On Fri, Jan 13, 2023 at 12:11:25PM +0100, Paul Heidekrüger wrote:
> Hi all,
>
> FWIW, here are two more broken address dependencies, both very similar to the
> one discussed in this thread. From what I can tell, both are protected by a
> lock, so, again, nothing to worry about right now? Would you agree?
FWIW, my opinion is that in both cases the broken dependency can be
removed entirely.
> Comments marked with "AD:" were added by me for readability.
>
> 1. drivers/hwtracing/stm/core.c::1050 - 1085
>
> /**
> * __stm_source_link_drop() - detach stm_source from an stm device
> * @src: stm_source device
> * @stm: stm device
> *
> * If @stm is @src::link, disconnect them from one another and put the
> * reference on the @stm device.
> *
> * Caller must hold stm::link_mutex.
> */
> static int __stm_source_link_drop(struct stm_source_device *src,
> struct stm_device *stm)
> {
> struct stm_device *link;
> int ret = 0;
>
> lockdep_assert_held(&stm->link_mutex);
>
> /* for stm::link_list modification, we hold both mutex and spinlock */
> spin_lock(&stm->link_lock);
> spin_lock(&src->link_lock);
>
> /* AD: Beginning of the address dependency. */
> link = srcu_dereference_check(src->link, &stm_source_srcu, 1);
>
> /*
> * The linked device may have changed since we last looked, because
> * we weren't holding the src::link_lock back then; if this is the
> * case, tell the caller to retry.
> */
> if (link != stm) {
> ret = -EAGAIN;
> goto unlock;
> }
>
> /* AD: Compiler deduces that "link" and "stm" are exchangeable at this point. */
> stm_output_free(link, &src->output); list_del_init(&src->link_entry);
>
> /* AD: Leads to WRITE_ONCE() to (&link->dev)->power.last_busy. */
> pm_runtime_mark_last_busy(&link->dev);
In both of these statements, link can safely be replaced by stm.
(There's also a control dependency which the LKMM isn't aware of. This
makes it all the more safe.)
> 2. kernel/locking/lockdep.c::6319 - 6348
>
> /*
> * Unregister a dynamically allocated key.
> *
> * Unlike lockdep_register_key(), a search is always done to find a matching
> * key irrespective of debug_locks to avoid potential invalid access to freed
> * memory in lock_class entry.
> */
> void lockdep_unregister_key(struct lock_class_key *key)
> {
> struct hlist_head *hash_head = keyhashentry(key);
> struct lock_class_key *k;
> struct pending_free *pf;
> unsigned long flags;
> bool found = false;
>
> might_sleep();
>
> if (WARN_ON_ONCE(static_obj(key)))
> return;
>
> raw_local_irq_save(flags);
> lockdep_lock();
>
> /* AD: Address dependency begins here with an rcu_dereference_raw() into k. */
> hlist_for_each_entry_rcu(k, hash_head, hash_entry) {
> /* AD: Compiler deduces that k and key are exchangable iff the if condition evaluates to true.
> if (k == key) {
> /* AD: Leads to WRITE_ONCE() to (&k->hash_entry)->pprev. */
> hlist_del_rcu(&k->hash_entry);
And here k could safely be replaced with key. (And again there is a
control dependency, but this is one that the LKMM would detect.)
Alan Stern
> found = true;
> break;
> }
> }
>
> Many thanks,
> Paul
