From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S932645AbbERTga (ORCPT ); Mon, 18 May 2015 15:36:30 -0400 Received: from mail-wg0-f50.google.com ([74.125.82.50]:36665 "EHLO mail-wg0-f50.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753919AbbERTg2 (ORCPT ); Mon, 18 May 2015 15:36:28 -0400 Message-ID: <555A3F39.8020907@gmail.com> Date: Mon, 18 May 2015 21:36:25 +0200 From: Patrick Marlier User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:31.0) Gecko/20100101 Thunderbird/31.6.0 MIME-Version: 1.0 To: NeilBrown CC: Steven Rostedt , "Paul E. McKenney" , linux-kernel@vger.kernel.org, mingo@kernel.org, laijs@cn.fujitsu.com, dipankar@in.ibm.com, akpm@linux-foundation.org, Mathieu Desnoyers , Josh Triplett , tglx@linutronix.de, peterz@infradead.org, dhowells@redhat.com, Eric Dumazet , dvhart@linux.intel.com, =?UTF-8?B?RnLDqWTDqXJpYyBXZWlzYmVja2Vy?= , oleg@redhat.com, pranith kumar , wangyun@linux.vnet.ibm.com Subject: Re: [PATCH tip/core/rcu 3/4] md/bitmap: Fix list_entry_rcu usage References: <20150512224603.GA4531@linux.vnet.ibm.com> <1431470787-4702-1-git-send-email-paulmck@linux.vnet.ibm.com> <1431470787-4702-3-git-send-email-paulmck@linux.vnet.ibm.com> <20150512223853.55e0ed90@grimm.local.home> <20150513125839.371ef677@notabene.brown> <5557819E.1060001@gmail.com> <20150518120647.0c3cecd8@notabene.brown> In-Reply-To: Content-Type: text/plain; charset=utf-8; format=flowed Content-Transfer-Encoding: 8bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 05/18/2015 03:53 PM, Patrick Marlier wrote: > On Mon, May 18, 2015 at 4:06 AM, NeilBrown wrote: >> On Sat, 16 May 2015 19:42:54 +0200 Patrick Marlier >> wrote: >>> On 05/13/2015 04:58 AM, NeilBrown wrote: >>>> On Tue, 12 May 2015 22:38:53 -0400 Steven Rostedt wrote: >>>>> On Tue, 12 May 2015 15:46:26 -0700 >>>>> "Paul E. McKenney" wrote: >>>>> What comes after this is: >>>>> >>>>> list_for_each_entry_continue_rcu(rdev, &mddev->disks, same_set) { >>>>> if (rdev->raid_disk >= 0 && >>>>> >>>>> Now the original code had: >>>>> >>>>> rdev = list_entry_rcu(&mddev->disks, struct md_rdev, same_set); >>>>> >>>>> Where &mddev->disks would return the address of the disks field of >>>>> mddev which is a list head. Then it would get the 'same_set' offset, >>>>> which is 0, and rdev is pointing to a makeshift md_rdev struct. But it >>>>> isn't used, as the list_for_each_entry_continue_rcu() has: >>>>> >>>>> #define list_for_each_entry_continue_rcu(pos, head, member) \ >>>>> for (pos = list_entry_rcu(pos->member.next, typeof(*pos), member); \ >>>>> &pos->member != (head); \ >>>>> pos = list_entry_rcu(pos->member.next, typeof(*pos), member)) >>>>> >>>>> Thus the first use of pos is pos->member.next or: >>>>> >>>>> mddev->disks.next >>>>> >>>>> But now you converted it to rdev = mddev->disks.next, which means the >>>>> first use is: >>>>> >>>>> pos = mddev->disks.next->next >>>>> >>>>> I think you are skipping the first element here. >>> >>> >>> struct mddev { >>> ... >>> struct list_head disks; >>> ...} >>> >>> struct list_head { >>> struct list_head *next, *prev; >>> }; >>> >>> The tricky thing is that "list_entry_rcu" before and after the patch is >>> reading the same thing. >> >> No it isn't. >> Before the patch it is passed the address of the 'next' field. After the >> patch it is passed the contents of the 'next' field. > > Here I meant "list_entry_rcu" (in include/linux/rculist.h) not the > change to drivers/md/bitmap.c. > > >>> However in your case, the change I proposed is probably wrong I trust >>> you on this side. :) What's your proposal to fix it with the rculist patch? >> >> What needs fixing? I don't see anything broken. >> >> Maybe there is something in this "rculist patch" that I'm missing. Can you >> point me at it? > > Do not apply the patch on drivers/md/bitmap.c but only on > include/linux/rculist.h and you will see that the compilation fails. Here an example of the compilation error in drivers/md/bitmap.c In file included from include/linux/sched.h:17:0, from include/linux/blkdev.h:4, from drivers/md/bitmap.c:18: drivers/md/bitmap.c: In function ‘next_active_rdev’: include/linux/compiler.h:469:24: error: lvalue required as unary ‘&’ operand (volatile typeof(x) *)&(x); }) ^ include/linux/kernel.h:800:49: note: in definition of macro ‘container_of’ const typeof( ((type *)0)->member ) *__mptr = (ptr); \ ^ include/linux/compiler.h:470:26: note: in expansion of macro ‘__ACCESS_ONCE’ #define ACCESS_ONCE(x) (*__ACCESS_ONCE(x)) ^ include/linux/rcupdate.h:630:26: note: in expansion of macro ‘ACCESS_ONCE’ typeof(p) _________p1 = ACCESS_ONCE(p); \ ^ include/linux/rcupdate.h:587:48: note: in expansion of macro ‘lockless_dereference’ typeof(*p) *________p1 = (typeof(*p) *__force)lockless_dereference(p); \ ^ include/linux/rcupdate.h:723:2: note: in expansion of macro ‘__rcu_dereference_check’ __rcu_dereference_check((p), rcu_read_lock_held() || (c), __rcu) ^ include/linux/rcupdate.h:746:32: note: in expansion of macro ‘rcu_dereference_check’ #define rcu_dereference_raw(p) rcu_dereference_check(p, 1) /*@@@ needed? @@@*/ ^ include/linux/rculist.h:251:28: note: in expansion of macro ‘rcu_dereference_raw’ container_of((typeof(ptr))rcu_dereference_raw(ptr), type, member); \ ^ drivers/md/bitmap.c:184:10: note: in expansion of macro ‘list_entry_rcu’ rdev = list_entry_rcu(&mddev->disks, struct md_rdev, same_set); ^ Thanks. -- Pat