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

[Patrick Marlier]

On 05/13/2015 04:58 AM, NeilBrown wrote:
> 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?

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
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