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From: Valentin Schneider <valentin.schneider@arm.com>
To: Thomas Gleixner <tglx@linutronix.de>,
	linux-kernel@vger.kernel.org,
	linux-arm-kernel@lists.infradead.org, rcu@vger.kernel.org,
	linux-rt-users@vger.kernel.org
Cc: Catalin Marinas <catalin.marinas@arm.com>,
	Will Deacon <will@kernel.org>, Ingo Molnar <mingo@kernel.org>,
	Peter Zijlstra <peterz@infradead.org>,
	Steven Rostedt <rostedt@goodmis.org>,
	Daniel Bristot de Oliveira <bristot@redhat.com>,
	Sebastian Andrzej Siewior <bigeasy@linutronix.de>,
	"Paul E. McKenney" <paulmck@kernel.org>,
	Frederic Weisbecker <frederic@kernel.org>,
	Josh Triplett <josh@joshtriplett.org>,
	Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
	Davidlohr Bueso <dave@stgolabs.net>,
	Lai Jiangshan <jiangshanlai@gmail.com>,
	Joel Fernandes <joel@joelfernandes.org>,
	Anshuman Khandual <anshuman.khandual@arm.com>,
	Vincenzo Frascino <vincenzo.frascino@arm.com>,
	Steven Price <steven.price@arm.com>,
	Ard Biesheuvel <ardb@kernel.org>,
	Boqun Feng <boqun.feng@gmail.com>, Mike Galbraith <efault@gmx.de>
Subject: Re: rcu/tree: Protect rcu_rdp_is_offloaded() invocations on RT
Date: Thu, 30 Sep 2021 10:00:39 +0100	[thread overview]
Message-ID: <87h7e21lqg.mognet@arm.com> (raw)
In-Reply-To: <87pmt163al.ffs@tglx>

Hi,

On 21/09/21 23:12, Thomas Gleixner wrote:
> Valentin reported warnings about suspicious RCU usage on RT kernels. Those
> happen when offloading of RCU callbacks is enabled:
>
>   WARNING: suspicious RCU usage
>   5.13.0-rt1 #20 Not tainted
>   -----------------------------
>   kernel/rcu/tree_plugin.h:69 Unsafe read of RCU_NOCB offloaded state!
>
>   rcu_rdp_is_offloaded (kernel/rcu/tree_plugin.h:69 kernel/rcu/tree_plugin.h:58)
>   rcu_core (kernel/rcu/tree.c:2332 kernel/rcu/tree.c:2398 kernel/rcu/tree.c:2777)
>   rcu_cpu_kthread (./include/linux/bottom_half.h:32 kernel/rcu/tree.c:2876)
>
> The reason is that rcu_rdp_is_offloaded() is invoked without one of the
> required protections on RT enabled kernels because local_bh_disable() does
> not disable preemption on RT.
>
> Valentin proposed to add a local lock to the code in question, but that's
> suboptimal in several aspects:
>
>   1) local locks add extra code to !RT kernels for no value.
>
>   2) All possible callsites have to audited and amended when affected
>      possible at an outer function level due to lock nesting issues.
>
>   3) As the local lock has to be taken at the outer functions it's required
>      to release and reacquire them in the inner code sections which might
>      voluntary schedule, e.g. rcu_do_batch().
>
> Both callsites of rcu_rdp_is_offloaded() which trigger this check invoke
> rcu_rdp_is_offloaded() in the variable declaration section right at the top
> of the functions. But the actual usage of the result is either within a
> section which provides the required protections or after such a section.
>
> So the obvious solution is to move the invocation into the code sections
> which provide the proper protections, which solves the problem for RT and
> does not have any impact on !RT kernels.
>

Thanks for taking a look at this!

