Based on kernel version 3.9. Page generated on 2013-05-02 23:13 EST.
1 RT-mutex subsystem with PI support 2 ---------------------------------- 3 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.] 8 9 This technology was developed in the -rt tree and streamlined for 10 pthread_mutex support. 11 12 Basic principles: 13 ----------------- 14 15 RT-mutexes extend the semantics of simple mutexes by the priority 16 inheritance protocol. 17 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. 24 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. 30 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.] 39 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] 45 46 The state of the rt-mutex is tracked via the owner field of the rt-mutex 47 structure: 48 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. 52 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 62 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. 69 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]. 75 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.