struct_futex_q (9)

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;
};

list

priority-sorted list of tasks waiting on this futex

task

the task waiting on the futex

lock_ptr

the hash bucket lock

key

the key the futex is hashed on

pi_state

optional priority inheritance state

rt_waiter

rt_waiter storage for use with requeue_pi

requeue_pi_key

the requeue_pi target futex key

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.

Rusty Russell <[email protected]>

Author.