| Mathieu Desnoyers | 5b25b13 | 2015-09-11 13:07:39 -0700 | [diff] [blame] | 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 | } |