Re: [PATCH tip/core/rcu 3/4] md/bitmap: Fix list_entry_rcu usage

["Paul E. McKenney" <paulmck@linux.vnet.ibm.com>]

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
>