From: Thomas Gleixner <tglx@linutronix.de>
To: paulmck@kernel.org, Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Cc: 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>,
Frederic Weisbecker <frederic@kernel.org>
Subject: Re: [PATCH] rcu/nocb: Extend checks for offloaded rdp by migrate_disable
Date: Tue, 27 Jul 2021 21:33:42 +0200 [thread overview]
Message-ID: <87wnpbpn55.ffs@nanos.tec.linutronix.de> (raw)
In-Reply-To: <20210727172351.GC4397@paulmck-ThinkPad-P17-Gen-1>
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.
>> kernel/rcu/tree_plugin.h | 6 ++++--
>> 1 file changed, 4 insertions(+), 2 deletions(-)
>>
>> diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
>> index 0ff5e4fb933e7..d8a623ba7d243 100644
>> --- a/kernel/rcu/tree_plugin.h
>> +++ b/kernel/rcu/tree_plugin.h
>> @@ -57,16 +57,18 @@ static bool rcu_rdp_is_offloaded(struct rcu_data *rdp)
>> * timers have their own means of synchronization against the
>> * offloaded state updaters.
>> */
>> +#ifdef CONFIG_SMP
>> RCU_LOCKDEP_WARN(
>> !(lockdep_is_held(&rcu_state.barrier_mutex) ||
>> (IS_ENABLED(CONFIG_HOTPLUG_CPU) && lockdep_is_cpus_held()) ||
>> rcu_lockdep_is_held_nocb(rdp) ||
>> (rdp == this_cpu_ptr(&rcu_data) &&
>> - !(IS_ENABLED(CONFIG_PREEMPT_COUNT) && preemptible())) ||
>> + (!(IS_ENABLED(CONFIG_PREEMPT_COUNT) && preemptible()) ||
>
>
>> + current->migration_disabled)) ||
>
> How does this change interact with the one proposed by Valentin?
>
> https://lore.kernel.org/lkml/20210721115118.729943-3-valentin.schneider@arm.com/
It does not interact, it conflicts and Valentin's is definitely the
better solution.
Thanks,
tglx
next prev parent reply other threads:[~2021-07-27 19:33 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 [this message]
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
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