Re: [PATCH 1/3] list_bl: Add hlist_bl_add_before/behind helpers

Mike Snitzer
2019-03-14T00:30:33Z

On Wed, Mar 13 2019 at 7:48pm -0400,
Paul E. McKenney <paulmck@linux.ibm.com> wrote:

> On Thu, Feb 28, 2019 at 04:32:02PM -0500, Mike Snitzer wrote:
> > On Thu, Dec 20 2018 at 1:06pm -0500,
> > Nikos Tsironis <ntsironis@arrikto.com> wrote:
> >
> > > Add hlist_bl_add_before/behind helpers to add an element before/after an
> > > existing element in a bl_list.
> > >
> > > Signed-off-by: Nikos Tsironis <ntsironis@arrikto.com>
> > > Signed-off-by: Ilias Tsitsimpis <iliastsi@arrikto.com>
> > > ---
> > > include/linux/list_bl.h | 27 +++++++++++++++++++++++++++
> > > 1 file changed, 27 insertions(+)
> > >
> > > diff --git a/include/linux/list_bl.h b/include/linux/list_bl.h
> > > index 3fc2cc57ba1b..2fd918e5fd48 100644
> > > --- a/include/linux/list_bl.h
> > > +++ b/include/linux/list_bl.h
> > > @@ -86,6 +86,33 @@ static inline void hlist_bl_add_head(struct hlist_bl_node *n,
> > > hlist_bl_set_first(h, n);
> > > }
> > >
> > > +static inline void hlist_bl_add_before(struct hlist_bl_node *n,
> > > + struct hlist_bl_node *next)
> > > +{
> > > + struct hlist_bl_node **pprev = next->pprev;
> > > +
> > > + n->pprev = pprev;
> > > + n->next = next;
> > > + next->pprev = &n->next;
> > > +
> > > + /* pprev may be `first`, so be careful not to lose the lock bit */
> > > + WRITE_ONCE(*pprev,
> > > + (struct hlist_bl_node *)
> > > + ((unsigned long)n |
> > > + ((unsigned long)*pprev & LIST_BL_LOCKMASK)));
>
> A nit, but use of uintptr_t shrinks things a bit:
>
> + (struct hlist_bl_node *)
> + ((uintptr_t)n | ((uintptr_t)*pprev & LIST_BL_LOCKMASK)));
>
> I am not too concerned about this, though.

I'm fine with folding in your suggestion.

> The WRITE_ONCE() is to handle races with hlist_bl_empty() (which does contain
> the corresponding READ_ONCE()) correct?

Correct.

> > > +}
> > > +
> > > +static inline void hlist_bl_add_behind(struct hlist_bl_node *n,
> > > + struct hlist_bl_node *prev)
> > > +{
> > > + n->next = prev->next;
> > > + n->pprev = &prev->next;
> > > + WRITE_ONCE(prev->next, n);
>
> I don't see what this WRITE_ONCE() is interacting with. The traversals
> use plain C-language reads, and hlist_bl_empty() can't get here. All
> uses of hlist_bl_for_each_entry() invoke hlist_bl_lock() before starting
> the traversal, and hlist_bl_for_each_entry_safe() looks to be unused.
> (Perhaps it should be removed? Or is there some anticipated use?)
>
> I don't believe that the WRITE_ONCE() is needed. What am I missing?
>
> Other than that, looks good.
>
> Thanx, Paul
>

I'd imagine it was just born out of symmetry with hlist_bl_add_before()
and/or caution. But let's see what Nikos has to say.

Thanks,
Mike