On Wed, May 20, 2015 at 03:09:19PM +1000, NeilBrown wrote:
> On Tue, 19 May 2015 15:07:25 -0700 "Paul E. McKenney"
> <paul...@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 <ne...@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.marl...@gmail.com>
> Signed-off-by: NeilBrown <ne...@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);
--
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