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McKenney" , Uladzislau Rezki , Zqiang , rcu References: <20250318135619.4300-1-frederic@kernel.org> <20250318135619.4300-3-frederic@kernel.org> Content-Language: en-US From: Joel Fernandes In-Reply-To: <20250318135619.4300-3-frederic@kernel.org> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit X-ClientProxiedBy: FR4P281CA0010.DEUP281.PROD.OUTLOOK.COM (2603:10a6:d10:c8::9) 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_|LV2PR12MB5918:EE_ X-MS-Office365-Filtering-Correlation-Id: 1a8e269e-e18a-46ce-2042-08dd69630639 X-MS-Exchange-SenderADCheck: 1 X-MS-Exchange-AntiSpam-Relay: 0 X-Microsoft-Antispam: BCL:0;ARA:13230040|1800799024|376014|366016|7053199007; X-Microsoft-Antispam-Message-Info: =?utf-8?B?TWlCck56L05venRMUG0xcEllUmJFd1h1ODRFMGVNUXRwT0xUUFNsdGRmQWsv?= =?utf-8?B?cWVYODg1NjVRQ1Z3UkYvR1VlVngwU0lTWHhOeW1waGZmQXN2a1QzeHQ3M3h2?= =?utf-8?B?RFVrWVFWVy9vdVF3b0dSYUZkWVBtOGNraHI2NUl3OTBsS3crWkxWUFlZQkZ4?= =?utf-8?B?NUtqRWtlVmk3cjJQUVdwVVFPZ2xzOWYyQmtvbFJhLy9RUHBDZkdSakZsQTU3?= =?utf-8?B?eEgvLy9ibUQ3UytFaUk2Rzh5SHNMM0hBN21ORnF3WnJEdS96eHRwVjh6bkt4?= =?utf-8?B?N0lyM3JXTjNaQzZ1OU9GaENBTGc3VUlkYUlwcmJmUmZDdXZsM0xZMFVNUUJO?= =?utf-8?B?aEVjcld2eXF5cDBxL2xOT0llam5tQ05uTnUyVXZTU1FVa3NpNVQ5SDg5K3dW?= =?utf-8?B?Z3R0MG52SHJ0RUlicElUcXZzZlBjSVVnMXE2VjU0d0wvR3hZZGp6L0lCVUQr?= =?utf-8?B?NHhtUnhObmNpM0xoTVhta0hJUE5hWnFLTjlmREpDWVRRdmF6UUgxaS81U1hO?= =?utf-8?B?QjlqaXZ2YnJveEp4RG9uWitVOE9aajlBc1EyZU9rTmxIUllWRCtheFg1TEo5?= =?utf-8?B?SjV4dms5aEpvVWxQQlpPRjIrNjBwQUxYTHQ4YWcxa2FscTZ4OTJUQS9GYzBu?= =?utf-8?B?RkQ1SjFWTmFZVjRVNFphQkwwVkNUNG1iOHppSXhrM2g5amNYN2V1b2dDbTk1?= =?utf-8?B?NC9hc05WMzVEN0NpVGFhR3AxR1ltTlVOdUFRckpaYWsxa1U1Z3FPWk9zQStV?= =?utf-8?B?RGNNUXRjcllEY3Znc29ERkc1NDY5bWcva0REWDhyWTArOWFVRlF4QklNYm5C?= =?utf-8?B?SmN6S2Rla0kvN0dncGdlVUdSV0NFWFM4MVJTTExXbkxWZzNHZHp3TUNoV3Fj?= =?utf-8?B?bjNFTkZtL2hOays3S2VRSkRrcVl6L1Rldkt3SXAyNXlCMlI3czQ5TW9WYit4?= =?utf-8?B?QS95dmN2aVFRUStHNXRaYnozZ1hBbGY2VTVJKzBaK3dUWE5Ba3c5N3NXRGNj?= =?utf-8?B?V2h0RlNZL2tyc0c5OC9IOHExZjNYeStYL3hBVnJicVBRTmp2OHlIMnBiclh3?= =?utf-8?B?Z28rTkpvVFBrY3hxSGhpMWxXRm42OFE3TGdkZlg5MWFtK1BZbk1UQTdyd2dO?= =?utf-8?B?QzEyaVVrRk1lUG8ySFNjVEZqZ0s1RWZNWlFCWnlkNXVOMEZ2YllUYTRaN2kr?= =?utf-8?B?aDhSZFAxODM2aVJkRXk3aWo4L3c3UHJJM0pqY3dBcDhtcW5odTJOeE9FR2d1?= =?utf-8?B?Q0pkd2Q4elhaeGFkVWFoNEtYSnV5VUN4ZjdpZDk5M2dsY1EwVU9obG1DOGw5?= =?utf-8?B?NTh5S1VlbGc1Vi9QZDU1cENSVjNEUU1QVGVQajVaeWdmT2p6Si9XZXF2OWdF?= =?utf-8?B?cGg3MlFCZzJUYk9kNGxTWmZEL05ac0NUSC9pVHBRSm1nK1pwYklQK0ExQ3U5?= =?utf-8?B?c1ZTejFrSkN4NmRYWHJVblc5Z0JTZ1NoUnhFMmJ3NjdEc1lMZmI0Zy9lTGdv?= =?utf-8?B?T3VPVEVXUzFmOUxEb3pHcEtEMXN1ZDR0QnlwQWRpYjU2ZUFSako2TXN2STdP?= =?utf-8?B?U2V3M095UUpLSUhWNUV0amM4K25YZTV1WW1zMDIxK3dkUWJHaTFJWFE3ZWU2?= =?utf-8?B?MDBpZzBpd0FyRENGeFpuUFpNOVA0TW1sYWFhQytMNGJWeW9wZ3hPbGNydjRE?= =?utf-8?B?Tzl4MHdPZkF5aHNiMGp4WW83WVR6SG1nOE5kM0RwbzUrOTZjcHdWT2RGVWF0?= =?utf-8?B?OTJOandQSjBUb0RtNXVIdTZIaGJnRU9SV2t3cmtQUENIcVdjRW1TRXFsRnFB?= =?utf-8?B?Yzh1K3kwdWk0dTNIQ0xhZEsyYVNNMSs0OTlzcmxBMXo0RW5QRUZwOGttMGVY?= =?utf-8?Q?TpeO22UzNI2Iw?