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McKenney" , Frederic Weisbecker , Neeraj Upadhyay , Joel Fernandes , Josh Triplett , Boqun Feng , Uladzislau Rezki , Steven Rostedt , Mathieu Desnoyers , Lai Jiangshan , Zqiang Cc: Joel Fernandes , rcu@vger.kernel.org Subject: [PATCH 1/3] rcu: Replace magic number with meaningful constant in rcu_seq_done_exact() Date: Mon, 24 Mar 2025 13:01:53 -0400 Message-ID: <20250324170156.469763-2-joelagnelf@nvidia.com> X-Mailer: git-send-email 2.43.0 In-Reply-To: <20250324170156.469763-1-joelagnelf@nvidia.com> References: <20250324170156.469763-1-joelagnelf@nvidia.com> X-ClientProxiedBy: BN9PR03CA0856.namprd03.prod.outlook.com (2603:10b6:408:13d::21) To SN7PR12MB8059.namprd12.prod.outlook.com (2603:10b6:806:32b::7) Precedence: bulk X-Mailing-List: rcu@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 X-MS-PublicTrafficType: Email X-MS-TrafficTypeDiagnostic: SN7PR12MB8059:EE_|IA1PR12MB9061:EE_ X-MS-Office365-Filtering-Correlation-Id: b9788194-d031-42b6-f716-08dd6af59cd5 X-MS-Exchange-SenderADCheck: 1 X-MS-Exchange-AntiSpam-Relay: 0 X-Microsoft-Antispam: BCL:0;ARA:13230040|7416014|366016|376014|1800799024|921020; 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X-MS-Exchange-AntiSpam-MessageData-ChunkCount: 1 X-MS-Exchange-AntiSpam-MessageData-0: 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 X-OriginatorOrg: Nvidia.com X-MS-Exchange-CrossTenant-Network-Message-Id: b9788194-d031-42b6-f716-08dd6af59cd5 X-MS-Exchange-CrossTenant-AuthSource: SN7PR12MB8059.namprd12.prod.outlook.com X-MS-Exchange-CrossTenant-AuthAs: Internal X-MS-Exchange-CrossTenant-OriginalArrivalTime: 24 Mar 2025 17:02:08.8498 (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: 9Z1hhc83wSo2ekFe3xQkwyreS1Ybom56D+FvSqAJKi0w0fUY7/8SzE8FKML1lbr6t+5y4EaUWT1TGSDzWaosbQ== X-MS-Exchange-Transport-CrossTenantHeadersStamped: IA1PR12MB9061 The rcu_seq_done_exact() function checks if a grace period has completed by comparing sequence numbers. It includes a guard band to handle sequence number wraparound, which was previously expressed using the magic number calculation '3 * RCU_SEQ_STATE_MASK + 1'. This magic number is not immediately obvious in terms of what it represents. Instead, the reason we need this tiny guardband is because of the lag between the setting of rcu_state.gp_seq_polled and root rnp's gp_seq in rcu_gp_init(). This guardband needs to be at least 2 GPs worth of counts, to avoid recognizing the newly started GP as completed immediately, due to the following sequence which arises due to the delay between update of rcu_state.gp_seq_polled and root rnp's gp_seq: rnp->gp_seq = rcu_state.gp_seq = 0 CPU 0 CPU 1 ----- ----- // rcu_state.gp_seq = 1 rcu_seq_start(&rcu_state.gp_seq) // snap = 8 snap = rcu_seq_snap(&rcu_state.gp_seq) // Two full GP differences rcu_seq_done_exact(&rnp->gp_seq, snap) // rnp->gp_seq = 1 WRITE_ONCE(rnp->gp_seq, rcu_state.gp_seq); This can happen due to get_state_synchronize_rcu_full() sampling rcu_state.gp_seq_polled, however the poll_state_synchronize_rcu_full() sampling the root rnp's gp_seq. The delay between the update of the 2 counters occurs in rcu_gp_init() during which the counters briefly go out of sync. Make the guardband explictly 2 GPs. This improves code readability and maintainability by making the intent clearer as well. Suggested-by: Frederic Weisbecker Signed-off-by: Joel Fernandes Reviewed-by: Paul E. McKenney --- kernel/rcu/rcu.h | 5 ++++- 1 file changed, 4 insertions(+), 1 deletion(-) diff --git a/kernel/rcu/rcu.h b/kernel/rcu/rcu.h