blob: 9f9284f37f8d8b6d3149ecc22634c613123e792d
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 | */ |
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 |