* cmpxchg is protected from ABA races by holding a RCU read lock between
* s->head read and cmpxchg modifying s->head and requiring that dequeuers wait
* for a grace period before freeing the returned node.
* cmpxchg is protected from ABA races by holding a RCU read lock between
* s->head read and cmpxchg modifying s->head and requiring that dequeuers wait
* for a grace period before freeing the returned node.