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RT-mutex subsystem with PI support

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RT-mutexes with priority inheritance are used to support PI-futexes,

which enable pthread_mutex_t priority inheritance attributes

(PTHREAD_PRIO_INHERIT). [See Documentation/pi-futex.txt for more details

about PI-futexes.]

This technology was developed in the -rt tree and streamlined for

pthread_mutex support.

Basic principles:

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RT-mutexes extend the semantics of simple mutexes by the priority

inheritance protocol.

A low priority owner of a rt-mutex inherits the priority of a higher

priority waiter until the rt-mutex is released. If the temporarily

boosted owner blocks on a rt-mutex itself it propagates the priority

boosting to the owner of the other rt_mutex it gets blocked on. The

priority boosting is immediately removed once the rt_mutex has been

unlocked.

This approach allows us to shorten the block of high-prio tasks on

mutexes which protect shared resources. Priority inheritance is not a

magic bullet for poorly designed applications, but it allows

well-designed applications to use userspace locks in critical parts of

an high priority thread, without losing determinism.

The enqueueing of the waiters into the rtmutex waiter list is done in

priority order. For same priorities FIFO order is chosen. For each

rtmutex, only the top priority waiter is enqueued into the owner's

priority waiters list. This list too queues in priority order. Whenever

the top priority waiter of a task changes (for example it timed out or

got a signal), the priority of the owner task is readjusted. [The

priority enqueueing is handled by "plists", see include/linux/plist.h

for more details.]

RT-mutexes are optimized for fastpath operations and have no internal

locking overhead when locking an uncontended mutex or unlocking a mutex

without waiters. The optimized fastpath operations require cmpxchg

support. [If that is not available then the rt-mutex internal spinlock

is used]

The state of the rt-mutex is tracked via the owner field of the rt-mutex

structure:

rt_mutex->owner holds the task_struct pointer of the owner. Bit 0 and 1

are used to keep track of the "owner is pending" and "rtmutex has

waiters" state.

 owner		bit1	bit0

 NULL		0	0	mutex is free (fast acquire possible)

 NULL		0	1	invalid state

 NULL		1	0	Transitional state*

 NULL		1	1	invalid state

 taskpointer	0	0	mutex is held (fast release possible)

 taskpointer	0	1	task is pending owner

 taskpointer	1	0	mutex is held and has waiters

 taskpointer	1	1	task is pending owner and mutex has waiters

Pending-ownership handling is a performance optimization:

pending-ownership is assigned to the first (highest priority) waiter of

the mutex, when the mutex is released. The thread is woken up and once

it starts executing it can acquire the mutex. Until the mutex is taken

by it (bit 0 is cleared) a competing higher priority thread can "steal"

the mutex which puts the woken up thread back on the waiters list.

The pending-ownership optimization is especially important for the

uninterrupted workflow of high-prio tasks which repeatedly

takes/releases locks that have lower-prio waiters. Without this

optimization the higher-prio thread would ping-pong to the lower-prio

task [because at unlock time we always assign a new owner].

(*) The "mutex has waiters" bit gets set to take the lock. If the lock

doesn't already have an owner, this bit is quickly cleared if there are

no waiters.  So this is a transitional state to synchronize with looking

at the owner field of the mutex and the mutex owner releasing the lock.