Re: [PATCH] rcu/nocb: Extend checks for offloaded rdp by migrate_disable

From: Frederic Weisbecker <frederic@kernel.org>
To: Thomas Gleixner <tglx@linutronix.de>
Cc: paulmck@kernel.org,
	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>
Subject: Re: [PATCH] rcu/nocb: Extend checks for offloaded rdp by migrate_disable
Date: Wed, 28 Jul 2021 13:50:54 +0200	[thread overview]
Message-ID: <20210728115054.GB293265@lothringen> (raw)
In-Reply-To: <87mtq63khy.ffs@nanos.tec.linutronix.de>

On Wed, Jul 28, 2021 at 10:34:01AM +0200, Thomas Gleixner wrote:
> On Wed, Jul 28 2021 at 01:32, Frederic Weisbecker wrote:
> > On Tue, Jul 27, 2021 at 09:33:42PM +0200, Thomas Gleixner wrote:
> >> On Tue, Jul 27 2021 at 10:23, Paul E. McKenney wrote:
> >> > On Tue, Jul 27, 2021 at 06:38:15PM +0200, Sebastian Andrzej Siewior wrote:
> >> >> 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.
> >> 
> >> It only prevents unplugging of a CPU, but not plugging a CPU.
> >
> > Hmm, but both _cpu_down() and _cpu_up() do cpus_write_lock().
> > What did I overlook?
> 
> I meant, that preemption disable does not prevent plugging a CPU, but
> the final unplug step is prevented because the stomp machine thread
> cannot run. Similar for migrate_disable(). The final unplug step can
> only happen when all non-pinned tasks have left the migrate disabled
> section.

Ah ok so that should be fine since we rely on cpu_hotplug_lock to protect
against CPU hotplug here, preemption disabled is there for other purposes.

Thanks.