From: Frederic Weisbecker <frederic@kernel.org>
To: Valentin Schneider <valentin.schneider@arm.com>
Cc: 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,
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>,
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 12:53:40 +0200 [thread overview]
Message-ID: <20210930105340.GA232241@lothringen> (raw)
In-Reply-To: <87h7e21lqg.mognet@arm.com>
On Thu, Sep 30, 2021 at 10:00:39AM +0100, Valentin Schneider wrote:
> 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
I was wrong about that BTW!
Because rcu_nocb_lock() always require IRQs to be disabled, which of course disables
preemption, so the offloaded state can't change between
rcu_nocb_lock[_irqsave]() and rcu_nocb_unlock[_irqrestore]() but anyway there
were many other issues to fix :-)
>
> Anywho, I see Frederic has sent a fancy new series, let me go stare at it.
next prev parent reply other threads:[~2021-09-30 10:53 UTC|newest]
Thread overview: 51+ 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
2021-08-17 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
2021-09-30 10:53 ` Frederic Weisbecker [this message]
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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