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

Tejun Heo
2016-01-08T15:57:27Z

On Fri, Jan 08, 2016 at 07:53:41AM -0800, Paul E. McKenney wrote:
> commit 61822ff81839ee3c5b6094ce348944d972b87892
> Author: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
> 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@canb.auug.org.au>
> Reported-by: kbuild test robot <fengguang.wu@intel.com>
> Suggested-by: Boqun Feng <boqun.feng@gmail.com>
> Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
>
> 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@kernel.org>

Thanks Stephen for posting the rules around static percpu definitions.
I wonder whether there is a better way to tell people what's going on.

Thanks.

--
tejun