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-RT-mutex subsystem with PI support
-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:
-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
-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
-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.