On Tue, Mar 24, 2020 at 05:38:30PM +0100, Jann Horn wrote:
> On Tue, Mar 24, 2020 at 5:26 PM Greg KH <greg@xxxxxxxxx> wrote:
> > 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.
>
> If the list becomes empty in between, the loop just iterates over
> nothing, and the effect is no different from what you'd get if you had
> bailed out before. But sure, you have to be aware that that can
> happen.
Doh, yeah, so it is safe, crazy, but safe :)
