linux-kernel - Re: [RFC PATCH RT 3/4] rcu: unlock special: Treat irq and preempt disabled the same

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Date:   Mon, 24 Jun 2019 12:40:19 -0500
From:   Scott Wood <swood@...hat.com>
To:     paulmck@...ux.ibm.com
Cc:     Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        Peter Zijlstra <peterz@...radead.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Juri Lelli <juri.lelli@...hat.com>,
        Clark Williams <williams@...hat.com>,
        linux-rt-users@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH RT 3/4] rcu: unlock special: Treat irq and preempt
 disabled the same

On Sat, 2019-06-22 at 12:13 -0700, Paul E. McKenney wrote:
> On Fri, Jun 21, 2019 at 05:26:06PM -0700, Paul E. McKenney wrote:
> > On Thu, Jun 20, 2019 at 06:08:19PM -0500, Scott Wood wrote:
> > > On Thu, 2019-06-20 at 15:25 -0700, Paul E. McKenney wrote:
> > > > On Thu, Jun 20, 2019 at 04:59:30PM -0500, Scott Wood wrote:
> > > > > On Thu, 2019-06-20 at 14:10 -0700, Paul E. McKenney wrote:
> > > > > > On Tue, Jun 18, 2019 at 08:19:07PM -0500, Scott Wood wrote:
> > > > > > > [Note: Just before posting this I noticed that the
> > > > > > > invoke_rcu_core
> > > > > > > stuff
> > > > > > >  is part of the latest RCU pull request, and it has a patch
> > > > > > > that
> > > > > > >  addresses this in a more complicated way that appears to deal
> > > > > > > with
> > > > > > > the
> > > > > > >  bare irq-disabled sequence as well.
> > > > > > 
> > > > > > Far easier to deal with it than to debug the lack of it.  ;-)
> > > > > > 
> > > > > > >  Assuming we need/want to support such sequences, is the
> > > > > > >  invoke_rcu_core() call actually going to result in scheduling
> > > > > > > any
> > > > > > >  sooner?  resched_curr() just does the same setting of
> > > > > > > need_resched
> > > > > > >  when it's the same cpu.
> > > > > > > ]
> > > > > > 
> > > > > > Yes, invoke_rcu_core() can in some cases invoke the scheduler
> > > > > > sooner.
> > > > > > Setting the CPU-local bits might not have effect until the next
> > > > > > interrupt.
> > > > > 
> > > > > Maybe I'm missing something, but I don't see how (in the non-
> > > > > use_softirq
> > > > > case).  It just calls wake_up_process(), which in resched_curr()
> > > > > will
> > > > > set
> > > > > need_resched but not do an IPI-to-self.
> > > > 
> > > > The common non-rt case will be use_softirq.  Or are you referring
> > > > specifically to this block of code in current -rcu?
> > > > 
> > > > 		} else if (exp && irqs_were_disabled && !use_softirq
> > > > &&
> > > > 			   !t-
> > > > >rcu_read_unlock_special.b.deferred_qs) {
> > > > 			// Safe to awaken and we get no help from
> > > > enabling
> > > > 			// irqs, unlike bh/preempt.
> > > > 			invoke_rcu_core();
> > > 
> > > Yes, that one.  If that block is removed the else path should be
> > > sufficient,
> > > now that an IPI-to-self has been added.
> > 
> > I will give it a try and let you know what happens.
> 
> How about the following?

Looks good, thanks.

-Scott