linux-stable/kernel/membarrier.c
Mathieu Desnoyers 907565337e Fix: Disable sys_membarrier when nohz_full is enabled
Userspace applications should be allowed to expect the membarrier system
call with MEMBARRIER_CMD_SHARED command to issue memory barriers on
nohz_full CPUs, but synchronize_sched() does not take those into
account.

Given that we do not want unrelated processes to be able to affect
real-time sensitive nohz_full CPUs, simply return ENOSYS when membarrier
is invoked on a kernel with enabled nohz_full CPUs.

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
CC: Josh Triplett <josh@joshtriplett.org>
CC: Steven Rostedt <rostedt@goodmis.org>
CC: <stable@vger.kernel.org>	[3.10+]
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Chris Metcalf <cmetcalf@mellanox.com>
Cc: Rik van Riel <riel@redhat.com>
Acked-by: Lai Jiangshan <jiangshanlai@gmail.com>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>
2017-01-23 11:32:16 -08:00

70 lines
2.5 KiB
C

/*
* 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;
}
}