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
next prev parent reply other threads:[~2021-09-22 11:38 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 [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
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=20210922113820.GC106513@lothringen \
--to=frederic@kernel.org \
--cc=anshuman.khandual@arm.com \
--cc=ardb@kernel.org \
--cc=bigeasy@linutronix.de \
--cc=boqun.feng@gmail.com \
--cc=bristot@redhat.com \
--cc=catalin.marinas@arm.com \
--cc=dave@stgolabs.net \
--cc=efault@gmx.de \
--cc=jiangshanlai@gmail.com \
--cc=joel@joelfernandes.org \
--cc=josh@joshtriplett.org \
--cc=linux-arm-kernel@lists.infradead.org \
--cc=linux-kernel@vger.kernel.org \
--cc=linux-rt-users@vger.kernel.org \
--cc=mathieu.desnoyers@efficios.com \
--cc=mingo@kernel.org \
--cc=paulmck@kernel.org \
--cc=peterz@infradead.org \
--cc=rcu@vger.kernel.org \
--cc=rostedt@goodmis.org \
--cc=steven.price@arm.com \
--cc=tglx@linutronix.de \
--cc=valentin.schneider@arm.com \
--cc=vincenzo.frascino@arm.com \
--cc=will@kernel.org \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).