure if it is super valuable to remove it at this point. But we/I should definitely understand it. I was wondering if you could also point to the fastpath that this is racing with, it is not immediately clear (to me) what this smp_mb() is pairing with :( thanks, - Joel > np = rcu_next_node_entry(t, rnp); > list_del_init(&t->rcu_node_entry); > t->rcu_blocked_node = NULL; > -- > 2.46.0 >