Re: [RFC PATCH 03/21] list: Annotate lockless list primitives with data_race()

[Paul E. McKenney]

On Tue, Mar 24, 2020 at 09:32:01PM +0000, Will Deacon wrote:
> [mutt crashed while I was sending this; apologies if you receive it twice]
> 
> On Tue, Mar 24, 2020 at 05:56:15PM +0100, Jann Horn wrote:
> > On Tue, Mar 24, 2020 at 5:51 PM Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
> > > On Tue, Mar 24, 2020 at 03:36:25PM +0000, Will Deacon wrote:
> > > > diff --git a/include/linux/list.h b/include/linux/list.h
> > > > index 4fed5a0f9b77..4d9f5f9ed1a8 100644
> > > > --- a/include/linux/list.h
> > > > +++ b/include/linux/list.h
> > > > @@ -279,7 +279,7 @@ static inline int list_is_last(const struct list_head *list,
> > > >   */
> > > >  static inline int list_empty(const struct list_head *head)
> > > >  {
> > > > -     return READ_ONCE(head->next) == head;
> > > > +     return data_race(READ_ONCE(head->next) == head);
> > > >  }
> > >
> > > list_empty() isn't lockless safe, that's what we have
> > > list_empty_careful() for.
> > 
> > That thing looks like it could also use some READ_ONCE() sprinkled in...
> 
> Crikey, how did I miss that? I need to spend some time understanding the
> ordering there.
> 
> So it sounds like the KCSAN splats relating to list_empty() and loosely
> referred to by 1c97be677f72 ("list: Use WRITE_ONCE() when adding to lists
> and hlists") are indicative of real bugs and we should actually restore
> list_empty() to its former glory prior to 1658d35ead5d ("list: Use
> READ_ONCE() when testing for empty lists"). Alternatively, assuming
> list_empty_careful() does what it says on the tin, we could just make that
> the default.

The list_empty_careful() function (suitably annotated) returns false if
the list is non-empty, including when it is in the process of becoming
either empty or non-empty.  It would be fine for the lockless use cases
I have come across.

							Thanx, Paul