index eed2951a4962..5e1ee570bb27 100644 --- a/kernel/rcu/rcu.h +++ b/kernel/rcu/rcu.h @@ -57,6 +57,9 @@ /* Low-order bit definition for polled grace-period APIs. */ #define RCU_GET_STATE_COMPLETED 0x1 +/* A complete grace period count */ +#define RCU_SEQ_GP (RCU_SEQ_STATE_MASK + 1) + extern int sysctl_sched_rt_runtime; /* @@ -162,7 +165,7 @@ static inline bool rcu_seq_done_exact(unsigned long *sp, unsigned long s) { unsigned long cur_s = READ_ONCE(*sp); - return ULONG_CMP_GE(cur_s, s) || ULONG_CMP_LT(cur_s, s - (3 * RCU_SEQ_STATE_MASK + 1)); + return ULONG_CMP_GE(cur_s, s) || ULONG_CMP_LT(cur_s, s - (2 * RCU_SEQ_GP)); } /* From patchwork Mon Mar 24 17:01:54 2025 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Patchwork-Submitter: Joel Fernandes X-Patchwork-Id: 14027544 Received: from NAM10-BN7-obe.outbound.protection.outlook.com (mail-bn7nam10on2060.outbound.protection.outlook.com [40.107.92.60]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 12A3B2641ED; 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Ensure that this is working for the future by a quick check during rcu_gp_init(). Signed-off-by: Joel Fernandes --- kernel/rcu/tree.c | 5 +++++ 1 file changed, 5 insertions(+) diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c index 659f83e71048..29ddbcbea25e 100644 --- a/kernel/rcu/tree.c +++ b/kernel/rcu/tree.c @@ -1798,6 +1798,7 @@ static noinline_for_stack bool rcu_gp_init(void) struct rcu_data *rdp; struct rcu_node *rnp = rcu_get_root(); bool start_new_poll; + unsigned long old_gp_seq; WRITE_ONCE(rcu_state.gp_activity, jiffies); raw_spin_lock_irq_rcu_node(rnp); @@ -1825,7 +1826,11 @@ static noinline_for_stack bool rcu_gp_init(void) */ start_new_poll = rcu_sr_normal_gp_init(); /* Record GP times before starting GP, hence rcu_seq_start(). */ + old_gp_seq = rcu_state.gp_seq; rcu_seq_start(&rcu_state.gp_seq); + /* Ensure that rcu_seq_done_exact() guardband doesn't give false positives. */ + WARN_ON_ONCE(rcu_seq_done_exact(&old_gp_seq, rcu_seq_snap(&rcu_state.gp_seq))); + ASSERT_EXCLUSIVE_WRITER(rcu_state.gp_seq); trace_rcu_grace_period(rcu_state.name, rcu_state.gp_seq, TPS("start")); 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McKenney" , Frederic Weisbecker , Neeraj Upadhyay , Joel Fernandes , Josh Triplett , Boqun Feng , Uladzislau Rezki , Steven Rostedt , Mathieu Desnoyers , Lai Jiangshan , Zqiang Cc: Joel Fernandes , rcu@vger.kernel.org Subject: [PATCH 3/3] rcu: Comment on the extraneous delta test on rcu_seq_done_exact() Date: Mon, 24 Mar 2025 13:01:55 -0400 Message-ID: <20250324170156.469763-4-joelagnelf@nvidia.com> X-Mailer: git-send-email 2.43.0 In-Reply-To: <20250324170156.469763-1-joelagnelf@nvidia.com> References: <20250324170156.469763-1-joelagnelf@nvidia.com> X-ClientProxiedBy: MN2PR11CA0021.namprd11.prod.outlook.com (2603:10b6:208:23b::26) To SN7PR12MB8059.namprd12.prod.outlook.com (2603:10b6:806:32b::7) Precedence: bulk X-Mailing-List: rcu@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 X-MS-PublicTrafficType: Email X-MS-TrafficTypeDiagnostic: SN7PR12MB8059:EE_|IA1PR12MB9061:EE_ X-MS-Office365-Filtering-Correlation-Id: ea48f218-7135-4ee2-fb5c-08dd6af59f66 X-MS-Exchange-SenderADCheck: 1 X-MS-Exchange-AntiSpam-Relay: 0 X-Microsoft-Antispam: BCL:0;ARA:13230040|7416014|366016|376014|1800799024|921020; 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X-MS-Exchange-AntiSpam-MessageData-ChunkCount: 1 X-MS-Exchange-AntiSpam-MessageData-0: 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 X-OriginatorOrg: Nvidia.com