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

Paul E. McKenney
2019-03-14T15:03:00Z

On Thu, Mar 14, 2019 at 07:07:50AM -0700, Paul E. McKenney wrote:
> On Thu, Mar 14, 2019 at 03:28:23PM +0200, Nikos Tsironis wrote:
> > On 3/14/19 2:30 AM, Mike Snitzer wrote:
> > > On Wed, Mar 13 2019 at 7:48pm -0400,
> > > Paul E. McKenney <paulmck@linux.ibm.com> wrote:
> > >
> > Hi Paul,
> >
> > Thanks a lot for your feedback!
>
> NP, and apologies for the delay.
>
> > >> 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.
> >
> > Indeed, this looks better.
> >
> > >> The WRITE_ONCE() is to handle races with hlist_bl_empty() (which does contain
> > >> the corresponding READ_ONCE()) correct?
> > >
> > > Correct.
> >
> > Yes that's 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 am using hlist_bl_for_each_entry_safe() in this proposed patch for
> > dm-snapshot: https://patchwork.kernel.org/patch/10835709/
>
> Probably should keep it, then. ;-)
>
> > >>
> > >> 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.
> >
> > I also don't believe that this WRITE_SAME() is needed. But, looking at
> > hlist_add_behind() in include/linux/list.h, which, if I am not missing
> > something, is used in the same way as hlist_bl_add_behind(), it also
> > uses WRITE_ONCE() to update prev->next:
> >
> > static inline void hlist_add_behind(struct hlist_node *n,
> > struct hlist_node *prev)
> > {
> > n->next = prev->next;
> > WRITE_ONCE(prev->next, n);
> > n->pprev = &prev->next;
> >
> > if (n->next)
> > n->next->pprev = &n->next;
> > }
> >
> > Could it be the case that the WRITE_ONCE() in hlist_add_behind() is also
> > not needed? This WRITE_ONCE() was introduced by commit 1c97be677f72b3
> > ("list: Use WRITE_ONCE() when adding to lists and hlists").
>
> Looks like I have no one to blame but myself!
>
> Would you like to remove that as part of your patch series?
>
> > But, since I am not an expert in lockless programming, I opted to be on
> > the safe side and followed the example of hlist_add_behind().
> >
> > That said, I will follow up with a new version of the patch removing the
> > WRITE_ONCE() and using uintptr_t instead of unsigned long.
>
> Sounds good!

Oh, and of course intptr_t is one character shorter than uintptr_t, and
looks to work just as well in this context. ;-)

Thanx, Paul