linux-kernel - Re: [PATCH v2 3/6] rcu: limit PREEMPT

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Message-ID: <ZzXgpv2ywa6rIK5u@localhost.localdomain>
Date: Thu, 14 Nov 2024 12:36:06 +0100
From: Frederic Weisbecker <frederic@...nel.org>
To: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Cc: Ankur Arora <ankur.a.arora@...cle.com>, linux-kernel@...r.kernel.org,
	peterz@...radead.org, tglx@...utronix.de, paulmck@...nel.org,
	mingo@...nel.org, juri.lelli@...hat.com, vincent.guittot@...aro.org,
	dietmar.eggemann@....com, rostedt@...dmis.org, bsegall@...gle.com,
	mgorman@...e.de, vschneid@...hat.com, efault@....de,
	sshegde@...ux.ibm.com, boris.ostrovsky@...cle.com
Subject: Re: [PATCH v2 3/6] rcu: limit PREEMPT_RCU configurations

Le Thu, Nov 14, 2024 at 09:25:34AM +0100, Sebastian Andrzej Siewior a écrit :
> On 2024-11-13 16:23:03 [-0800], Ankur Arora wrote:
> > > But looking at !CONFIG_PREEMPT_RCU code on tree_plugin.h, I see
> > > some issues now that the code can be preemptible. Well I think
> > > it has always been preemptible but PREEMPTION && !PREEMPT_RCU
> > > has seldom been exerciced (or was it even possible?).
> > >
> > > For example rcu_read_unlock_strict() can be called with preemption
> > > enabled so we need the following otherwise the rdp is unstable, the
> > > norm value becomes racy
> > 
> > I think I see your point about rdp being racy, but given that
> > rcu_read_unlock_strict() would always be called with a non-zero
> > preemption count (with CONFIG_PREEMPTION), wouldn't the preempt_count()
> > check defeat any calls to rcu_read_unlock_strict()?
> > 
> >     void rcu_read_unlock_strict(void)
> >     {
> >             struct rcu_data *rdp;
> > 
> >             if (irqs_disabled() || preempt_count() || !rcu_state.gp_kthread)
> >                     return;
> > 
> > Or am I missing something?

Haha, right!

> 
> This is indeed broken. By moving preempt_disable() as Frederic suggested
> then rcu_read_unlock_strict() becomes a NOP. Keeping this as-is results
> in spats due to this_cpu_ptr() in preemptible regions. Looking further
> we have "rdp->cpu != smp_processor_id()" as the next candidate.
> 
> That preempt_disable() should go to rcu_read_unlock_strict() after the
> check.

Yeah that looks better ;-)

> 
> > Ankur
> 
> Sebastian