Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752018AbbEPRm7 (ORCPT ); Sat, 16 May 2015 13:42:59 -0400 Received: from mail-wi0-f174.google.com ([209.85.212.174]:33216 "EHLO mail-wi0-f174.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751209AbbEPRm6 (ORCPT ); Sat, 16 May 2015 13:42:58 -0400 Message-ID: <5557819E.1060001@gmail.com> Date: Sat, 16 May 2015 19:42:54 +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 , Steven Rostedt CC: "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@efficios.com, josh@joshtriplett.org, tglx@linutronix.de, peterz@infradead.org, dhowells@redhat.com, edumazet@google.com, dvhart@linux.intel.com, fweisbec@gmail.com, oleg@redhat.com, bobby.prani@gmail.com, 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> In-Reply-To: <20150513125839.371ef677@notabene.brown> Content-Type: text/plain; charset=windows-1252; format=flowed Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 3926 Lines: 123 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: >> >>> From: Patrick Marlier >>> >>> Signed-off-by: Patrick Marlier >>> Signed-off-by: Paul E. McKenney >>> --- >>> drivers/md/bitmap.c | 2 +- >>> 1 file changed, 1 insertion(+), 1 deletion(-) >>> >>> diff --git a/drivers/md/bitmap.c b/drivers/md/bitmap.c >>> index 2bc56e2a3526..32901772e4ee 100644 >>> --- a/drivers/md/bitmap.c >>> +++ b/drivers/md/bitmap.c >>> @@ -181,7 +181,7 @@ static struct md_rdev *next_active_rdev(struct md_rdev *rdev, struct mddev *mdde >>> rcu_read_lock(); >>> if (rdev == NULL) >>> /* start at the beginning */ >>> - rdev = list_entry_rcu(&mddev->disks, struct md_rdev, same_set); >>> + rdev = list_entry_rcu(mddev->disks.next, struct md_rdev, same_set); >> >> Hmm, this changes the semantics. >> >> The original code looks nasty, I first thought it was broken, but it >> seems to work out of sheer luck (or clever hack) > > Definitely a clever hack - no question of "luck" here :-) > > It might makes sense to change it to use list_for_each_entry_from_rcu() > > if (rdev == NULL) > rdev = list_entry_rcu(mddev->disks.next, struct md_rdev, same_set); > else { > rdev_dec_pending(rdev, mddev); > rdev = list_next_entry_rcu(rdev->same_set.next, struct md_rdev, same_set); > } > list_for_each_entry_from_rcu(rdev, ....) > > but there isn't a "list_next_entry_rcu".... > > > Also, it would have been polity to at least 'cc' them Maintainer of this code > in the original patch - no? Sure my bad. I hesitated to CC maintainers. I was almost sure that it will be rejected so I wanted to avoid noise. > > Thanks, > NeilBrown > >> >>> else { >>> /* release the previous rdev and start from there. */ >>> rdev_dec_pending(rdev, mddev); >> >> >> 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. 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? PS: In the rculist patch I proposed, I avoid the store and the atomic reload in the stack variable __ptr. (yeap, the rcu_dereference_raw/ACCESS_ONCE is a bit confusing because it implicitly do & on the parameter). Thanks. -- Pat -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/