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

[Will Deacon <will@kernel.org>]

On Tue, Mar 24, 2020 at 05:23:30PM +0100, Marco Elver wrote:
> On Tue, 24 Mar 2020 at 16:37, Will Deacon <will@kernel.org> wrote:
> >
> > Some list predicates can be used locklessly even with the non-RCU list
> > implementations, since they effectively boil down to a test against
> > NULL. For example, checking whether or not a list is empty is safe even
> > in the presence of a concurrent, tearing write to the list head pointer.
> > Similarly, checking whether or not an hlist node has been hashed is safe
> > as well.
> >
> > Annotate these lockless list predicates with data_race() and READ_ONCE()
> > so that KCSAN and the compiler are aware of what's going on. The writer
> > side can then avoid having to use WRITE_ONCE() in the non-RCU
> > implementation.
> >
> > Cc: Paul E. McKenney <paulmck@kernel.org>
> > Cc: Peter Zijlstra <peterz@infradead.org>
> > Cc: Marco Elver <elver@google.com>
> > Signed-off-by: Will Deacon <will@kernel.org>
> > ---
> >  include/linux/list.h       | 10 +++++-----
> >  include/linux/list_bl.h    |  5 +++--
> >  include/linux/list_nulls.h |  6 +++---
> >  include/linux/llist.h      |  2 +-
> >  4 files changed, 12 insertions(+), 11 deletions(-)
> >
> > 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);
> 
> Double-marking should never be necessary, at least if you want to make
> KCSAN happy. From what I gather there is an unmarked write somewhere,
> correct? In that case, KCSAN will still complain because if it sees a
> race between this read and the other write, then at least one is still
> plain (the write).
> 
> Then, my suggestion would be to mark the write with data_race() and
> just leave this as a READ_ONCE(). Having a data_race() somewhere only
> makes KCSAN stop reporting the race if the paired access is also
> marked (be it with data_race() or _ONCE, etc.).
> 
> Alternatively, if marking the write is impossible, you can surround
> the access with kcsan_disable_current()/kcsan_enable_current(). Or, as
> a last resort, just leaving as-is is fine too, because KCSAN's default
> config (still) has KCSAN_ASSUME_PLAIN_WRITES_ATOMIC selected.

Right, it looks like this is a bit of a smoking gun and we need to decide
on whether list_empty() is actually usable without synchronisation first.
Based on the outcome of that discussion, I'll update this patch accordingly.

The main thing I want to avoid is marking parts of the non-RCU list
implementation with data_race() or {READ,WRITE}_ONCE() because that's a
sure-fire way to hide real bugs.

Cheers,

Will