Subject: Re: [0/8] netpoll/bridge fixes

From: "Paul E. McKenney" <[email-masked]>
Date: Thu, 17 Jun 2010 14:26:43 -0700

On Thu, Jun 17, 2010 at 01:18:30PM +0300, Michael S. Tsirkin wrote:
> On Wed, Jun 16, 2010 at 04:02:49PM -0700, Paul E. McKenney wrote:
> > On Tue, Jun 15, 2010 at 09:47:02PM -0700, David Miller wrote:
> > > From: Herbert Xu <[email-masked]>
> > > Date: Wed, 16 Jun 2010 13:33:36 +1000
> > > 
> > > > On Wed, Jun 16, 2010 at 05:03:20AM +0200, Eric Dumazet wrote:
> > > >>
> > > >> I wonder how these patches were tested, Herbert ?
> > > > 
> > > > You know, not everyone enables RCU debugging...
> > > 
> > > Even though I'm as guilty as you, I have to agree with Eric that
> > > especially us core folks should be running with the various lock
> > > debugging options on all the time.
> > > 
> > > Maybe someone should add the RCU debugging config option to
> > > Documentation/SubmitChecklist :-)
> > 
> > How about the following added to Documentation/RCU/checklist.txt?
> > 
> > The first is in mainline, the second partly there, and the third
> > is still languishing in my tree.  I did manage to remove a dependency
> > on other maintainers, so things will hopefully move a bit faster.
> > 
> > 							Thanx, Paul
> > 
> > diff --git a/Documentation/RCU/checklist.txt b/Documentation/RCU/checklist.txt
> > index 790d1a8..c7c6788 100644
> > --- a/Documentation/RCU/checklist.txt
> > +++ b/Documentation/RCU/checklist.txt
> > @@ -365,3 +365,26 @@ over a rather long period of time, but improvements are always welcome!
> >  	and the compiler to freely reorder code into and out of RCU
> >  	read-side critical sections.  It is the responsibility of the
> >  	RCU update-side primitives to deal with this.
> > +
> > +17.	Use CONFIG_PROVE_RCU, CONFIG_DEBUG_OBJECTS_RCU_HEAD, and
> > +	the __rcu sparse checks to validate your RCU code.  These
> > +	can help find problems as follows:
> > +
> > +	CONFIG_PROVE_RCU: check that accesses to RCU-protected data
> > +		structures are carried out under the proper RCU
> > +		read-side critical section, while holding the right
> > +		combination of locks, or whatever other conditions
> > +		are appropriate.
> > +
> > +	CONFIG_DEBUG_OBJECTS_RCU_HEAD: check that you don't pass the
> > +		same object to call_rcu() (or friends) before an RCU
> > +		grace period has elapsed since the last time that you
> > +		passed that same object to call_rcu() (or friends).
> > +
> 
> Cool, will this also work with synchronize etc?

Unfortunately, it will not.  With call_rcu() and friends you can tag
the struct rcu_head and track it.  With synchronize_rcu() and friends,
there is nothing to track.  :-(

							Thanx, Paul

> > +	__rcu sparse checks: tag the pointer to the RCU-protected data
> > +		structure with __rcu, and sparse will warn you if you
> > +		access that pointer without the services of one of the
> > +		variants of rcu_dereference().
> > +
> > +	These debugging aids can help you find problems that are
> > +	otherwise extremely difficult to spot.