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From: Frederic Weisbecker <frederic@kernel.org>
To: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Cc: Thomas Gleixner <tglx@linutronix.de>,
	Valentin Schneider <valentin.schneider@arm.com>,
	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>,
	"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: Wed, 22 Sep 2021 13:38:20 +0200	[thread overview]
Message-ID: <20210922113820.GC106513@lothringen> (raw)
In-Reply-To: <20210922112731.dvauvxlhx5suc7qd@linutronix.de>

On Wed, Sep 22, 2021 at 01:27:31PM +0200, Sebastian Andrzej Siewior wrote:
> On 2021-09-22 13:10:12 [+0200], Frederic Weisbecker wrote:
> > On Wed, Sep 22, 2021 at 08:32:08AM +0200, Sebastian Andrzej Siewior wrote:
> > > On 2021-09-22 01:45:18 [+0200], Frederic Weisbecker wrote:
> > > > 
> > > > Also while at it, I'm asking again: traditionally softirqs could assume that
> > > > manipulating a local state was safe against !irq_count() code fiddling with
> > > > the same state on the same CPU.
> > > > 
> > > > Now with preemptible softirqs, that assumption can be broken anytime. RCU was
> > > > fortunate enough to have a warning for that. But who knows how many issues like
> > > > this are lurking?
> > > 
> > > If "local state" is modified then it is safe as long as it is modified
> > > within a local_bh_disable() section. And we are in this section while
> > > invoking a forced-threaded interrupt. The special part about RCU is
> > > that it is used in_irq() as part of core-code.
> > 
> > But local_bh_disable() was deemed for protecting from interrupting softirqs,
> > not the other way around (softirqs being preempted by other tasks). The latter
> > semantic is new and nobody had that in mind until softirqs have been made
> > preemptible.
> > 
> > For example:
> > 
> >                              CPU 0
> >           -----------------------------------------------
> >           SOFTIRQ                            RANDOM TASK
> >           ------                             -----------
> >           int *X = &per_cpu(CPUX, 0)         int *X = &per_cpu(CPUX, 0)
> >           int A, B;                          WRITE_ONCE(*X, 0);
> >                                              WRITE_ONCE(*X, 1);
> >           A = READ_ONCE(*X);
> >           B = READ_ONCE(*X);
> > 
> > 
> > We used to have the guarantee that A == B. That's not true anymore. Now
> > some new explicit local_bh_disable() should be carefully placed on RANDOM_TASK
> > where it wasn't necessary before. RCU is not that special in this regard.
> 
> The part with rcutree.use_softirq=0 on RT does not make it any better,
> right?

The rcuc kthread disables softirqs before calling rcu_core(), so it behaves
pretty much the same as a softirq. Or am I missing something?

> So you rely on some implicit behaviour which breaks with RT such as:
> 
>                               CPU 0
>            -----------------------------------------------
>            RANDOM TASK-A                      RANDOM TASK-B
>            ------                             -----------
>            int *X = &per_cpu(CPUX, 0)         int *X = &per_cpu(CPUX, 0)
>            int A, B;                          
> 					      spin_lock(&D);
>            spin_lock(&C);
> 	   				      WRITE_ONCE(*X, 0);
>            A = READ_ONCE(*X);
>                                               WRITE_ONCE(*X, 1);
>            B = READ_ONCE(*X);
> 
> while spinlock C and D are just random locks not related to CPUX but it
> just happens that they are held at that time. So for !RT you guarantee
> that A == B while it is not the case on RT.

Not sure which spinlocks you are referring to here. Also most RCU spinlocks
are raw.

> 
> Sebastian

  reply	other threads:[~2021-09-22 11:38 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 [this message]
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
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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