linux-kernel - Re: linux-next-20110923: warning kernel/rcutree.c:1833

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Message-ID: <20111003130335.GD1835@somewhere>
Date:	Mon, 3 Oct 2011 15:03:48 +0200
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc:	"Kirill A. Shutemov" <kirill@...temov.name>,
	linux-kernel@...r.kernel.org, Dipankar Sarma <dipankar@...ibm.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...e.hu>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Lai Jiangshan <laijs@...fujitsu.com>
Subject: Re: linux-next-20110923: warning kernel/rcutree.c:1833

On Sun, Oct 02, 2011 at 05:32:47PM -0700, Paul E. McKenney wrote:
> > > -void rcu_irq_enter(void)
> > > +int rcu_is_cpu_idle(void)
> > >  {
> > > -	rcu_exit_nohz();
> > > +	return (atomic_read(&__get_cpu_var(rcu_dynticks).dynticks) & 0x1) == 0;
> > >  }
> > 
> > So that's not used in this patch but it's interesting for me
> > to backport "rcu: Detect illegal rcu dereference in extended quiescent state".
> 
> Yep, that is why it is there.

Ok.

> 
> > The above should be read from a preempt disabled section though
> > (remember "rcu: Fix preempt-unsafe debug check of rcu extended quiescent state")
> 
> Yes, and that is why the last line of the header comment reads "The
> caller must have at least disabled preemption."  Disabling preemption
> is not necessary in Tiny RCU because there is no other CPU for the task
> to go to.  (Right?)

Right.

> > Those functions should probably lay in a separate patch. But I don't mind
> > much keeping the things as is and use these APIs in my next patches though.
> > I'll just fix the preempt enabled thing above.
> 
> Or were you saying that you wish to make calls to rcu_is_cpu_idle()
> that have preemption enabled?

Yeah. That's going to be called from places like rcu_read_lock_held()
and things like this that don't need to disable preemption themselves.

Would be better to disable preemption from that function.

> And I can split the patch easily enough while keeping the diff the same,
> so you should be able to do your porting on top of the existing code.

No I'm actually pretty fine with the current state. Whether that's defined
in this patch or a following one is actually not important.

Thanks!
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