[GitHub] [incubator-nuttx] pkarashchenko commented on pull request #5070: libc/semaphore:sem_init change defult protocol

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pkarashchenko commented on pull request #5070:
URL: https://github.com/apache/incubator-nuttx/pull/5070#issuecomment-1006118747


   > > Another real case: #5170, both case will generate hard fault.
   > 
   > I run ostest with https://github.com/apache/incubator-nuttx/pull/5171, no such assertions were found.
   > 
   > I think priority inheritance should only be valid for mutex, so there is only one holder per semphore at most.
   > 
   > The current implementation still cannot avoid priority flipping completely.
   > For example, there are 3 tasks t2 > t1 > t0, t0 held sem1, t1 held sem2 and wait sem1, then t2 wait sem2. In this case, only t1's priority is raised.
   > If multiple holders are supported, this problem is very difficult to solve, which is the O(n^2) level.
   > 
   > If only one holder is supported, many things can be simplified.
   > 
   > 
   
   The semaphores are the building blocks for the pthread mutexes and here is some description for pthread mutexes:
   > When a thread makes a call to pthread_mutex_lock(), the mutex was initialized with the protocol attribute having the value PTHREAD_PRIO_INHERIT, when the calling thread is blocked because the mutex is owned by another thread, that owner thread shall inherit the priority level of the calling thread as long as it continues to own the mutex. The implementation shall update its execution priority to the maximum of its assigned priority and all its inherited priorities. Furthermore, if this owner thread itself becomes blocked on another mutex with the protocol attribute having the value PTHREAD_PRIO_INHERIT, the same priority inheritance effect shall be propagated to this other owner thread, in a recursive manner.


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