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diff --git a/Documentation/locking/rt-mutex.rst b/Documentation/locking/rt-mutex.rst new file mode 100644 index 0000000000..3b5097a380 --- /dev/null +++ b/Documentation/locking/rt-mutex.rst @@ -0,0 +1,77 @@ +================================== +RT-mutex subsystem with PI support +================================== + +RT-mutexes with priority inheritance are used to support PI-futexes, +which enable pthread_mutex_t priority inheritance attributes +(PTHREAD_PRIO_INHERIT). [See Documentation/locking/pi-futex.rst for more details +about PI-futexes.] + +This technology was developed in the -rt tree and streamlined for +pthread_mutex support. + +Basic principles: +----------------- + +RT-mutexes extend the semantics of simple mutexes by the priority +inheritance protocol. + +A low priority owner of a rt-mutex inherits the priority of a higher +priority waiter until the rt-mutex is released. If the temporarily +boosted owner blocks on a rt-mutex itself it propagates the priority +boosting to the owner of the other rt_mutex it gets blocked on. The +priority boosting is immediately removed once the rt_mutex has been +unlocked. + +This approach allows us to shorten the block of high-prio tasks on +mutexes which protect shared resources. Priority inheritance is not a +magic bullet for poorly designed applications, but it allows +well-designed applications to use userspace locks in critical parts of +an high priority thread, without losing determinism. + +The enqueueing of the waiters into the rtmutex waiter tree is done in +priority order. For same priorities FIFO order is chosen. For each +rtmutex, only the top priority waiter is enqueued into the owner's +priority waiters tree. This tree too queues in priority order. Whenever +the top priority waiter of a task changes (for example it timed out or +got a signal), the priority of the owner task is readjusted. The +priority enqueueing is handled by "pi_waiters". + +RT-mutexes are optimized for fastpath operations and have no internal +locking overhead when locking an uncontended mutex or unlocking a mutex +without waiters. The optimized fastpath operations require cmpxchg +support. [If that is not available then the rt-mutex internal spinlock +is used] + +The state of the rt-mutex is tracked via the owner field of the rt-mutex +structure: + +lock->owner holds the task_struct pointer of the owner. Bit 0 is used to +keep track of the "lock has waiters" state: + + ============ ======= ================================================ + owner bit0 Notes + ============ ======= ================================================ + NULL 0 lock is free (fast acquire possible) + NULL 1 lock is free and has waiters and the top waiter + is going to take the lock [1]_ + taskpointer 0 lock is held (fast release possible) + taskpointer 1 lock is held and has waiters [2]_ + ============ ======= ================================================ + +The fast atomic compare exchange based acquire and release is only +possible when bit 0 of lock->owner is 0. + +.. [1] It also can be a transitional state when grabbing the lock + with ->wait_lock is held. To prevent any fast path cmpxchg to the lock, + we need to set the bit0 before looking at the lock, and the owner may + be NULL in this small time, hence this can be a transitional state. + +.. [2] There is a small time when bit 0 is set but there are no + waiters. This can happen when grabbing the lock in the slow path. + To prevent a cmpxchg of the owner releasing the lock, we need to + set this bit before looking at the lock. + +BTW, there is still technically a "Pending Owner", it's just not called +that anymore. The pending owner happens to be the top_waiter of a lock +that has no owner and has been woken up to grab the lock. |