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 01:32:52 +0200 [thread overview]
Message-ID: <20210727233252.GF283787@lothringen> (raw)
In-Reply-To: <87wnpbpn55.ffs@nanos.tec.linutronix.de>
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?
PS: just had a quick look and no RCU cpu up operation seem to even check
RCU nocb offload state. So we should be fine ( -ENOSPC for further
famous last words to engrave).
Thanks.
next prev parent reply other threads:[~2021-07-27 23:32 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 [this message]
2021-07-28 8:34 ` Thomas Gleixner
2021-07-28 11:50 ` Frederic Weisbecker
2021-07-27 23:20 ` Frederic Weisbecker
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