My reasoning for adding protection in the outer functions was to prevent
impaired unlocks of rcu_nocb_{un}lock_irqsave(), as that too depends on the
offload state. Cf. Frederic's writeup:

  http://lore.kernel.org/r/20210727230814.GC283787@lothringen

Anywho, I see Frederic has sent a fancy new series, let me go stare at it.

  parent reply	other threads:[~2021-09-30  9:00 UTC|newest]

Thread overview: 52+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-08-11 20:13 [PATCH v3 0/4] rcu, arm64: PREEMPT_RT fixlets Valentin Schneider
2021-08-11 20:13 ` [PATCH v3 1/4] rcutorture: Don't disable softirqs with preemption disabled when PREEMPT_RT Valentin Schneider
2021-08-12 16:47   ` Paul E. McKenney
2021-08-17 12:13   ` Sebastian Andrzej Siewior
2021-08-17 13:17     ` Sebastian Andrzej Siewior
2021-08-17 14:40       ` [PATCH] rcutorture: Avoid problematic critical section nesting on RT Sebastian Andrzej Siewior
2021-08-18 22:46         ` Paul E. McKenney
2021-08-19 15:35           ` Sebastian Andrzej Siewior
2021-08-19 15:39           ` Sebastian Andrzej Siewior
2021-08-19 15:47             ` Sebastian Andrzej Siewior
2021-08-19 18:20               ` Paul E. McKenney
2021-08-19 18:45                 ` Sebastian Andrzej Siewior
2021-08-20  4:11                 ` Scott Wood
2021-08-20  7:11                   ` Sebastian Andrzej Siewior
2021-08-20  7:42                   ` [PATCH v2] rcutorture: Avoid problematic critical section nesting on PREEMPT_RT Sebastian Andrzej Siewior
2021-08-20 22:10                     ` Paul E. McKenney
2021-08-20  3:23         ` [PATCH] rcutorture: Avoid problematic critical section nesting on RT Scott Wood
2021-08-20  6:54           ` Sebastian Andrzej Siewior
2021-08-11 20:13 ` [PATCH v3 2/4] sched: Introduce migratable() Valentin Schneider
2021-08-17 14:43   ` Sebastian Andrzej Siewior
2021-08-22 17:31     ` Valentin Schneider
2021-08-17 17:09   ` Sebastian Andrzej Siewior
2021-08-17 19:30     ` Phil Auld
2021-08-22 18:14     ` Valentin Schneider
2022-01-26 16:56       ` Sebastian Andrzej Siewior
2022-01-26 18:10         ` Valentin Schneider
2022-01-27 10:07           ` Sebastian Andrzej Siewior
2022-01-27 18:23             ` Valentin Schneider
2022-01-27 19:27         ` Valentin Schneider
2022-02-04  9:24           ` Sebastian Andrzej Siewior
2021-08-11 20:13 ` [PATCH v3 3/4] rcu/nocb: Protect NOCB state via local_lock() under PREEMPT_RT Valentin Schneider
2021-08-13  0:20   ` Paul E. McKenney
2021-08-13 18:48     ` Valentin Schneider
2021-08-24 13:00     ` Frederic Weisbecker
2021-08-17 15:36   ` Sebastian Andrzej Siewior
2021-08-22 18:15     ` Valentin Schneider
2021-09-21 14:05   ` Thomas Gleixner
2021-09-21 21:12     ` rcu/tree: Protect rcu_rdp_is_offloaded() invocations on RT Thomas Gleixner
2021-09-21 23:36       ` Frederic Weisbecker
2021-09-22  2:18         ` Paul E. McKenney
2021-09-22 11:31           ` Frederic Weisbecker
2021-09-21 23:45       ` Frederic Weisbecker
2021-09-22  6:32         ` Sebastian Andrzej Siewior
2021-09-22 11:10           ` Frederic Weisbecker
2021-09-22 11:27             ` Sebastian Andrzej Siewior
2021-09-22 11:38               ` Frederic Weisbecker
2021-09-22 13:02                 ` Sebastian Andrzej Siewior
2021-09-23 10:02                   ` Frederic Weisbecker
2021-09-30  9:00       ` Valentin Schneider [this message]
2021-09-30 10:53         ` Frederic Weisbecker
2021-09-30 13:22           ` Valentin Schneider
2021-08-11 20:13 ` [PATCH v3 4/4] arm64: mm: Make arch_faults_on_old_pte() check for migratability Valentin Schneider

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