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5b25b13a MD |
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 | */ | |
52 | SYSCALL_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 | } |