kernel/membarrier.c

   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 }