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