struct futex_q — The hashed futex queue entry, one per waiting task
struct futex_q {
struct plist_node list;
struct task_struct * task;
spinlock_t * lock_ptr;
union futex_key key;
struct futex_pi_state * pi_state;
struct rt_mutex_waiter * rt_waiter;
union futex_key * requeue_pi_key;
u32 bitset;
}; priority-sorted list of tasks waiting on this futex
the task waiting on the futex
the hash bucket lock
the key the futex is hashed on
optional priority inheritance state
rt_waiter storage for use with requeue_pi
the requeue_pi target futex key
bitset for the optional bitmasked wakeup
We use this hashed waitqueue, instead of a normal wait_queue_t, so we can wake only the relevant ones (hashed queues may be shared).
A futex_q has a woken state, just like tasks have TASK_RUNNING. It is considered woken when plist_node_empty(q->list) || q->lock_ptr == 0. The order of wakeup is always to make the first condition true, then the second.
PI futexes are typically woken before they are removed from the hash list via
the rt_mutex code. See unqueue_me_pi.