On Tue, Mar 24, 2020 at 05:20:45PM +0100, Jann Horn wrote:
> On Tue, Mar 24, 2020 at 4:37 PM Will Deacon <will@xxxxxxxxxx> 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.
> [...]
> > static inline int list_empty(const struct list_head *head)
> > {
> > - return READ_ONCE(head->next) == head;
> > + return data_race(READ_ONCE(head->next) == head);
> > }
> [...]
> > static inline int hlist_unhashed(const struct hlist_node *h)
> > {
> > - return !READ_ONCE(h->pprev);
> > + return data_race(!READ_ONCE(h->pprev));
> > }
>
> This is probably valid in practice for hlist_unhashed(), which
> compares with NULL, as long as the most significant byte of all kernel
> pointers is non-zero; but I think list_empty() could realistically
> return false positives in the presence of a concurrent tearing store?
> This could break the following code pattern:
>
> /* optimistic lockless check */
> if (!list_empty(&some_list)) {
> /* slowpath */
> mutex_lock(&some_mutex);
> list_for_each(tmp, &some_list) {
> ...
> }
> mutex_unlock(&some_mutex);
> }
>
> (I'm not sure whether patterns like this appear commonly though.)
I would hope not as the list could go "empty" before the lock is
grabbed. That pattern would be wrong.
thanks,
greg k-h
