From: Frederic Weisbecker <frederic@kernel.org>
To: "Paul E. McKenney" <paulmck@kernel.org>
Cc: Sebastian Andrzej Siewior <bigeasy@linutronix.de>,
rcu@vger.kernel.org, Josh Triplett <josh@joshtriplett.org>,
Steven Rostedt <rostedt@goodmis.org>,
Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
Lai Jiangshan <jiangshanlai@gmail.com>,
Joel Fernandes <joel@joelfernandes.org>,
Thomas Gleixner <tglx@linutronix.de>
Subject: Re: [PATCH] rcu/nocb: Extend checks for offloaded rdp by migrate_disable
Date: Wed, 28 Jul 2021 01:20:23 +0200 [thread overview]
Message-ID: <20210727232023.GD283787@lothringen> (raw)
In-Reply-To: <20210727172351.GC4397@paulmck-ThinkPad-P17-Gen-1>
On Tue, Jul 27, 2021 at 10:23:51AM -0700, Paul E. McKenney wrote:
> On Tue, Jul 27, 2021 at 06:38:15PM +0200, Sebastian Andrzej Siewior wrote:
> > Commit 3820b513a2e33 ("rcu/nocb: Detect unsafe checks for offloaded
> > rdp") added checks for safe rdp usage in the nocb case.
> >
> > On PREEMPT_RT this checks triggers in the
> > rcu_cpu_kthread() -> rcu_core() -> rcu_check_quiescent_state() ->
> > rcu_rdp_is_offloaded()
> >
> > call chain. On !PREEMPT_RT this warnings is suppressed because rcu_core()
> > is invoked with disabled BH which also disables preemption and
> > "preemptible()" is part of the checks which are considered safe.
> > On PREEMPT_RT disabling BH does not disable preemption and since there
> > is no other criteria the warning triggers.
> >
> > According to the mentioned commit the goal is to check if the rcu data
> > pointer is "stable … and prevent from its value to be changed under us".
> > My understanding is that this may happen if the task is preemptible and
> > therefore free to be migrated to another CPU which would then lead to
> > another value of `rcu_data'.
> > One thing that has been overseen is that a task within a migrate-disable
> > region (as on PREEMPT_RT with disabled BH) is fully preemptible but may
> > not be migrated to another CPU which should be enough to guarantee that
> > rdp remains stable.
> >
> > Check also disabled migration of the task if the RCU data pointer is
> > from current CPU. Put the whole check within an SMP ifdef block since
> > without SMP there are not CPU migrations to worry about (also
> > task_struct::migration_disabled is missing).
> >
> > Cc: Frederic Weisbecker <frederic@kernel.org>
> > Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
> > ---
> > I don't fully understand why the CPU-hotplug lock matters here but this
> > is beside the point ;)
>
> If I remember correctly, any attempt to change the offloaded state
> must hold off CPU-hotplug operations. So if the current thread is
> holding off CPU-hotplug operations, no other thread can be doing
> an offload or de-offload operation.
Exactly! :)
prev parent reply other threads:[~2021-07-27 23:20 UTC|newest]
Thread overview: 9+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-07-27 16:38 [PATCH] rcu/nocb: Extend checks for offloaded rdp by migrate_disable Sebastian Andrzej Siewior
2021-07-27 17:23 ` Paul E. McKenney
2021-07-27 19:26 ` Sebastian Andrzej Siewior
2021-07-27 23:26 ` Frederic Weisbecker
2021-07-27 19:33 ` Thomas Gleixner
2021-07-27 23:32 ` Frederic Weisbecker
2021-07-28 8:34 ` Thomas Gleixner
2021-07-28 11:50 ` Frederic Weisbecker
2021-07-27 23:20 ` Frederic Weisbecker [this message]
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