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