= 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)(1800799024)(376014)(366016)(7053199007);DIR:OUT;SFP:1101; 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I agree with patch, but this line is a bit confusing ->nesting is set to 1 *after* the RCU state is set to "watching". Did you mean "watching" ? But I think you meant "After RCU transitions from a state of not-watching to watching' instead of 'actual state is set to RCU not watching'.. ct_kernel_entry(): // RCU is not watching here ... ct_kernel_enter_state(offset); // ... but is watching here. WRITE_ONCE(ct->nesting, 1); > (then we know for sure we interrupted RCU not watching) > > * The nesting counter is set to 0 or decremented further _before_ the > actual state is set to RCU watching. > > Therefore it is safe to assume that if ct_nesting() > 0, RCU is not > watching. But if ct_nesting() <= 0, RCU is watching except for a tiny > window. > > This hasn't been a problem so far because rcu_is_cpu_rrupt_from_idle() > has only been called from interrupts. However the code is confusing Agreed, and I could also see the existing code's snippet: WARN_ON_ONCE(!nesting && !is_idle_task(current)); .. not working if this function were to be called from non-interrupt kernel context. > and abuses the role of the context tracking nesting counter while there > are more accurate indicators available. > > Clarify and robustify accordingly. > > Signed-off-by: Frederic Weisbecker > --- > kernel/rcu/tree.c | 27 +++++++++++++++++---------- > 1 file changed, 17 insertions(+), 10 deletions(-) > > diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c > index 79dced5fb72e..90c43061c981 100644 > --- a/kernel/rcu/tree.c > +++ b/kernel/rcu/tree.c > @@ -367,7 +367,7 @@ EXPORT_SYMBOL_GPL(rcu_momentary_eqs); > */ > static int rcu_is_cpu_rrupt_from_idle(void) > { > - long nesting; > + long nmi_nesting = ct_nmi_nesting(); > > /* > * Usually called from the tick; but also used from smp_function_call() > @@ -379,21 +379,28 @@ static int rcu_is_cpu_rrupt_from_idle(void) > /* Check for counter underflows */ > RCU_LOCKDEP_WARN(ct_nesting() < 0, > "RCU nesting counter underflow!"); > - RCU_LOCKDEP_WARN(ct_nmi_nesting() <= 0, > - "RCU nmi_nesting counter underflow/zero!"); > > - /* Are we at first interrupt nesting level? */ > - nesting = ct_nmi_nesting(); > - if (nesting > 1) > + /* Non-idle interrupt or nested idle interrupt */ > + if (nmi_nesting > 1) > return false; > > /* > - * If we're not in an interrupt, we must be in the idle task! > + * Non nested idle interrupt (interrupting section where RCU > + * wasn't watching). > */ > - WARN_ON_ONCE(!nesting && !is_idle_task(current)); > + if (nmi_nesting == 1) > + return true; > > - /* Does CPU appear to be idle from an RCU standpoint? */ > - return ct_nesting() == 0; > + /* Not in an interrupt */ > + if (!nmi_nesting) { > + RCU_LOCKDEP_WARN(!in_task() || !is_idle_task(current), > + "RCU nmi_nesting counter not in idle task!"); > + return !rcu_is_watching_curr_cpu(); Makes sense to me and it is also consistent with rcu_watching_snap_in_eqs(). thanks, - Joel