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

[Paul E. McKenney]

On Fri, Apr 24, 2020 at 06:39:33PM +0100, Will Deacon wrote:
> On Mon, Mar 30, 2020 at 04:13:15PM -0700, Paul E. McKenney wrote:
> > 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.
> 
> Hmm, I had a look at the implementation and I'm not at all convinced that
> it's correct. First of all, the comment above it states:
> 
>  * NOTE: using list_empty_careful() without synchronization
>  * can only be safe if the only activity that can happen
>  * to the list entry is list_del_init(). Eg. it cannot be used
>  * if another CPU could re-list_add() it.

Huh.  This thing is unchanged since 2.6.12-rc2, back in 2005:

static inline int list_empty_careful(const struct list_head *head)
{
	struct list_head *next = head->next;
	return (next == head) && (next == head->prev);
}

I can imagine compiler value-caching optimizations that would cause
trouble, for example, if a previous obsolete fetch from head->prev was
lying around in a register, causing this function to say "not empty" when
it was in fact empty.  Of course, if obsolete values for both head->next
and head->prev were lying around, pretty much anything could happen.

> but it seems that people disregard this note and instead use it as a
> general-purpose lockless test, taking a lock and rechecking if it returns
> non-empty. It would also mean we'd have to keep the WRITE_ONCE() in
> INIT_LIST_HEAD, which is something that I've been trying to remove.
> 
> In the face of something like a concurrent list_add(); list_add_tail()
> sequence, then the tearing writes to the head->{prev,next} pointers could
> cause list_empty_careful() to indicate that the list is momentarily empty.
> 
> I've started looking at whether we can use a NULL next pointer to indicate
> an empty list, which might allow us to kill the __list_del_clearprev() hack
> at the same time, but I've not found enough time to really get my teeth into
> it yet.

In the delete-only case, I kind of get it, other than the potential for
optimization.  Once the list becomes empty, it will forever remain empty.
And the additional test of head->prev avoids this returning true while the
deletion is half done (again, aside from the potential for optimization).

If insertions are allowed, the thing I haven't quite figured out yet is
what is being gained by the additional check of head->prev.  After all,
if updates are not excluded, the return value can become obsolete
immediately anyhow.  Yes, it could be used as a heuristic, but it could
report empty immediately before a list_add(), so there would need to
either be a careful wakeup protocol or a periodic poll of the list.

Or am I missing a trick here?

							Thanx, Paul