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

[Paul E. McKenney]

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@xxxxxxxxxxx> 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@xxxxxxxxxxx>
> > Signed-off-by: Ilias Tsitsimpis <iliastsi@xxxxxxxxxxx>
> > ---
> >  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.

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

> > +
> > +	if (n->next)
> > +		n->next->pprev = &n->next;
> > +}
> > +
> >  static inline void __hlist_bl_del(struct hlist_bl_node *n)
> >  {
> >  	struct hlist_bl_node *next = n->next;
> > -- 
> > 2.11.0
> 
> Hi Paul and Christoph,
> 
> You've added your Signed-off-by to include/linux/list_bl.h commits in
> the past.  I'm not sure how this proposed patch should be handled.
> 
> These new hlist_bl_add_{before,behind} changes are a prereq for
> dm-snapshot changes that Nikos has proposed, please see:
> https://patchwork.kernel.org/patch/10739265/
> 
> Any assistance/review you, or others on LKML, might be able to provide
> would be appreciated.
> 
> Thanks,
> Mike
>