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From: Frederic Weisbecker <frederic@kernel.org>
To: "Paul E. McKenney" <paulmck@kernel.org>
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>,
	Sebastian Andrzej Siewior <bigeasy@linutronix.de>,
	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 13:31:17 +0200	[thread overview]
Message-ID: <20210922113117.GB106513@lothringen> (raw)
In-Reply-To: <20210922021837.GV880162@paulmck-ThinkPad-P17-Gen-1>

On Tue, Sep 21, 2021 at 07:18:37PM -0700, Paul E. McKenney wrote:
> On Wed, Sep 22, 2021 at 01:36:27AM +0200, Frederic Weisbecker wrote:
> > Doing the local_irq_save() before checking that the segcblist is offloaded
> > protect that state from being changed (provided we lock the local rdp). Then we
> > can safely manipulate cblist, whether locked or unlocked.
> > 
> > 2) The actual call to rcu_do_batch(). If we are preempted between
> > rcu_segcblist_completely_offloaded() and rcu_do_batch() with a deoffload in
> > the middle, we miss the callback invocation. Invoking rcu_core by the end of
> > deoffloading process should solve that.
> 
> Maybe invoke rcu_core() at that point?  My concern is that there might
> be an extended time between the missed rcu_do_batch() and the end of
> the deoffloading process.

Agreed!

> 
> > > Reported-by: Valentin Schneider <valentin.schneider@arm.com>
> > > Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
> > > ---
> > >  kernel/rcu/tree.c |    7 ++++---
> > >  1 file changed, 4 insertions(+), 3 deletions(-)
> > > 
> > > --- a/kernel/rcu/tree.c
> > > +++ b/kernel/rcu/tree.c
> > > @@ -2278,13 +2278,13 @@ rcu_report_qs_rdp(struct rcu_data *rdp)
> > >  {
> > >  	unsigned long flags;
> > >  	unsigned long mask;
> > > -	bool needwake = false;
> > > -	const bool offloaded = rcu_rdp_is_offloaded(rdp);
> > > +	bool offloaded, needwake = false;
> > >  	struct rcu_node *rnp;
> > >  
> > >  	WARN_ON_ONCE(rdp->cpu != smp_processor_id());
> > >  	rnp = rdp->mynode;
> > >  	raw_spin_lock_irqsave_rcu_node(rnp, flags);
> > > +	offloaded = rcu_rdp_is_offloaded(rdp);
> > >  	if (rdp->cpu_no_qs.b.norm || rdp->gp_seq != rnp->gp_seq ||
> > >  	    rdp->gpwrap) {
> > 
> > BTW Paul, if we happen to switch to non-NOCB (deoffload) some time after
> > rcu_report_qs_rdp(), it's possible that the rcu_accelerate_cbs()
> > that was supposed to be handled by nocb kthreads on behalf of
> > rcu_core() -> rcu_report_qs_rdp() would not happen. At least not until
> > we invoke rcu_core() again. Not sure how much harm that could cause.
> 
> Again, can we just invoke rcu_core() as soon as this is noticed?

Right. So I'm going to do things a bit differently. I'm going to add
a new segcblist state flag so that during the deoffloading process,
the first very step is an invoke_rcu_core() on the target after setting a
flag that requires handling all this things: accelerate/do_batch, etc...

Then will remain the "do we still have pending callbacks after do_batch?"
in which case we'll need to invoke the rcu_core again as long as we are in
the middle of deoffloading.

Ok, now to write the patches.

  reply	other threads:[~2021-09-22 11:31 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 [this message]
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
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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