X-MS-Exchange-CrossTenant-Network-Message-Id: ea48f218-7135-4ee2-fb5c-08dd6af59f66 X-MS-Exchange-CrossTenant-AuthSource: SN7PR12MB8059.namprd12.prod.outlook.com X-MS-Exchange-CrossTenant-AuthAs: Internal X-MS-Exchange-CrossTenant-OriginalArrivalTime: 24 Mar 2025 17:02:13.1990 (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: /psp3BxxPeRZnRf2Or/+/SGE0Zm3jfwlYA1MuXB5Axg08gK/EQ0YJoltm1lKWZ8FM8fMNeBOLDK8ro6ygzWmTQ== X-MS-Exchange-Transport-CrossTenantHeadersStamped: IA1PR12MB9061 From: Frederic Weisbecker The numbers used in rcu_seq_done_exact() lack some explanation behind their magic. Especially after the commit: 85aad7cc4178 ("rcu: Fix get_state_synchronize_rcu_full() GP-start detection") which reported a subtle issue where a new GP sequence snapshot was taken on the root node state while a grace period had already been started and reflected on the global state sequence but not yet on the root node sequence, making a polling user waiting on a wrong already started grace period that would ignore freshly online CPUs. The fix involved taking the snaphot on the global state sequence and waiting on the root node sequence. And since a grace period is first started on the global state and only afterward reflected on the root node, a snapshot taken on the global state sequence might be two full grace periods ahead of the root node as in the following example: rnp->gp_seq = rcu_state.gp_seq = 0 CPU 0 CPU 1 ----- ----- // rcu_state.gp_seq = 1 rcu_seq_start(&rcu_state.gp_seq) // snap = 8 snap = rcu_seq_snap(&rcu_state.gp_seq) // Two full GP differences rcu_seq_done_exact(&rnp->gp_seq, snap) // rnp->gp_seq = 1 WRITE_ONCE(rnp->gp_seq, rcu_state.gp_seq); Add a comment about those expectations and to clarify the magic within the relevant function. Note that the issue arises mainly with the use of rcu_seq_done_exact() which has a much tigher guardband (of 2 GPs) to ensure the false-negative window of the API during wraparound is limited to just 2 GPs. rcu_seq_done() does not have such strict requirements, however its large false-negative window of ULONG_MAX/2 is not ideal for the polling API. However, this also means care is needed to ensure the guardband is as large as needed to avoid the example scenario describe above which a warning added in an earlier patch does. [ Comment wordsmithing by Joel ] Signed-off-by: Frederic Weisbecker Reviewed-by: Paul E. McKenney Signed-off-by: Joel Fernandes --- kernel/rcu/rcu.h | 9 +++++++++ 1 file changed, 9 insertions(+) diff --git a/kernel/rcu/rcu.h b/kernel/rcu/rcu.h index 5e1ee570bb27..db63f330768c 100644 --- a/kernel/rcu/rcu.h +++ b/kernel/rcu/rcu.h @@ -160,6 +160,15 @@ static inline bool rcu_seq_done(unsigned long *sp, unsigned long s) * Given a snapshot from rcu_seq_snap(), determine whether or not a * full update-side operation has occurred, but do not allow the * (ULONG_MAX / 2) safety-factor/guard-band. + * + * The token returned by get_state_synchronize_rcu_full() is based on + * rcu_state.gp_seq but it is tested in poll_state_synchronize_rcu_full() + * against the root rnp->gp_seq. Since rcu_seq_start() is first called + * on rcu_state.gp_seq and only later reflected on the root rnp->gp_seq, + * it is possible that rcu_seq_snap(rcu_state.gp_seq) returns 2 full grace + * periods ahead of the root rnp->gp_seq. To prevent false-positives with the + * full polling API that a wrap around instantly completed the GP, when nothing + * like that happened, adjust for the 2 GPs in the ULONG_CMP_LT(). */ static inline bool rcu_seq_done_exact(unsigned long *sp, unsigned long s) {