From mboxrd@z Thu Jan 1 00:00:00 1970 From: "Paul E. McKenney" Subject: Re: linux-next: build failure after merge of the rcu tree Date: Fri, 8 Jan 2016 07:53:41 -0800 Message-ID: <20160108155341.GC3818@linux.vnet.ibm.com> References: <20160107195725.3a130b4d@canb.auug.org.au> <20160107180244.GR3818@linux.vnet.ibm.com> <20160108071932.060a9bcc@canb.auug.org.au> <20160107205220.GU3818@linux.vnet.ibm.com> <20160108013631.GA11410@fixme-laptop.cn.ibm.com> <20160108034157.GZ3818@linux.vnet.ibm.com> <20160108150850.1ec97e78@canb.auug.org.au> <20160108044835.GB3818@linux.vnet.ibm.com> <20160108045453.GB4300@fixme-laptop.cn.ibm.com> Reply-To: paulmck@linux.vnet.ibm.com Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Return-path: Received: from e31.co.us.ibm.com ([32.97.110.149]:47207 "EHLO e31.co.us.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1755497AbcAHPxh (ORCPT ); Fri, 8 Jan 2016 10:53:37 -0500 Received: from localhost by e31.co.us.ibm.com with IBM ESMTP SMTP Gateway: Authorized Use Only! Violators will be prosecuted for from ; Fri, 8 Jan 2016 08:53:36 -0700 Content-Disposition: inline In-Reply-To: <20160108045453.GB4300@fixme-laptop.cn.ibm.com> Sender: linux-next-owner@vger.kernel.org List-ID: To: Boqun Feng Cc: Stephen Rothwell , linux-next@vger.kernel.org, linux-kernel@vger.kernel.org, Tejun Heo , Christoph Lameter On Fri, Jan 08, 2016 at 12:54:53PM +0800, Boqun Feng wrote: > On Thu, Jan 07, 2016 at 08:48:35PM -0800, Paul E. McKenney wrote: > > On Fri, Jan 08, 2016 at 03:08:50PM +1100, Stephen Rothwell wrote: > > > Hi Paul, > > > > > > On Thu, 7 Jan 2016 19:41:57 -0800 "Paul E. McKenney" wrote: > > > > > > > > Indeed, I suspect that SRCU might not be the only thing that would like > > > > static per-CPU variables. ;-) > > > > > > from include/linux/percpu-defs.h: > > > > > > * s390 and alpha modules require percpu variables to be defined as > > > * weak to force the compiler to generate GOT based external > > > * references for them. This is necessary because percpu sections > > > * will be located outside of the usually addressable area. > > > * > > > * This definition puts the following two extra restrictions when > > > * defining percpu variables. > > > * > > > * 1. The symbol must be globally unique, even the static ones. > > > * 2. Static percpu variables cannot be defined inside a function. > > > * > > > * Archs which need weak percpu definitions should define > > > * ARCH_NEEDS_WEAK_PER_CPU in asm/percpu.h when necessary. > > > * > > > * To ensure that the generic code observes the above two > > > * restrictions, if CONFIG_DEBUG_FORCE_WEAK_PER_CPU is set weak > > > * definition is used for all cases. > > > > > > so they can be static, but must be globally unique. > > Interesting ;-) Thank you. > > > Thank you for the tutorial! I have updated the patch to use a unique > > name for the srcu_struct added by rcuperf.c. > > Plus a comment for DEFINE_STATIC_SRCU saying srcu can be defined as > static but must have a unique name for this reason? Yes, plus some words about how to work around the limitation. How about the patch below? Thanx, Paul ------------------------------------------------------------------------ commit 61822ff81839ee3c5b6094ce348944d972b87892 Author: Paul E. McKenney 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 Reported-by: kbuild test robot Suggested-by: Boqun Feng Signed-off-by: Paul E. McKenney 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. */ #define __DEFINE_SRCU(name, is_static) \ static DEFINE_PER_CPU(struct srcu_struct_array, name##_srcu_array);\