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 20, 2015 at 03:09:19PM +1000, NeilBrown wrote:
> On Tue, 19 May 2015 15:07:25 -0700 "Paul E. McKenney"
> <paulmck@linux.vnet.ibm.com> wrote:
> 
> 
> > The code in md probably needs to change in any case, as otherwise we are
> > invoking rcu_dereference_whatever() on a full struct list_head rather
> > than on a single pointer.  Or am I missing something here?
> 
> I think it would be
>    rcu_dereference_whatever(&mddev->disks)
> 
> I don't know what you mean by "on a full struct list_head", but there is
> nothing actually being dereferenced here - right?  Just pointer arithmetic on
> 'mddev'.

It really does dereference.  Strange but true.

> I should probably just
> 
> diff --git a/drivers/md/bitmap.c b/drivers/md/bitmap.c
> index 2bc56e2a3526..b1d237bf8b3b 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(&mddev->disks, struct md_rdev, same_set);
>  	else {
>  		/* release the previous rdev and start from there. */
>  		rdev_dec_pending(rdev, mddev);
> 
> as there really are no RCU issues with getting that address.  Maybe I should
> move it outside the rcu_read_lock() just to be blatant.... but that would
> make the code a lot more clumsy as the rdev_dec_pending must be inside the
> rcu_read_lock..
> 
> So this.

Fair enough -- if you aren't using RCU, there is really no point in using
the RCU API.  I will drop this patch from my tree.  You are pushing yours,
I am guessing?

							Thanx, Paul

> Thanks,
> NeilBrown
> 
> From: NeilBrown <neilb@suse.de>
> Date: Wed, 20 May 2015 15:05:09 +1000
> Subject: [PATCH] md/bitmap: remove rcu annotation from pointer arithmetic.
> 
> Evaluating  "&mddev->disks" is simple pointer arithmetic, so
> it does not need 'rcu' annotations - no dereferencing is happening.
> 
> Also enhance the comment to explain that 'rdev' in that case
> is not actually a pointer to an rdev.
> 
> Reported-by: Patrick Marlier <patrick.marlier@gmail.com>
> Signed-off-by: NeilBrown <neilb@suse.de>
> 
> diff --git a/drivers/md/bitmap.c b/drivers/md/bitmap.c
> index 2bc56e2a3526..135a0907e9de 100644
> --- a/drivers/md/bitmap.c
> +++ b/drivers/md/bitmap.c
> @@ -177,11 +177,16 @@ static struct md_rdev *next_active_rdev(struct md_rdev *rdev, struct mddev *mdde
>  	 * nr_pending is 0 and In_sync is clear, the entries we return will
>  	 * still be in the same position on the list when we re-enter
>  	 * list_for_each_entry_continue_rcu.
> +	 *
> +	 * Note that if entered with 'rdev == NULL' to start at the
> +	 * beginning, we temporarily assign 'rdev' to an address which
> +	 * isn't really an rdev, but which can be used by
> +	 * list_for_each_entry_continue_rcu() to find the first entry.
>  	 */
>  	rcu_read_lock();
>  	if (rdev == NULL)
>  		/* start at the beginning */
> -		rdev = list_entry_rcu(&mddev->disks, struct md_rdev, same_set);
> +		rdev = list_entry(&mddev->disks, struct md_rdev, same_set);
>  	else {
>  		/* release the previous rdev and start from there. */
>  		rdev_dec_pending(rdev, mddev);