futex: add requeue-pi documentation

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-desgin.txt] 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 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_REQUEUE_CMP_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_REQUEUE_CMP_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_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_REQUEUE_PI requires that

nr_wake=1.  nr_requeue should be INT_MAX for broadcast and 0 for

signal.