From: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
To: NeilBrown <neilb@suse.de>
Cc: Steven Rostedt <rostedt@goodmis.org>,
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,
Patrick Marlier <patrick.marlier@gmail.com>,
wangyun@linux.vnet.ibm.com
Subject: Re: [PATCH tip/core/rcu 3/4] md/bitmap: Fix list_entry_rcu usage
Date: Wed, 13 May 2015 06:17:37 -0700 [thread overview]
Message-ID: <20150513131737.GZ6776@linux.vnet.ibm.com> (raw)
In-Reply-To: <20150513125839.371ef677@notabene.brown>
On Wed, May 13, 2015 at 12:58:39PM +1000, NeilBrown wrote:
> On Tue, 12 May 2015 22:38:53 -0400 Steven Rostedt <rostedt@goodmis.org> wrote:
>
> > On Tue, 12 May 2015 15:46:26 -0700
> > "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> wrote:
> >
> > > From: Patrick Marlier <patrick.marlier@gmail.com>
> > >
> > > Signed-off-by: Patrick Marlier <patrick.marlier@gmail.com>
> > > Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
> > > ---
> > > 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"....
Patrick, this one is for you. As are all of the questions from Steven.
> Also, it would have been polity to at least 'cc' them Maintainer of this code
> in the original patch - no?
Rest assured that this set is not going to -tip without at least an
Acked-by from at least one of the maintainers. In some cases, I will
interpret silence as assent, but this is not one of those cases. ;-)
Thanx, Paul
> 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.
> >
> > -- Steve
>
next prev parent reply other threads:[~2015-05-13 13:17 UTC|newest]
Thread overview: 24+ messages / expand[flat|nested] mbox.gz Atom feed top
2015-05-12 22:46 [PATCH tip/core/rcu 0/4] RCU-protected list updates for 4.2 Paul E. McKenney
2015-05-12 22:46 ` [PATCH tip/core/rcu 1/4] rculist: Fix another sparse warning Paul E. McKenney
2015-05-12 22:46 ` [PATCH tip/core/rcu 2/4] rculist: Fix list_entry_rcu to read ptr with rcu_dereference_raw Paul E. McKenney
2015-05-12 22:46 ` [PATCH tip/core/rcu 3/4] md/bitmap: Fix list_entry_rcu usage Paul E. McKenney
2015-05-13 2:38 ` Steven Rostedt
2015-05-13 2:58 ` NeilBrown
2015-05-13 13:17 ` Paul E. McKenney [this message]
2015-05-16 17:42 ` Patrick Marlier
2015-05-18 2:06 ` NeilBrown
2015-05-18 13:43 ` Steven Rostedt
2015-05-19 22:07 ` Paul E. McKenney
2015-05-20 5:09 ` NeilBrown
2015-05-20 13:28 ` Paul E. McKenney
2015-05-21 0:07 ` NeilBrown
2015-09-11 23:05 ` Paul E. McKenney
2015-09-13 10:06 ` Patrick Marlier
2015-09-13 16:10 ` Paul E. McKenney
2015-09-22 20:50 ` Paul E. McKenney
2015-09-23 17:57 ` Patrick Marlier
2015-09-24 4:45 ` Paul E. McKenney
2015-05-18 13:53 ` Patrick Marlier
2015-05-18 19:36 ` Patrick Marlier
2015-05-12 22:46 ` [PATCH tip/core/rcu 4/4] netfilter: " Paul E. McKenney
2015-05-13 2:42 ` Steven Rostedt
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