LXR linux/kernel/membarrier.c

   1/*
   2 * Copyright (C) 2010, 2015 Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
   3 *
   4 * membarrier system call
   5 *
   6 * This program is free software; you can redistribute it and/or modify
   7 * it under the terms of the GNU General Public License as published by
   8 * the Free Software Foundation; either version 2 of the License, or
   9 * (at your option) any later version.
  10 *
  11 * This program is distributed in the hope that it will be useful,
  12 * but WITHOUT ANY WARRANTY; without even the implied warranty of
  13 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
  14 * GNU General Public License for more details.
  15 */
  16
  17#include <linux/syscalls.h>
  18#include <linux/membarrier.h>
  19
  20/*
  21 * Bitmask made from a "or" of all commands within enum membarrier_cmd,
  22 * except MEMBARRIER_CMD_QUERY.
  23 */
  24#define MEMBARRIER_CMD_BITMASK  (MEMBARRIER_CMD_SHARED)
  25
  26/**
  27 * sys_membarrier - issue memory barriers on a set of threads
  28 * @cmd:   Takes command values defined in enum membarrier_cmd.
  29 * @flags: Currently needs to be 0. For future extensions.
  30 *
  31 * If this system call is not implemented, -ENOSYS is returned. If the
  32 * command specified does not exist, or if the command argument is invalid,
  33 * this system call returns -EINVAL. For a given command, with flags argument
  34 * set to 0, this system call is guaranteed to always return the same value
  35 * until reboot.
  36 *
  37 * All memory accesses performed in program order from each targeted thread
  38 * is guaranteed to be ordered with respect to sys_membarrier(). If we use
  39 * the semantic "barrier()" to represent a compiler barrier forcing memory
  40 * accesses to be performed in program order across the barrier, and
  41 * smp_mb() to represent explicit memory barriers forcing full memory
  42 * ordering across the barrier, we have the following ordering table for
  43 * each pair of barrier(), sys_membarrier() and smp_mb():
  44 *
  45 * The pair ordering is detailed as (O: ordered, X: not ordered):
  46 *
  47 *                        barrier()   smp_mb() sys_membarrier()
  48 *        barrier()          X           X            O
  49 *        smp_mb()           X           O            O
  50 *        sys_membarrier()   O           O            O
  51 */
  52SYSCALL_DEFINE2(membarrier, int, cmd, int, flags)
  53{
  54        if (unlikely(flags))
  55                return -EINVAL;
  56        switch (cmd) {
  57        case MEMBARRIER_CMD_QUERY:
  58                return MEMBARRIER_CMD_BITMASK;
  59        case MEMBARRIER_CMD_SHARED:
  60                if (num_online_cpus() > 1)
  61                        synchronize_sched();
  62                return 0;
  63        default:
  64                return -EINVAL;
  65        }
  66}
  67