linux-kernel - Re: [PATCH] rcu: Is it safe to enter an RCU read-side critical section?

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Message-ID: <20130906131631.6a0a192d@gandalf.local.home>
Date:	Fri, 6 Sep 2013 13:16:31 -0400
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Frederic Weisbecker <fweisbec@...il.com>
Cc:	paulmck@...ux.vnet.ibm.com, linux-kernel@...r.kernel.org,
	mingo@...e.hu, laijs@...fujitsu.com, dipankar@...ibm.com,
	akpm@...ux-foundation.org, mathieu.desnoyers@...icios.com,
	josh@...htriplett.org, niv@...ibm.com, tglx@...utronix.de,
	peterz@...radead.org, dhowells@...hat.com, edumazet@...gle.com,
	darren@...art.com, sbw@....edu
Subject: Re: [PATCH] rcu: Is it safe to enter an RCU read-side critical
 section?

On Fri, 6 Sep 2013 19:00:08 +0200
Frederic Weisbecker <fweisbec@...il.com> wrote:

> On Fri, Sep 06, 2013 at 12:52:38PM -0400, Steven Rostedt wrote:
> > On Fri, 6 Sep 2013 18:40:18 +0200
> > Frederic Weisbecker <fweisbec@...il.com> wrote:
> >  
> > > > I can't use plain preempt_disable() in function tracing.
> > > > 
> > > > Also, since it's a misnomer to say the cpu is idle in NO_HZ_FULL when
> > > > we are coming from userspace, can we rename that?
> > > > 
> > > > Perhaps we can also have a __rcu_is_cpu_tracking() (or whatever), with
> > > > the "__" appended that does not do the preempt disable.
> > > 
> > > rcu_is_cpu_eqs() is probably better. It refers to other related "eqs" naming
> > > in RCU APIs.
> > 
> > But that will just confuse the heck out of people. When I see "eqs" I
> > equate that with "equals". What does the rcu cpu equal?
> 
> It's "extended quiescent state". There is already rcu_eqs_enter() and rcu_eqs_exit().
> You're right, may be we can rename that to avoid confusion with "equals". I don't mind much.
> I'm happy as long as the reader rcu_is_cpu_foo() and the writers rcu_foo_enter() and
> rcu_foo_exit() have consistant naming.
> 

What exactly does "extended quiescent state" mean? (Note, that's a
rhetorical question)


I wonder if we should change "rcu_cpu_ignore()" for "rcu_eqs_enter()"
and "rcu_cpu_heed()" for "rcu_eqs_exit()", as IMHO that's much more
straight forward to understand than trying to wrap you head around what
a quiescent state is, and why we are entering it or exiting it.

It also flat out explains to people that rcu is not processing that
current CPU, and things like rcu_read_lock() should not be used.

Then we can say "rcu_cpu_is_ignored()" for things like
"rcu_is_cpu_eqs()".

-- Steve
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