On Tue, 12 May 2015 22:38:53 -0400 Steven Rostedt <rost...@goodmis.org> wrote:
> On Tue, 12 May 2015 15:46:26 -0700
> "Paul E. McKenney" <paul...@linux.vnet.ibm.com> wrote:
>
> > From: Patrick Marlier <patrick.marl...@gmail.com>
> >
> > Signed-off-by: Patrick Marlier <patrick.marl...@gmail.com>
> > Signed-off-by: Paul E. McKenney <paul...@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"....
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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