/*
* linux/ipc/sem.c
* Copyright (C) 1992 Krishna Balasubramanian
* Copyright (C) 1995 Eric Schenk, Bruno Haible
*
* /proc/sysvipc/sem support (c) 1999 Dragos Acostachioaie <dragos@iname.com>
*
* SMP-threaded, sysctl's added
* (c) 1999 Manfred Spraul <manfred@colorfullife.com>
* Enforced range limit on SEM_UNDO
* (c) 2001 Red Hat Inc
* Lockless wakeup
* (c) 2003 Manfred Spraul <manfred@colorfullife.com>
* Further wakeup optimizations, documentation
* (c) 2010 Manfred Spraul <manfred@colorfullife.com>
*
* support for audit of ipc object properties and permission changes
* Dustin Kirkland <dustin.kirkland@us.ibm.com>
*
* namespaces support
* OpenVZ, SWsoft Inc.
* Pavel Emelianov <xemul@openvz.org>
*
* Implementation notes: (May 2010)
* This file implements System V semaphores.
*
* User space visible behavior:
* - FIFO ordering for semop() operations (just FIFO, not starvation
* protection)
* - multiple semaphore operations that alter the same semaphore in
* one semop() are handled.
* - sem_ctime (time of last semctl()) is updated in the IPC_SET, SETVAL and
* SETALL calls.
* - two Linux specific semctl() commands: SEM_STAT, SEM_INFO.
* - undo adjustments at process exit are limited to 0..SEMVMX.
* - namespace are supported.
* - SEMMSL, SEMMNS, SEMOPM and SEMMNI can be configured at runtine by writing
* to /proc/sys/kernel/sem.
* - statistics about the usage are reported in /proc/sysvipc/sem.
*
* Internals:
* - scalability:
* - all global variables are read-mostly.
* - semop() calls and semctl(RMID) are synchronized by RCU.
* - most operations do write operations (actually: spin_lock calls) to
* the per-semaphore array structure.
* Thus: Perfect SMP scaling between independent semaphore arrays.
* If multiple semaphores in one array are used, then cache line
* trashing on the semaphore array spinlock will limit the scaling.
* - semncnt and semzcnt are calculated on demand in count_semncnt() and
* count_semzcnt()
* - the task that performs a successful semop() scans the list of all
* sleeping tasks and completes any pending operatio
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