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From: Valentin Schneider <valentin.schneider@arm.com>
To: Frederic Weisbecker <frederic@kernel.org>,
	"Paul E . McKenney" <paulmck@kernel.org>
Cc: LKML <linux-kernel@vger.kernel.org>,
	Thomas Gleixner <tglx@linutronix.de>,
	Sebastian Andrzej Siewior <bigeasy@linutronix.de>,
	Peter Zijlstra <peterz@infradead.org>,
	Uladzislau Rezki <urezki@gmail.com>,
	Frederic Weisbecker <frederic@kernel.org>,
	Boqun Feng <boqun.feng@gmail.com>,
	Neeraj Upadhyay <neeraju@codeaurora.org>,
	Josh Triplett <josh@joshtriplett.org>,
	Joel Fernandes <joel@joelfernandes.org>,
	rcu@vger.kernel.org
Subject: Re: [PATCH 04/11] rcu/nocb: Make rcu_core() callbacks acceleration preempt-safe
Date: Fri, 01 Oct 2021 18:50:22 +0100	[thread overview]
Message-ID: <87bl48my75.mognet@arm.com> (raw)
In-Reply-To: <20210929221012.228270-5-frederic@kernel.org>

On 30/09/21 00:10, Frederic Weisbecker wrote:
> From: Thomas Gleixner <tglx@linutronix.de>
>
> While reporting a quiescent state for a given CPU, rcu_core() takes
> advantage of the freshly loaded grace period sequence number and the
> locked rnp to accelerate the callbacks whose sequence number have been
> assigned a stale value.
>
> This action is only necessary when the rdp isn't offloaded, otherwise
> the NOCB kthreads already take care of the callbacks progression.
>
> However the check for the offloaded state is volatile because it is
> performed outside the IRQs disabled section. It's possible for the
> offloading process to preempt rcu_core() at that point on PREEMPT_RT.
>
> This is dangerous because rcu_core() may end up accelerating callbacks
> concurrently with NOCB kthreads without appropriate locking.
>
> Fix this with moving the offloaded check inside the rnp locking section.
>
> Reported-by: Valentin Schneider <valentin.schneider@arm.com>
> Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
> Cc: Valentin Schneider <valentin.schneider@arm.com>
> Cc: Peter Zijlstra <peterz@infradead.org>
> Cc: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
> Cc: Josh Triplett <josh@joshtriplett.org>
> Cc: Joel Fernandes <joel@joelfernandes.org>
> Cc: Boqun Feng <boqun.feng@gmail.com>
> Cc: Neeraj Upadhyay <neeraju@codeaurora.org>
> Cc: Uladzislau Rezki <urezki@gmail.com>
> Cc: Thomas Gleixner <tglx@linutronix.de>
> Signed-off-by: Frederic Weisbecker <frederic@kernel.org>

Reviewed-by: Valentin Schneider <valentin.schneider@arm.com>

  reply	other threads:[~2021-10-01 17:50 UTC|newest]

Thread overview: 38+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-09-29 22:10 [PATCH 00/11] rcu: Make rcu_core() safe in PREEMPT_RT with NOCB + a few other fixes Frederic Weisbecker
2021-09-29 22:10 ` [PATCH 01/11] rcu/nocb: Make local rcu_nocb_lock_irqsave() safe against concurrent deoffloading Frederic Weisbecker
2021-09-29 22:10 ` [PATCH 02/11] rcu/nocb: Prepare state machine for a new step Frederic Weisbecker
2021-10-01 17:48   ` Valentin Schneider
2021-10-04 11:35     ` Frederic Weisbecker
2021-09-29 22:10 ` [PATCH 03/11] rcu/nocb: Invoke rcu_core() at the start of deoffloading Frederic Weisbecker
2021-10-01 17:50   ` Valentin Schneider
2021-10-04 12:41     ` Frederic Weisbecker
2021-09-29 22:10 ` [PATCH 04/11] rcu/nocb: Make rcu_core() callbacks acceleration preempt-safe Frederic Weisbecker
2021-10-01 17:50   ` Valentin Schneider [this message]
2021-09-29 22:10 ` [PATCH 05/11] rcu/nocb: Make rcu_core() callbacks acceleration (de-)offloading safe Frederic Weisbecker
2021-10-01 17:50   ` Valentin Schneider
2021-10-04 13:14     ` Frederic Weisbecker
2021-09-29 22:10 ` [RFC PATCH 06/11] rcu/nocb: Check a stable offloaded state to manipulate qlen_last_fqs_check Frederic Weisbecker
2021-09-29 22:10 ` [PATCH 07/11] rcu/nocb: Use appropriate rcu_nocb_lock_irqsave() Frederic Weisbecker
2021-10-01 17:50   ` Valentin Schneider
2021-10-04 13:33     ` Frederic Weisbecker
2021-09-29 22:10 ` [PATCH 08/11] rcu/nocb: Limit number of softirq callbacks only on softirq Frederic Weisbecker
2021-10-01 17:51   ` Valentin Schneider
2021-10-04 13:42     ` Frederic Weisbecker
2021-10-05 20:55       ` Paul E. McKenney
2021-09-29 22:10 ` [PATCH 09/11] rcu: Fix callbacks processing time limit retaining cond_resched() Frederic Weisbecker
2021-10-01 17:51   ` Valentin Schneider
2021-09-29 22:10 ` [PATCH 10/11] rcu: Apply callbacks processing time limit only on softirq Frederic Weisbecker
2021-10-01 17:51   ` Valentin Schneider
2021-10-04 13:47     ` Frederic Weisbecker
2021-10-06 15:12       ` Valentin Schneider
2021-10-07  0:18         ` Frederic Weisbecker
2021-09-29 22:10 ` [PATCH 11/11] rcu/nocb: Don't invoke local rcu core on callback overload from nocb kthread Frederic Weisbecker
2021-09-30 15:37   ` Sebastian Andrzej Siewior
2021-10-11 12:11     ` Frederic Weisbecker
2021-10-01 17:47 ` [PATCH 00/11] rcu: Make rcu_core() safe in PREEMPT_RT with NOCB + a few other fixes Valentin Schneider
2021-10-08 14:03   ` Valentin Schneider
2021-10-08 15:49     ` Paul E. McKenney
2021-10-06 15:13 ` Paul E. McKenney
2021-10-07  8:49   ` Sebastian Andrzej Siewior
2021-10-07 19:12     ` Paul E. McKenney
2021-10-11 14:51 [PATCH 00/11] rcu: Make rcu_core() safe in PREEMPT_RT with NOCB + a few other fixes v2 Frederic Weisbecker
2021-10-11 14:51 ` [PATCH 04/11] rcu/nocb: Make rcu_core() callbacks acceleration preempt-safe Frederic Weisbecker

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