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From: Frederic Weisbecker <frederic@kernel.org>
To: "Paul E. McKenney" <paulmck@kernel.org>
Cc: Valentin Schneider <valentin.schneider@arm.com>,
	linux-kernel@vger.kernel.org,
	linux-arm-kernel@lists.infradead.org, rcu@vger.kernel.org,
	linux-rt-users@vger.kernel.org,
	Catalin Marinas <catalin.marinas@arm.com>,
	Will Deacon <will@kernel.org>, Ingo Molnar <mingo@kernel.org>,
	Peter Zijlstra <peterz@infradead.org>,
	Thomas Gleixner <tglx@linutronix.de>,
	Steven Rostedt <rostedt@goodmis.org>,
	Daniel Bristot de Oliveira <bristot@redhat.com>,
	Sebastian Andrzej Siewior <bigeasy@linutronix.de>,
	Josh Triplett <josh@joshtriplett.org>,
	Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
	Davidlohr Bueso <dave@stgolabs.net>,
	Lai Jiangshan <jiangshanlai@gmail.com>,
	Joel Fernandes <joel@joelfernandes.org>,
	Anshuman Khandual <anshuman.khandual@arm.com>,
	Vincenzo Frascino <vincenzo.frascino@arm.com>,
	Steven Price <steven.price@arm.com>,
	Ard Biesheuvel <ardb@kernel.org>,
	Boqun Feng <boqun.feng@gmail.com>, Mike Galbraith <efault@gmx.de>
Subject: Re: [PATCH v3 3/4] rcu/nocb: Protect NOCB state via local_lock() under PREEMPT_RT
Date: Tue, 24 Aug 2021 15:00:33 +0200	[thread overview]
Message-ID: <20210824130033.GA880822@lothringen> (raw)
In-Reply-To: <20210813002045.GZ4126399@paulmck-ThinkPad-P17-Gen-1>

[-- Attachment #1: Type: text/plain, Size: 2366 bytes --]

On Thu, Aug 12, 2021 at 05:20:45PM -0700, Paul E. McKenney wrote:
> Another concern...
> 
> During the preemption of rcu_check_quiescent_state() someone might report
> a quiescent state on behalf of CPU x (perhaps due to its having recently
> been idle) and then the RCU grace-period kthread might start running on
> CPU x, where it might initialize a new grace period in rcu_gp_init().
> It can then invoke __note_gp_changes(), also on CPU x.
> 
> If preempted as shown above just after checking >core_needs_qs, the
> ->cpu_no_qs.b.norm field will be set by the grace-period kthread, which
> will cause the rcu_check_quiescent_state() function's subsequent check
> of ->cpu_no_qs.b.norm to take an early exit.  So OK here.
> 
> On the other hand, if preempted just after the rcu_check_quiescent_state()
> function's check of ->cpu_no_qs.b.norm, the later invocation of
> rcu_report_qs_rdp() should take an early exit due to ->gp_seq mismatch.
> So OK here.

It looks like this could already happen through call_rcu() interrupting
rcu_core(). call_rcu() can also call note_gp_changes().

> 
> However, this should be added to the commit log.  Might be a big commit
> log, but mass storage is cheap these days.  ;-)

Agreed, and a comment.

> 
> This needs a review of each and every manipulation of ->core_needs_qs
> and ->cpu_no_qs.b.norm.  For example, the preemptions will cause the
> scheduler to invoke RCU's context-switch hooks, which also mess with
> ->cpu_no_qs.b.norm.

I think those situations should be fine. In the worst case rcu_core() misses
an opportunity to propagate an rdp quiescent state, waiting for the next core
invocation. The most important is that we don't report a quiescent state that
applied to a previous gp_seq but rcu_report_qs_rdp() should take care about that.

Of course I can easily miss something. Btw I'm attaching a map of all the
interactions with rdp.cpu_no_qs if that can help anyone. I used that to stop
getting lost alone with my headlamp.

> I can get to that some time next week (or tomorrow,
> if things go better than expected), but it would be good for you (and
> others) to check as well.
> 
> Frederic should look this over, but I am taking a quick pass in the
> meantime.  Please see below.

I have yet to check again the nocb bits. They look fine from a quick glance
but I need to sit down a bit longer.

Thanks.

