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;


struct plist_node list

priority-sorted list of tasks waiting on this futex

struct task_struct * task

the task waiting on the futex

spinlock_t * lock_ptr

the hash bucket lock

union futex_key key

the key the futex is hashed on

struct futex_pi_state * pi_state

optional priority inheritance state

struct rt_mutex_waiter * rt_waiter

rt_waiter storage for use with requeue_pi

union futex_key * requeue_pi_key

the requeue_pi target futex key

u32 bitset

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.