/* * Copyright (C) 2010, 2015 Mathieu Desnoyers <mathieu.desnoyers@efficios.com> * * membarrier system call * * This program is free software; you can redistribute it and/or modify * it under the terms of the GNU General Public License as published by * the Free Software Foundation; either version 2 of the License, or * (at your option) any later version. * * This program is distributed in the hope that it will be useful, * but WITHOUT ANY WARRANTY; without even the implied warranty of * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the * GNU General Public License for more details. */ #include <linux/syscalls.h>#include <linux/membarrier.h>#include <linux/tick.h> /* * Bitmask made from a "or" of all commands within enum membarrier_cmd, * except MEMBARRIER_CMD_QUERY. */#define MEMBARRIER_CMD_BITMASK	(MEMBARRIER_CMD_SHARED) /** * sys_membarrier - issue memory barriers on a set of threads * @cmd:   Takes command values defined in enum membarrier_cmd. * @flags: Currently needs to be 0. For future extensions. * * If this system call is not implemented, -ENOSYS is returned. If the * command specified does not exist, or if the command argument is invalid, * this system call returns -EINVAL. For a given command, with flags argument * set to 0, this system call is guaranteed to always return the same value * until reboot. * * All memory accesses performed in program order from each targeted thread * is guaranteed to be ordered with respect to sys_membarrier(). If we use * the semantic "barrier()" to represent a compiler barrier forcing memory * accesses to be performed in program order across the barrier, and * smp_mb() to represent explicit memory barriers forcing full memory * ordering across the barrier, we have the following ordering table for * each pair of barrier(), sys_membarrier() and smp_mb(): * * The pair ordering is detailed as (O: ordered, X: not ordered): * *                        barrier()   smp_mb() sys_membarrier() *        barrier()          X           X            O *        smp_mb()           X           O            O *        sys_membarrier()   O           O            O */SYSCALL_DEFINE2(membarrier, int, cmd, int, flags){	/* MEMBARRIER_CMD_SHARED is not compatible with nohz_full. */	if (tick_nohz_full_enabled())		return -ENOSYS;	if (unlikely(flags))		return -EINVAL;	switch (cmd) {	case MEMBARRIER_CMD_QUERY:		return MEMBARRIER_CMD_BITMASK;	case MEMBARRIER_CMD_SHARED:		if (num_online_cpus() > 1)			synchronize_sched();		return 0;	default:		return -EINVAL;	}}