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/*
 * 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;
	}
}