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

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Message-ID: <20160108155727.GP1898@mtj.duckdns.org>
Date:	Fri, 8 Jan 2016 10:57:27 -0500
From:	Tejun Heo <tj@...nel.org>
To:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc:	Boqun Feng <boqun.feng@...il.com>,
	Stephen Rothwell <sfr@...b.auug.org.au>,
	linux-next@...r.kernel.org, linux-kernel@...r.kernel.org,
	Christoph Lameter <cl@...ux-foundation.org>
Subject: Re: linux-next: build failure after merge of the rcu tree

On Fri, Jan 08, 2016 at 07:53:41AM -0800, Paul E. McKenney wrote:
> commit 61822ff81839ee3c5b6094ce348944d972b87892
> Author: Paul E. McKenney <paulmck@...ux.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@...b.auug.org.au>
>     Reported-by: kbuild test robot <fengguang.wu@...el.com>
>     Suggested-by: Boqun Feng <boqun.feng@...il.com>
>     Signed-off-by: Paul E. McKenney <paulmck@...ux.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@...nel.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