On Wed, 20 May 2015 06:28:35 -0700 "Paul E. McKenney" <paul...@linux.vnet.ibm.com> wrote:
> 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.
Well... your the expert. But without an lvalue, I can't see it.
>
> > 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?
Excellent guess :-)
Hopefully for the next -rc.
Thanks,
NeilBrown
>
> 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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