Linux Driver Verification » Linux Kernel Safety RuleDB

From documentation:
Note that the value returned by rcu_dereference() is valid
only within the enclosing RCU read-side critical section.
For example, the following is not legal:

rcu_read_lock();
                p = rcu_dereference(head.next);
                rcu_read_unlock();
                x = p->address;
                rcu_read_lock();
                y = p->data;
                rcu_read_unlock();

Holding a reference from one RCU read-side critical section
to another is just as illegal as holding a reference from
one lock-based critical section to another! Similarly,
using a reference outside of the critical section in which
it was acquired is just as illegal as doing so with normal
locking.