From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754344AbbETN2p (ORCPT ); Wed, 20 May 2015 09:28:45 -0400 Received: from e31.co.us.ibm.com ([32.97.110.149]:48258 "EHLO e31.co.us.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S932076AbbETN2n (ORCPT ); Wed, 20 May 2015 09:28:43 -0400 Date: Wed, 20 May 2015 06:28:35 -0700 From: "Paul E. McKenney" To: NeilBrown Cc: Steven Rostedt , Patrick Marlier , 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 Message-ID: <20150520132835.GJ6776@linux.vnet.ibm.com> Reply-To: paulmck@linux.vnet.ibm.com 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> <20150518094321.2012a66a@gandalf.local.home> <20150519220725.GA6776@linux.vnet.ibm.com> <20150520150919.7f5aa0e9@notabene.brown> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20150520150919.7f5aa0e9@notabene.brown> User-Agent: Mutt/1.5.21 (2010-09-15) X-TM-AS-MML: disable X-Content-Scanned: Fidelis XPS MAILER x-cbid: 15052013-8236-0000-0000-00000BA76CDB Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, May 20, 2015 at 03:09:19PM +1000, NeilBrown wrote: > On Tue, 19 May 2015 15:07:25 -0700 "Paul E. McKenney" > wrote: > > > > The code in md probably needs to change in any case, as otherwise we are > > invoking rcu_dereference_whatever() on a full struct list_head rather > > than on a single pointer. Or am I missing something here? > > I think it would be > rcu_dereference_whatever(&mddev->disks) > > I don't know what you mean by "on a full struct list_head", but there is > nothing actually being dereferenced here - right? Just pointer arithmetic on > 'mddev'. It really does dereference. Strange but true. > I should probably just > > diff --git a/drivers/md/bitmap.c b/drivers/md/bitmap.c > index 2bc56e2a3526..b1d237bf8b3b 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(&mddev->disks, struct md_rdev, same_set); > else { > /* release the previous rdev and start from there. */ > rdev_dec_pending(rdev, mddev); > > as there really are no RCU issues with getting that address. Maybe I should > move it outside the rcu_read_lock() just to be blatant.... but that would > make the code a lot more clumsy as the rdev_dec_pending must be inside the > rcu_read_lock.. > > So this. Fair enough -- if you aren't using RCU, there is really no point in using the RCU API. I will drop this patch from my tree. You are pushing yours, I am guessing? Thanx, Paul > Thanks, > NeilBrown > > From: NeilBrown > Date: Wed, 20 May 2015 15:05:09 +1000 > Subject: [PATCH] md/bitmap: remove rcu annotation from pointer arithmetic. > > Evaluating "&mddev->disks" is simple pointer arithmetic, so > it does not need 'rcu' annotations - no dereferencing is happening. > > Also enhance the comment to explain that 'rdev' in that case > is not actually a pointer to an rdev. > > Reported-by: Patrick Marlier > Signed-off-by: NeilBrown > > diff --git a/drivers/md/bitmap.c b/drivers/md/bitmap.c > index 2bc56e2a3526..135a0907e9de 100644 > --- a/drivers/md/bitmap.c > +++ b/drivers/md/bitmap.c > @@ -177,11 +177,16 @@ static struct md_rdev *next_active_rdev(struct md_rdev *rdev, struct mddev *mdde > * nr_pending is 0 and In_sync is clear, the entries we return will > * still be in the same position on the list when we re-enter > * list_for_each_entry_continue_rcu. > + * > + * Note that if entered with 'rdev == NULL' to start at the > + * beginning, we temporarily assign 'rdev' to an address which > + * isn't really an rdev, but which can be used by > + * list_for_each_entry_continue_rcu() to find the first entry. > */ > rcu_read_lock(); > if (rdev == NULL) > /* start at the beginning */ > - rdev = list_entry_rcu(&mddev->disks, struct md_rdev, same_set); > + rdev = list_entry(&mddev->disks, struct md_rdev, same_set); > else { > /* release the previous rdev and start from there. */ > rdev_dec_pending(rdev, mddev);