Fast mutexes in Visual Studio .NET

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17.3.1.3 Fast mutexes
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One of the main purposes of the nanokernel is to localize all the nonpreemptible sections of code in the system. For this to be practical, the nanokernel has to provide a very fast mutual exclusion primitive that still allows preemption of the critical section by unrelated threads. To be useful for real time applications, this mechanism also needs to provide a means of limiting priority inversion times. Because EKA2 must support an open system, the priority ceiling protocol cannot be used, and so some form of priority inheritance is needed. We designed fast mutexes to provide the solution to this problem. A fast mutex consists of two elds a pointer to the nanokernel thread, which currently holds the mutex (or null if the mutex is free), and a ag (the waiting ag), which indicates that some action has been deferred because the mutex was locked. To lock a fast mutex, a thread rst inspects the holding thread pointer. If this is null, the thread simply claims the mutex by writing its own address into the holding thread pointer and then proceeds. This makes fast mutexes very fast in the case where there is no contention. If the mutex is held by another thread, the nanokernel sets the waiting ag and then performs an immediate context switch directly to the holding thread (which must be of lower priority than the blocked thread). The blocked thread remains on the ready list this is
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not the case when blocking on other wait objects. So, this is how we get priority inheritance because the blocked thread is still on the ready list, a reschedule will only be triggered if a thread with a higher priority than the blocked thread is made ready. Otherwise the holding thread will continue running until it releases the mutex. At that point it checks the waiting ag and, if it is set, triggers a reschedule. This will then switch to the blocked thread. There are two restrictions on the use of fast mutexes that we impose to guarantee predictable real time behavior: 1. Fast mutexes cannot be nested 2. A thread may not block on another wait object while holding a fast mutex. We impose these restrictions so that the scheduler can always nd the next thread to run in a bounded time, even when the preferred thread (that is, the highest priority ready thread) is currently blocked on a fast mutex. We have placed assertions in the debug build of the kernel to ag violations of these rules to those porting Symbian OS and writing device drivers.
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17.3.1.4 Context switching
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Under the moving memory model (used on ARM architecture 4 and 5 processors), a context switch between threads belonging to different user-side processes can be a time consuming procedure. It may involve, in the worst case: Moving all the chunks attached to the current data section process to the home section Protecting all chunks attached to the last user process to run Moving all chunks attached to the new process from the home section to the data section Flushing the processor data cache. On processors with large data caches and slow memory interfaces, this could take more than 500 s this is a measured value from one such system. If all this work were done directly by the scheduler, with preemption disabled, this would add 500 s or more to the worst case thread latency. We didn t consider this to be acceptable performance not all context switches require the full list of actions listed. Switches to kernel threads and threads in certain user processes can occur much faster and so should have lower guaranteed latency. To achieve this goal, we perform the modi cation of page directory entries and the ushing of the
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