futex_requeue.md

mx_futex_requeue

NAME

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

SYNOPSIS

#include <magenta/syscalls.h>

mx_status_t mx_futex_requeue(mx_futex_t* value_ptr, uint32_t wake_count,
                             int current_value, mx_futex_t* requeue_ptr,
                             uint32_t requeue_count);

DESCRIPTION

Requeuing is a generalization of waking. First, the kernel verifies that the value in wake_count matches the value of the futex at value_ptr, and if not reports MX_ERR_ALREADY_BOUND. 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.

RETURN VALUE

futex_requeue() returns MX_OK on success.

ERRORS

MX_ERR_INVALID_ARGS value_ptr isn't a valid userspace pointer, or value_ptr is the same futex as requeue_ptr, or value_ptr or requeue_ptr is not aligned, or requeue_ptr is NULL but requeue_count is positive.

MX_ERR_BAD_STATE current_value does not match the value at value_ptr.

SEE ALSO

futex_wait, futex_wake.