80 lines
3.5 KiB
Text
80 lines
3.5 KiB
Text
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RT-mutex subsystem with PI support
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----------------------------------
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RT-mutexes with priority inheritance are used to support PI-futexes,
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which enable pthread_mutex_t priority inheritance attributes
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(PTHREAD_PRIO_INHERIT). [See Documentation/pi-futex.txt for more details
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about PI-futexes.]
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This technology was developed in the -rt tree and streamlined for
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pthread_mutex support.
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Basic principles:
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-----------------
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RT-mutexes extend the semantics of simple mutexes by the priority
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inheritance protocol.
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A low priority owner of a rt-mutex inherits the priority of a higher
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priority waiter until the rt-mutex is released. If the temporarily
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boosted owner blocks on a rt-mutex itself it propagates the priority
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boosting to the owner of the other rt_mutex it gets blocked on. The
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priority boosting is immediately removed once the rt_mutex has been
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unlocked.
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This approach allows us to shorten the block of high-prio tasks on
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mutexes which protect shared resources. Priority inheritance is not a
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magic bullet for poorly designed applications, but it allows
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well-designed applications to use userspace locks in critical parts of
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an high priority thread, without losing determinism.
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The enqueueing of the waiters into the rtmutex waiter list is done in
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priority order. For same priorities FIFO order is chosen. For each
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rtmutex, only the top priority waiter is enqueued into the owner's
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priority waiters list. This list too queues in priority order. Whenever
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the top priority waiter of a task changes (for example it timed out or
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got a signal), the priority of the owner task is readjusted. [The
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priority enqueueing is handled by "plists", see include/linux/plist.h
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for more details.]
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RT-mutexes are optimized for fastpath operations and have no internal
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locking overhead when locking an uncontended mutex or unlocking a mutex
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without waiters. The optimized fastpath operations require cmpxchg
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support. [If that is not available then the rt-mutex internal spinlock
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is used]
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The state of the rt-mutex is tracked via the owner field of the rt-mutex
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structure:
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rt_mutex->owner holds the task_struct pointer of the owner. Bit 0 and 1
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are used to keep track of the "owner is pending" and "rtmutex has
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waiters" state.
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owner bit1 bit0
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NULL 0 0 mutex is free (fast acquire possible)
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NULL 0 1 invalid state
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NULL 1 0 Transitional state*
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NULL 1 1 invalid state
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taskpointer 0 0 mutex is held (fast release possible)
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taskpointer 0 1 task is pending owner
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taskpointer 1 0 mutex is held and has waiters
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taskpointer 1 1 task is pending owner and mutex has waiters
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Pending-ownership handling is a performance optimization:
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pending-ownership is assigned to the first (highest priority) waiter of
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the mutex, when the mutex is released. The thread is woken up and once
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it starts executing it can acquire the mutex. Until the mutex is taken
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by it (bit 0 is cleared) a competing higher priority thread can "steal"
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the mutex which puts the woken up thread back on the waiters list.
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The pending-ownership optimization is especially important for the
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uninterrupted workflow of high-prio tasks which repeatedly
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takes/releases locks that have lower-prio waiters. Without this
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optimization the higher-prio thread would ping-pong to the lower-prio
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task [because at unlock time we always assign a new owner].
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(*) The "mutex has waiters" bit gets set to take the lock. If the lock
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doesn't already have an owner, this bit is quickly cleared if there are
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no waiters. So this is a transitional state to synchronize with looking
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at the owner field of the mutex and the mutex owner releasing the lock.
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