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 #include <linux/tick.h>
20
21 /*
22 * Bitmask made from a "or" of all commands within enum membarrier_cmd,
23 * except MEMBARRIER_CMD_QUERY.
24 */
25 #define MEMBARRIER_CMD_BITMASK (MEMBARRIER_CMD_SHARED)
26
27 /**
28 * sys_membarrier - issue memory barriers on a set of threads
29 * @cmd: Takes command values defined in enum membarrier_cmd.
30 * @flags: Currently needs to be 0. For future extensions.
31 *
32 * If this system call is not implemented, -ENOSYS is returned. If the
33 * command specified does not exist, or if the command argument is invalid,
34 * this system call returns -EINVAL. For a given command, with flags argument
35 * set to 0, this system call is guaranteed to always return the same value
36 * until reboot.
37 *
38 * All memory accesses performed in program order from each targeted thread
39 * is guaranteed to be ordered with respect to sys_membarrier(). If we use
40 * the semantic "barrier()" to represent a compiler barrier forcing memory
41 * accesses to be performed in program order across the barrier, and
42 * smp_mb() to represent explicit memory barriers forcing full memory
43 * ordering across the barrier, we have the following ordering table for
44 * each pair of barrier(), sys_membarrier() and smp_mb():
45 *
46 * The pair ordering is detailed as (O: ordered, X: not ordered):
47 *
48 * barrier() smp_mb() sys_membarrier()
49 * barrier() X X O
50 * smp_mb() X O O
51 * sys_membarrier() O O O
52 */
SYSCALL_DEFINE2(membarrier,int,cmd,int,flags)53 SYSCALL_DEFINE2(membarrier, int, cmd, int, flags)
54 {
55 /* MEMBARRIER_CMD_SHARED is not compatible with nohz_full. */
56 if (tick_nohz_full_enabled())
57 return -ENOSYS;
58 if (unlikely(flags))
59 return -EINVAL;
60 switch (cmd) {
61 case MEMBARRIER_CMD_QUERY:
62 return MEMBARRIER_CMD_BITMASK;
63 case MEMBARRIER_CMD_SHARED:
64 if (num_online_cpus() > 1)
65 synchronize_sched();
66 return 0;
67 default:
68 return -EINVAL;
69 }
70 }
71