[-- Attachment #2: rdp.cpu_no_qs.norm.jpg --]
[-- Type: image/jpeg, Size: 472684 bytes --]

  parent reply	other threads:[~2021-08-24 13:00 UTC|newest]

Thread overview: 52+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-08-11 20:13 [PATCH v3 0/4] rcu, arm64: PREEMPT_RT fixlets Valentin Schneider
2021-08-11 20:13 ` [PATCH v3 1/4] rcutorture: Don't disable softirqs with preemption disabled when PREEMPT_RT Valentin Schneider
2021-08-12 16:47   ` Paul E. McKenney
2021-08-17 12:13   ` Sebastian Andrzej Siewior
2021-08-17 13:17     ` Sebastian Andrzej Siewior
2021-08-17 14:40       ` [PATCH] rcutorture: Avoid problematic critical section nesting on RT Sebastian Andrzej Siewior
2021-08-18 22:46         ` Paul E. McKenney
2021-08-19 15:35           ` Sebastian Andrzej Siewior
2021-08-19 15:39           ` Sebastian Andrzej Siewior
2021-08-19 15:47             ` Sebastian Andrzej Siewior
2021-08-19 18:20               ` Paul E. McKenney
2021-08-19 18:45                 ` Sebastian Andrzej Siewior
2021-08-20  4:11                 ` Scott Wood
2021-08-20  7:11                   ` Sebastian Andrzej Siewior
2021-08-20  7:42                   ` [PATCH v2] rcutorture: Avoid problematic critical section nesting on PREEMPT_RT Sebastian Andrzej Siewior
2021-08-20 22:10                     ` Paul E. McKenney
2021-08-20  3:23         ` [PATCH] rcutorture: Avoid problematic critical section nesting on RT Scott Wood
2021-08-20  6:54           ` Sebastian Andrzej Siewior
2021-08-11 20:13 ` [PATCH v3 2/4] sched: Introduce migratable() Valentin Schneider
2021-08-17 14:43   ` Sebastian Andrzej Siewior
2021-08-22 17:31     ` Valentin Schneider
2021-08-17 17:09   ` Sebastian Andrzej Siewior
2021-08-17 19:30     ` Phil Auld
2021-08-22 18:14     ` Valentin Schneider
2022-01-26 16:56       ` Sebastian Andrzej Siewior
2022-01-26 18:10         ` Valentin Schneider
2022-01-27 10:07           ` Sebastian Andrzej Siewior
2022-01-27 18:23             ` Valentin Schneider
2022-01-27 19:27         ` Valentin Schneider
2022-02-04  9:24           ` Sebastian Andrzej Siewior
2021-08-11 20:13 ` [PATCH v3 3/4] rcu/nocb: Protect NOCB state via local_lock() under PREEMPT_RT Valentin Schneider
2021-08-13  0:20   ` Paul E. McKenney
2021-08-13 18:48     ` Valentin Schneider
2021-08-24 13:00     ` Frederic Weisbecker [this message]
2021-08-17 15:36   ` Sebastian Andrzej Siewior
2021-08-22 18:15     ` Valentin Schneider
2021-09-21 14:05   ` Thomas Gleixner
2021-09-21 21:12     ` rcu/tree: Protect rcu_rdp_is_offloaded() invocations on RT Thomas Gleixner
2021-09-21 23:36       ` Frederic Weisbecker
2021-09-22  2:18         ` Paul E. McKenney
2021-09-22 11:31           ` Frederic Weisbecker
2021-09-21 23:45       ` Frederic Weisbecker
2021-09-22  6:32         ` Sebastian Andrzej Siewior
2021-09-22 11:10           ` Frederic Weisbecker
2021-09-22 11:27             ` Sebastian Andrzej Siewior
2021-09-22 11:38               ` Frederic Weisbecker
2021-09-22 13:02                 ` Sebastian Andrzej Siewior
2021-09-23 10:02                   ` Frederic Weisbecker
2021-09-30  9:00       ` Valentin Schneider
2021-09-30 10:53         ` Frederic Weisbecker
2021-09-30 13:22           ` Valentin Schneider
2021-08-11 20:13 ` [PATCH v3 4/4] arm64: mm: Make arch_faults_on_old_pte() check for migratability Valentin Schneider

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