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futex_requeue.md

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zx_futex_requeue

SUMMARY

Wake some number of threads waiting on a futex, and move more waiters to another wait queue.

DECLARATION

#include <zircon/syscalls.h>

zx_status_t zx_futex_requeue(const zx_futex_t* value_ptr,
                             uint32_t wake_count,
                             zx_futex_t current_value,
                             const zx_futex_t* requeue_ptr,
                             uint32_t requeue_count,
                             zx_handle_t new_requeue_owner);

DESCRIPTION

Requeuing is a generalization of waking. First, the kernel verifies that the value in current_value matches the value of the futex at value_ptr, and if not reports ZX_ERR_BAD_STATE. After waking wake_count threads, requeue_count threads are moved from the original futex's wait queue to the wait queue corresponding to requeue_ptr, another futex.

This requeueing behavior may be used to avoid thundering herds on wake.

OWNERSHIP

A requeue operation targets two futexes, the wake futex and the requeue futex. The ownership implications for each are discussed separately. Generally, if the call fails for any reason, no changes to ownership for either futex are made.

See Ownership and Priority Inheritance in futex for details.

Effects on the wake futex target

A successful call to zx_futex_requeue() results in the owner of the futex being set to nothing, regardless of the wake count. In order to transfer ownership of a futex, use the zx_futex_requeue_single_owner() variant instead. zx_futex_requeue_single_owner() will attempt to wake exactly one thread from the futex wait queue. If there is at least one thread to wake, the owner of the futex will be set to the thread that was woken. Otherwise, the futex will have no owner.

Effects on the requeue futex target

A successful call to zx_futex_requeue() or zx_futex_requeue_single_owner() results in the owner of the futex being set to the thread referenced by the new_requeue_owner handle, or to nothing if new_requeue_owner is ZX_HANDLE_INVALID.

RIGHTS

None.

RETURN VALUE

zx_futex_requeue() returns ZX_OK on success.

ERRORS

ZX_ERR_INVALID_ARGS One of the following is true:

  • Either value_ptr or requeue_ptr is not a valid userspace pointer
  • Either value_ptr or requeue_ptr is not aligned to a sizeof(zx_futex_t) boundary.
  • value_ptr is the same futex as requeue_ptr
  • new_requeue_owner is currently a member of the waiters for either value_ptr or requeue_ptr
  • new_requeue_owner has not been started yet.

ZX_ERR_BAD_HANDLE new_requeue_owner is not ZX_HANDLE_INVALID, and not a valid handle AND current_value matches the value at value_ptr

ZX_ERR_WRONG_TYPE new_requeue_owner is a valid handle, but is not a handle to a thread.

ZX_ERR_BAD_STATE current_value does not match the value at value_ptr.

SEE ALSO