This caused a problem when the thread calling sem_wait() was very low priority. When it received the count, there may be higher priority threads "hogging" the CPU that prevent the lower priority task from running and, as a result, the sem_addholder() may be delayed indefinitely.
The fix was to have sem_post() call sem_addholder() just before restarting the thread waiting for the semaphore count.
This problem was noted by Benix Vincent who also suggested the solution.
Gregory Nutt