78 lines
3.5 KiB
ReStructuredText
78 lines
3.5 KiB
ReStructuredText
==================================
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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 tree 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 tree. This tree 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 "pi_waiters".
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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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lock->owner holds the task_struct pointer of the owner. Bit 0 is used to
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keep track of the "lock has waiters" state:
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============ ======= ================================================
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owner bit0 Notes
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============ ======= ================================================
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NULL 0 lock is free (fast acquire possible)
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NULL 1 lock is free and has waiters and the top waiter
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is going to take the lock [1]_
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taskpointer 0 lock is held (fast release possible)
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taskpointer 1 lock is held and has waiters [2]_
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============ ======= ================================================
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The fast atomic compare exchange based acquire and release is only
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possible when bit 0 of lock->owner is 0.
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.. [1] It also can be a transitional state when grabbing the lock
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with ->wait_lock is held. To prevent any fast path cmpxchg to the lock,
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we need to set the bit0 before looking at the lock, and the owner may
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be NULL in this small time, hence this can be a transitional state.
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.. [2] There is a small time when bit 0 is set but there are no
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waiters. This can happen when grabbing the lock in the slow path.
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To prevent a cmpxchg of the owner releasing the lock, we need to
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set this bit before looking at the lock.
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BTW, there is still technically a "Pending Owner", it's just not called
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that anymore. The pending owner happens to be the top_waiter of a lock
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that has no owner and has been woken up to grab the lock.
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