Futex Requeue PI

Requeueing of tasks from a non-PI futex to a PI futex requires special handling in order to ensure the underlying rt_mutex is never left without an owner if it has waiters; doing so would break the PI boosting logic [see RT-mutex implementation design] For the purposes of brevity, this action will be referred to as “requeue_pi” throughout this document. Priority inheritance is abbreviated throughout as “PI”.

Motivation

Without requeue_pi, the glibc implementation of pthread_cond_broadcast() must resort to waking all the tasks waiting on a pthread_condvar and letting them try to sort out which task gets to run first in classic thundering-herd formation. An ideal implementation would wake the highest-priority waiter, and leave the rest to the natural wakeup inherent in unlocking the mutex associated with the condvar.

Consider the simplified glibc calls:

/* caller must lock mutex */
pthread_cond_wait(cond, mutex)
{
        lock(cond->__data.__lock);
        unlock(mutex);
        do {
        unlock(cond->__data.__lock);
        futex_wait(cond->__data.__futex);
        lock(cond->__data.__lock);
        } while(...)
        unlock(cond->__data.__lock);
        lock(mutex);
}

pthread_cond_broadcast(cond)
{
        lock(cond->__data.__lock);
        unlock(cond->__data.__lock);
        futex_requeue(cond->data.__futex, cond->mutex);
}

Once pthread_cond_broadcast() requeues the tasks, the cond->mutex has waiters. Note that pthread_cond_wait() attempts to lock the mutex only after it has returned to user space. This will leave the underlying rt_mutex with waiters, and no owner, breaking the previously mentioned PI-boosting algorithms.

In order to support PI-aware pthread_condvar’s, the kernel needs to be able to requeue tasks to PI futexes. This support implies that upon a successful futex_wait system call, the caller would return to user space already holding the PI futex. The glibc implementation would be modified as follows:

/* caller must lock mutex */
pthread_cond_wait_pi(cond, mutex)
{
        lock(cond->__data.__lock);
        unlock(mutex);
        do {
        unlock(cond->__data.__lock);
        futex_wait_requeue_pi(cond->__data.__futex);
        lock(cond->__data.__lock);
        } while(...)
        unlock(cond->__data.__lock);
        /* the kernel acquired the mutex for us */
}

pthread_cond_broadcast_pi(cond)
{
        lock(cond->__data.__lock);
        unlock(cond->__data.__lock);
        futex_requeue_pi(cond->data.__futex, cond->mutex);
}

The actual glibc implementation will likely test for PI and make the necessary changes inside the existing calls rather than creating new calls for the PI cases. Similar changes are needed for pthread_cond_timedwait() and pthread_cond_signal().

Implementation

In order to ensure the rt_mutex has an owner if it has waiters, it is necessary for both the requeue code, as well as the waiting code, to be able to acquire the rt_mutex before returning to user space. The requeue code cannot simply wake the waiter and leave it to acquire the rt_mutex as it would open a race window between the requeue call returning to user space and the waiter waking and starting to run. This is especially true in the uncontended case.

The solution involves two new rt_mutex helper routines, rt_mutex_start_proxy_lock() and rt_mutex_finish_proxy_lock(), which allow the requeue code to acquire an uncontended rt_mutex on behalf of the waiter and to enqueue the waiter on a contended rt_mutex. Two new system calls provide the kernel<->user interface to requeue_pi: FUTEX_WAIT_REQUEUE_PI and FUTEX_CMP_REQUEUE_PI.

FUTEX_WAIT_REQUEUE_PI is called by the waiter (pthread_cond_wait() and pthread_cond_timedwait()) to block on the initial futex and wait to be requeued to a PI-aware futex. The implementation is the result of a high-speed collision between futex_wait() and futex_lock_pi(), with some extra logic to check for the additional wake-up scenarios.

FUTEX_CMP_REQUEUE_PI is called by the waker (pthread_cond_broadcast() and pthread_cond_signal()) to requeue and possibly wake the waiting tasks. Internally, this system call is still handled by futex_requeue (by passing requeue_pi=1). Before requeueing, futex_requeue() attempts to acquire the requeue target PI futex on behalf of the top waiter. If it can, this waiter is woken. futex_requeue() then proceeds to requeue the remaining nr_wake+nr_requeue tasks to the PI futex, calling rt_mutex_start_proxy_lock() prior to each requeue to prepare the task as a waiter on the underlying rt_mutex. It is possible that the lock can be acquired at this stage as well, if so, the next waiter is woken to finish the acquisition of the lock.

FUTEX_CMP_REQUEUE_PI accepts nr_wake and nr_requeue as arguments, but their sum is all that really matters. futex_requeue() will wake or requeue up to nr_wake + nr_requeue tasks. It will wake only as many tasks as it can acquire the lock for, which in the majority of cases should be 0 as good programming practice dictates that the caller of either pthread_cond_broadcast() or pthread_cond_signal() acquire the mutex prior to making the call. FUTEX_CMP_REQUEUE_PI requires that nr_wake=1. nr_requeue should be INT_MAX for broadcast and 0 for signal.