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

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Message-ID: <20130906175238.GV3966@linux.vnet.ibm.com>
Date:	Fri, 6 Sep 2013 10:52:38 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	Frederic Weisbecker <fweisbec@...il.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, Sep 06, 2013 at 01:16:31PM -0400, Steven Rostedt wrote:
> 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)

In which case my rhetorical (and therefore useless) answer has to be
"it is a quiescent state that is extended".  ;-)

Sorry, couldn't resist...

> 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()".

Currently, none of RCU's _eqs functions are exported, so they have
the potential to confuse only people working on the RCU implementation
itself, who had better understand what "eqs" means.

But I do count your vote against "eqs" appearing in the name of any
function exported by RCU.

How about if I made rcu_is_cpu_idle() be as follows?

int rcu_is_cpu_idle(void)
{
	int ret;

	ret = (atomic_read(&per_cpu(rcu_dynticks.dynticks,
				    raw_smp_processor_id())) & 0x1) == 0;
	return ret;
}

This should allow existing uses to function properly and should allow
you to use it as well.

							Thanx, Paul

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