/*
   * Copyright (C) 2010, 2015 Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
   *
   * membarrier system call
   *
   * This program is free software; you can redistribute it and/or modify
   * it under the terms of the GNU General Public License as published by
   * the Free Software Foundation; either version 2 of the License, or
   * (at your option) any later version.
   *
   * This program is distributed in the hope that it will be useful,
   * but WITHOUT ANY WARRANTY; without even the implied warranty of
   * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
   * GNU General Public License for more details.
   */
  
  #include <linux/syscalls.h>
  #include <linux/membarrier.h>
  
  /*
   * Bitmask made from a "or" of all commands within enum membarrier_cmd,
   * except MEMBARRIER_CMD_QUERY.
   */
  #define MEMBARRIER_CMD_BITMASK	(MEMBARRIER_CMD_SHARED)
  
  /**
   * sys_membarrier - issue memory barriers on a set of threads
   * @cmd:   Takes command values defined in enum membarrier_cmd.
   * @flags: Currently needs to be 0. For future extensions.
   *
   * If this system call is not implemented, -ENOSYS is returned. If the
   * command specified does not exist, or if the command argument is invalid,
   * this system call returns -EINVAL. For a given command, with flags argument
   * set to 0, this system call is guaranteed to always return the same value
   * until reboot.
   *
   * All memory accesses performed in program order from each targeted thread
   * is guaranteed to be ordered with respect to sys_membarrier(). If we use
   * the semantic "barrier()" to represent a compiler barrier forcing memory
   * accesses to be performed in program order across the barrier, and
   * smp_mb() to represent explicit memory barriers forcing full memory
   * ordering across the barrier, we have the following ordering table for
   * each pair of barrier(), sys_membarrier() and smp_mb():
   *
   * The pair ordering is detailed as (O: ordered, X: not ordered):
   *
   *                        barrier()   smp_mb() sys_membarrier()
   *        barrier()          X           X            O
   *        smp_mb()           X           O            O
   *        sys_membarrier()   O           O            O
   */
  SYSCALL_DEFINE2(membarrier, int, cmd, int, flags)
  {
  	if (unlikely(flags))
  		return -EINVAL;
  	switch (cmd) {
  	case MEMBARRIER_CMD_QUERY:
  		return MEMBARRIER_CMD_BITMASK;
  	case MEMBARRIER_CMD_SHARED:
  		if (num_online_cpus() > 1)
  			synchronize_sched();
  		return 0;
  	default:
  		return -EINVAL;
  	}
  }