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

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Date:	Fri, 6 Sep 2013 14:21:35 -0400
From:	Steven Rostedt <rostedt@...dmis.org>
To:	paulmck@...ux.vnet.ibm.com
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, 6 Sep 2013 10:52:38 -0700
"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com> wrote:
 
> > 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...

Of course you couldn't ;)

> 
> > 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.

Yeah, that's what I thought, and never cared about the "eqs" meaning.

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

Right, their shouldn't be any "eqs" functions that are global to users
outside of the RCU infrastructure.

> 
> 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.
>

You already said it wont work, but I still would have been against
using it, because I wouldn't be checking if rcu thinks the CPU is idle,
as NO_HZ_FULL has nothing to do with idle.

-- Steve
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