/*
* linux/ipc/sem.c
* Copyright (C) 1992 Krishna Balasubramanian
* Copyright (C) 1995 Eric Schenk, Bruno Haible
*
* IMPLEMENTATION NOTES ON CODE REWRITE (Eric Schenk, January 1995):
* This code underwent a massive rewrite in order to solve some problems
* with the original code. In particular the original code failed to
* wake up processes that were waiting for semval to go to 0 if the
* value went to 0 and was then incremented rapidly enough. In solving
* this problem I have also modified the implementation so that it
* processes pending operations in a FIFO manner, thus give a guarantee
* that processes waiting for a lock on the semaphore won't starve
* unless another locking process fails to unlock.
* In addition the following two changes in behavior have been introduced:
* - The original implementation of semop returned the value
* last semaphore element examined on success. This does not
* match the manual page specifications, and effectively
* allows the user to read the semaphore even if they do not
* have read permissions. The implementation now returns 0
* on success as stated in the manual page.
* - There is some confusion over whether the set of undo adjustments
* to be performed at exit should be done in an atomic manner.
* That is, if we are attempting to decrement the semval should we queue
* up and wait until we can do so legally?
* The original implementation attempted to do this.
* The current implementation does not do so. This is because I don't
* think it is the right thing (TM) to do, and because I couldn't
* see a clean way to get the old behavior with the new design.
* The POSIX standard and SVID should be consulted to determine
* what behavior is mandated.
*
* Further notes on refinement (Christoph Rohland, December 1998):
* - The POSIX standard says, that the undo adjustments simply should
* redo. So the current implementation is o.K.
* - The previous code had two flaws:
* 1) It actively gave the semaphore to the next waiting process
* sleeping on the semaphore. Since this process did not have the
* cpu this led to many unnecessary context switches and bad
* performance. Now we only check which process should be able to
* get the semaphore and if this process wants to reduce some
* semaphore value we simply wake it up without doing the
* operation. So it has to try to
