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

[NeilBrown]

On Tue, 12 May 2015 22:38:53 -0400 Steven Rostedt <rostedt@xxxxxxxxxxx> wrote:

> On Tue, 12 May 2015 15:46:26 -0700
> "Paul E. McKenney" <paulmck@xxxxxxxxxxxxxxxxxx> wrote:
> 
> > From: Patrick Marlier <patrick.marlier@xxxxxxxxx>
> > 
> > Signed-off-by: Patrick Marlier <patrick.marlier@xxxxxxxxx>
> > Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
> > ---
> >  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?

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

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