> On Mar 31, 2023, at 9:34 AM, Gregory Nutt <spudan...@gmail.com> wrote: > > On 3/31/2023 8:56 AM, Gregory Nutt wrote: >> >>> Even more. In my previous example if semaphore is posted from the interrupt >>> we do not know which of TaskA or TaskB is no longer a "holder l" of a >>> semaphore. >>> >> You are right. In this usage case, the counting semaphore is not being used >> for locking; it is being used for signalling an event per >> https://cwiki.apache.org/confluence/display/NUTTX/Signaling+Semaphores+and+Priority+Inheritance >> >> In that case, priority inheritance must be turned off. >> > You example is really confusing because you are mixing two different > concepts, just subtly enough to be really confusing. If a counting semaphore > is used for locking a set of multiple resources, the posting the semaphore > does not release the resource. That is not the way that it is done. Here is > a more believable example of how this would work: > > 1. TaskA with priority 10 allocates DMA0 channel by calling a DMA > channel allocation function. If a DMA channel is available, it is > allocated and the allocation function takes the semaphore. TaskA > then starts DMA activity. > 2. TaskA waits on another signaling semaphore for the DMA to complete. > 3. TaskB with priority 20 does the same. > 4. TaskC with priority 30 wants to allocate a DMA channel. It calls > the channel allocation function which waits on the sempahore for a > count to be available. This boost the priority of TaskA and TaskB > to 30. > 5. When the DMA started by TaskA completes, it signals TaskA which > calls the resource deallocation function which increments the > counting semaphore, giving the count to TaskC and storing the base > priorities. > > The confusion arises because you are mixing the signaling logic with the > resource deallocation logic.
Greg, quite right. But Petro himself pointed out: > TaskC with priority 30 wants to allocate a DMA channel, so boosts priority > of TaskA and TaskB to 30 (even if that will not lead to fasted DMA > operation completion). So what is the problem here for which Priority Inheritance is the solution? Technically, there is priority inversion here, but 1) it’s not unbounded, the channel will be free as soon as DMA completes; and 2) boosting task priority will not reduce the latency of freeing a DMA channel. This scenario requires a counting semaphore that’s used as for signaling HW resource availability; and if there are any blocked callers, the highest priority caller is unblocked first.
