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

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Message-ID: <20111006121118.GB23375@somewhere.redhat.com>
Date:	Thu, 6 Oct 2011 14:11:28 +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>,
	arjan.van.de.ven@...el.com
Subject: Re: linux-next-20110923: warning kernel/rcutree.c:1833

On Wed, Oct 05, 2011 at 05:58:58PM -0700, Paul E. McKenney wrote:
> On Mon, Oct 03, 2011 at 09:30:36AM -0700, Paul E. McKenney wrote:
> > On Mon, Oct 03, 2011 at 03:03:48PM +0200, Frederic Weisbecker wrote:
> > > 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.
> > 
> > Hmmm...  This might be a good use for the "drive-by" per-CPU access
> > functions.
> > 
> > No, that doesn't work.  We could pick up the pointer, switch to another
> > CPU, the original CPU could run a task that blocks before we start running,
> > and then we could incorrectly decide that we were running in idle context,
> > issuing a spurious warning.  This approach would only work in environments
> > that (unlike the Linux kernel) mapped all the per-CPU variables to the
> > same virtual address on all CPUs.  (DYNIX/ptx did this, but this leads
> > to other problems, like being unable to reasonably access other CPUs'
> > variables.  Double mapping has other issues on some architectures.)
> > 
> > OK, agreed.  I will make this function disable preemption.
> > 
> > > > 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.
> > 
> > Fair enough!
> 
> And here is an update that might handle an irq entry/exit miscounting
> problem.  Thanks to Arjan van de Ven for pointing out that my earlier
> approach would in fact miscount irq entries/exits in face of things like
> upcalls to user-mode helpers.

I'm not sure what you mean. How could the current state miscount in user-mode?
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