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