linux-kernel - Re: [RFC Patch 1/10] Introducing generic hardware breakpoint handler interfaces

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Date:	Fri, 30 Jan 2009 10:55:39 -0500 (EST)
From:	Alan Stern <stern@...land.harvard.edu>
To:	"K.Prasad" <prasad@...ux.vnet.ibm.com>
cc:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Roland McGrath <roland@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>, <mingo@...e.hu>,
	<richardj_moore@...ibm.com>, <naren@...ux.vnet.ibm.com>
Subject: Re: [RFC Patch 1/10] Introducing generic hardware breakpoint handler
 interfaces

On Fri, 30 Jan 2009, K.Prasad wrote:

> > A few RCU-related questions below.
> > 
> > 							Thanx, Paul

Paul, you've got to learn to trim your replies!  It's not nice to have
to skim over hundreds and hundreds lines of quoted text while searching
for your interpolated comments.  In fact, the phrase "needle in a 
haystack" springs to mind...

> > > +	thr_kbpdata = chbi->cur_kbpdata;
> > > +	barrier();
> > 
> > Couldn't the above two lines instead be:
> > 
> > 	thr_kbpdata = ACCESS_ONCE(chbi->cur_kbpdata);
> > 
> > This would prevent the pointer aliasing, but would make it very clear
> > exactly how the compiler was to be restricted.
> 
> Ok. Using a barrier() could be an overkill. I will change it.

IIRC, the original code above was written before ACCESS_ONCE came into
being.  But I could be wrong about that...

> > > +/*
> > > + * Tell all CPUs to update their debug registers.
> > > + *
> > > + * The caller must hold hw_breakpoint_mutex.
> > > + */
> > > +static void update_all_cpus(void)
> > > +{
> > > +	/* We don't need to use any sort of memory barrier.  The IPI
> > > +	 * carried out by on_each_cpu() includes its own barriers.
> > > +	 */
> > > +	on_each_cpu(update_this_cpu, NULL, 0);
> > > +	synchronize_rcu();
> > 
> > Don't we need the rcu_read_lock() / rcu_read_unlock() pair from
> > load_debug_registers() to move down into update_this_cpu() in order
> > for this to be guaranteed to work?  As the code reads now, the
> > update_this_cpu() calls running on other CPUs are not running under
> > RCU protection, right?

Maybe I'm misunderstanding the question.  update_this_cpu() is called
from only two places: on_each_cpu() as shown above, and
load_debug_registers().  It seems clear that contexts resulting from
on_each_cpu() don't need RCU protection, because on_each_cpu() won't
return until those routines have completed.

This leaves only contexts resulting from load_debug_registers().  But
the first thing load_debug_registers() does is disable local
interrupts, thus blocking IPI delivery.  Hence any simultaneous
on_each_cpu() won't complete until after load_debug_registers() is
done.

So there doesn't seem to be any need for RCU protection in
update_this_cpu().

> Yes, indeed. With the current implementation, there's a possibility of
> two instances of update_this_cpu() function executing - one with an
> rcu_read_lock() taken (when called from load_debug_registers) while the
> other without (when invoked through update_all_cpus()).

No, this isn't possible unless I have misunderstood the nature of
IPIs.  Isn't is true that calling local_irq_save() will block delivery
of IPIs?

Alan Stern

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