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STRUCT FUTEX_Q(9) Futex API reference STRUCT FUTEX_Q(9)

NAME

struct_futex_q - The hashed futex queue entry, one per waiting task

SYNOPSIS

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;
};

MEMBERS

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

DESCRIPTION

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.

AUTHOR

Rusty Russell <rusty@rustcorp.com.au>
Author.

COPYRIGHT

January 2017 Kernel Hackers Manual 4.8.