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387b14684f
Convert the locking documents to ReST and add them to the kernel development book where it belongs. Most of the stuff here is just to make Sphinx to properly parse the text file, as they're already in good shape, not requiring massive changes in order to be parsed. The conversion is actually: - add blank lines and identation in order to identify paragraphs; - fix tables markups; - add some lists markups; - mark literal blocks; - adjust title markups. At its new index.rst, let's add a :orphan: while this is not linked to the main index.rst file, in order to avoid build warnings. Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org> Acked-by: Federico Vaga <federico.vaga@vaga.pv.it>
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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