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Documentation / rt-mutex.txt

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Based on kernel version 3.17.3. Page generated on 2014-11-14 22:19 EST.

1	RT-mutex subsystem with PI support
2	----------------------------------
4	RT-mutexes with priority inheritance are used to support PI-futexes,
5	which enable pthread_mutex_t priority inheritance attributes
6	(PTHREAD_PRIO_INHERIT). [See Documentation/pi-futex.txt for more details
7	about PI-futexes.]
9	This technology was developed in the -rt tree and streamlined for
10	pthread_mutex support.
12	Basic principles:
13	-----------------
15	RT-mutexes extend the semantics of simple mutexes by the priority
16	inheritance protocol.
18	A low priority owner of a rt-mutex inherits the priority of a higher
19	priority waiter until the rt-mutex is released. If the temporarily
20	boosted owner blocks on a rt-mutex itself it propagates the priority
21	boosting to the owner of the other rt_mutex it gets blocked on. The
22	priority boosting is immediately removed once the rt_mutex has been
23	unlocked.
25	This approach allows us to shorten the block of high-prio tasks on
26	mutexes which protect shared resources. Priority inheritance is not a
27	magic bullet for poorly designed applications, but it allows
28	well-designed applications to use userspace locks in critical parts of
29	an high priority thread, without losing determinism.
31	The enqueueing of the waiters into the rtmutex waiter list is done in
32	priority order. For same priorities FIFO order is chosen. For each
33	rtmutex, only the top priority waiter is enqueued into the owner's
34	priority waiters list. This list too queues in priority order. Whenever
35	the top priority waiter of a task changes (for example it timed out or
36	got a signal), the priority of the owner task is readjusted. [The
37	priority enqueueing is handled by "plists", see include/linux/plist.h
38	for more details.]
40	RT-mutexes are optimized for fastpath operations and have no internal
41	locking overhead when locking an uncontended mutex or unlocking a mutex
42	without waiters. The optimized fastpath operations require cmpxchg
43	support. [If that is not available then the rt-mutex internal spinlock
44	is used]
46	The state of the rt-mutex is tracked via the owner field of the rt-mutex
47	structure:
49	rt_mutex->owner holds the task_struct pointer of the owner. Bit 0 and 1
50	are used to keep track of the "owner is pending" and "rtmutex has
51	waiters" state.
53	 owner		bit1	bit0
54	 NULL		0	0	mutex is free (fast acquire possible)
55	 NULL		0	1	invalid state
56	 NULL		1	0	Transitional state*
57	 NULL		1	1	invalid state
58	 taskpointer	0	0	mutex is held (fast release possible)
59	 taskpointer	0	1	task is pending owner
60	 taskpointer	1	0	mutex is held and has waiters
61	 taskpointer	1	1	task is pending owner and mutex has waiters
63	Pending-ownership handling is a performance optimization:
64	pending-ownership is assigned to the first (highest priority) waiter of
65	the mutex, when the mutex is released. The thread is woken up and once
66	it starts executing it can acquire the mutex. Until the mutex is taken
67	by it (bit 0 is cleared) a competing higher priority thread can "steal"
68	the mutex which puts the woken up thread back on the waiters list.
70	The pending-ownership optimization is especially important for the
71	uninterrupted workflow of high-prio tasks which repeatedly
72	takes/releases locks that have lower-prio waiters. Without this
73	optimization the higher-prio thread would ping-pong to the lower-prio
74	task [because at unlock time we always assign a new owner].
76	(*) The "mutex has waiters" bit gets set to take the lock. If the lock
77	doesn't already have an owner, this bit is quickly cleared if there are
78	no waiters.  So this is a transitional state to synchronize with looking
79	at the owner field of the mutex and the mutex owner releasing the lock.
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