Re: linux-next: build failure after merge of the rcu tree

[Paul E. McKenney]

On Fri, Jan 08, 2016 at 10:57:27AM -0500, Tejun Heo wrote:
> On Fri, Jan 08, 2016 at 07:53:41AM -0800, Paul E. McKenney wrote:
> > commit 61822ff81839ee3c5b6094ce348944d972b87892
> > Author: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
> > Date:   Fri Jan 8 07:43:50 2016 -0800
> > 
> >     rcu: Document unique-name limitation for DEFINE_STATIC_SRCU()
> >     
> >     SRCU uses per-CPU variables, and DEFINE_STATIC_SRCU() uses a static
> >     per-CPU variable.  However, per-CPU variables have significant
> >     restrictions, for example, names of per-CPU variables must be globally
> >     unique, even if declared static.  These restrictions carry over to
> >     DEFINE_STATIC_SRCU(), and this commit therefore documents these
> >     restrictions.
> >     
> >     Reported-by: Stephen Rothwell <sfr@xxxxxxxxxxxxxxxx>
> >     Reported-by: kbuild test robot <fengguang.wu@xxxxxxxxx>
> >     Suggested-by: Boqun Feng <boqun.feng@xxxxxxxxx>
> >     Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
> > 
> > diff --git a/include/linux/srcu.h b/include/linux/srcu.h
> > index f5f80c5643ac..dc8eb63c6568 100644
> > --- a/include/linux/srcu.h
> > +++ b/include/linux/srcu.h
> > @@ -99,8 +99,23 @@ void process_srcu(struct work_struct *work);
> >  	}
> >  
> >  /*
> > - * define and init a srcu struct at build time.
> > - * dont't call init_srcu_struct() nor cleanup_srcu_struct() on it.
> > + * Define and initialize a srcu struct at build time.
> > + * Do -not- call init_srcu_struct() nor cleanup_srcu_struct() on it.
> > + *
> > + * Note that although DEFINE_STATIC_SRCU() hides the name from other
> > + * files, the per-CPU variable rules nevertheless require that the
> > + * chosen name be globally unique.  These rules also prohibit use of
> > + * DEFINE_STATIC_SRCU() within a function.  If these rules are too
> > + * restrictive, declare the srcu_struct manually.  For example, in
> > + * each file:
> > + *
> > + *	static struct srcu_struct my_srcu;
> > + *
> > + * Then, before the first use of each my_srcu, manually initialize it:
> > + *
> > + *	init_srcu_struct(&my_srcu);
> > + *
> > + * See include/linux/percpu-defs.h for the rules on per-CPU variables.
> 
> FWIW,
> 
>  Reviewed-by: Tejun Heo <tj@xxxxxxxxxx>

Applied, thank you!

> Thanks Stephen for posting the rules around static percpu definitions.

Me too!  Hey, it was building fine with all of my configurations,
so I was really beginning to wonder if this was some sort of early
April Fools joke.  ;-)

> I wonder whether there is a better way to tell people what's going on.

I hope so.  ;-)

							Thanx, Paul