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

Archive Download this file