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

[Will Deacon]

[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.

Will