Re: [PATCH 09/19] rcu/context-tracking: Remove rcu_irq_enter/exit()

From: Frederic Weisbecker <frederic@kernel.org>
To: Peter Zijlstra <peterz@infradead.org>
Cc: LKML <linux-kernel@vger.kernel.org>, Phil Auld <pauld@redhat.com>,
	Alex Belits <abelits@marvell.com>,
	Nicolas Saenz Julienne <nsaenz@kernel.org>,
	Xiongfeng Wang <wangxiongfeng2@huawei.com>,
	Neeraj Upadhyay <quic_neeraju@quicinc.com>,
	Thomas Gleixner <tglx@linutronix.de>,
	Yu Liao <liaoyu15@huawei.com>, Boqun Feng <boqun.feng@gmail.com>,
	"Paul E . McKenney" <paulmck@kernel.org>,
	Marcelo Tosatti <mtosatti@redhat.com>,
	Paul Gortmaker <paul.gortmaker@windriver.com>,
	Uladzislau Rezki <uladzislau.rezki@sony.com>,
	Joel Fernandes <joel@joelfernandes.org>
Subject: Re: [PATCH 09/19] rcu/context-tracking: Remove rcu_irq_enter/exit()
Date: Wed, 9 Mar 2022 23:25:20 +0100	[thread overview]
Message-ID: <20220309222520.GF68899@lothringen> (raw)
In-Reply-To: <20220305141603.GY11184@worktop.programming.kicks-ass.net>

On Sat, Mar 05, 2022 at 03:16:03PM +0100, Peter Zijlstra wrote:
> On Wed, Mar 02, 2022 at 04:48:00PM +0100, Frederic Weisbecker wrote:
> >  void ct_irq_enter_irqson(void)
> >  {
> > -	rcu_irq_enter_irqson();
> > +	unsigned long flags;
> > +
> > +	local_irq_save(flags);
> > +	ct_irq_enter();
> > +	local_irq_restore(flags);
> >  }
> >  
> >  void ct_irq_exit_irqson(void)
> >  {
> > -	rcu_irq_exit_irqson();
> > +	unsigned long flags;
> > +
> > +	local_irq_save(flags);
> > +	ct_irq_exit();
> > +	local_irq_restore(flags);
> >  }
> 
> I know you're just copying code around, but this is broken per
> construction :/
> 
> On the irq_enter site, local_irq_save() will hit a tracepoint, which
> requires RCU, which will only be made available by the ct_irq_enter().
> Same in reverse for the exit case.

Ouch. And playing a game similar to that in default_idle_call() is going to
be trickier because rcu_irq_enter() may or may not exit an RCU-off mode. But it's
feasible. Again I'll fix it ahead.

Thanks!