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

[Steven Rostedt <rostedt@goodmis.org>]

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)

>  	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