2 * Copyright (C) 2010-2017 Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
4 * membarrier system call
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.
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.
17 #include <linux/syscalls.h>
18 #include <linux/membarrier.h>
19 #include <linux/tick.h>
20 #include <linux/cpumask.h>
22 #include "sched.h" /* for cpu_rq(). */
25 * Bitmask made from a "or" of all commands within enum membarrier_cmd,
26 * except MEMBARRIER_CMD_QUERY.
28 #define MEMBARRIER_CMD_BITMASK \
29 (MEMBARRIER_CMD_SHARED | MEMBARRIER_CMD_PRIVATE_EXPEDITED)
31 static void ipi_mb(void *info)
33 smp_mb(); /* IPIs should be serializing but paranoid. */
36 static void membarrier_private_expedited(void)
39 bool fallback = false;
40 cpumask_var_t tmpmask;
42 if (num_online_cpus() == 1)
46 * Matches memory barriers around rq->curr modification in
49 smp_mb(); /* system call entry is not a mb. */
52 * Expedited membarrier commands guarantee that they won't
53 * block, hence the GFP_NOWAIT allocation flag and fallback
56 if (!zalloc_cpumask_var(&tmpmask, GFP_NOWAIT)) {
57 /* Fallback for OOM. */
62 for_each_online_cpu(cpu) {
63 struct task_struct *p;
66 * Skipping the current CPU is OK even through we can be
67 * migrated at any point. The current CPU, at the point
68 * where we read raw_smp_processor_id(), is ensured to
69 * be in program order with respect to the caller
70 * thread. Therefore, we can skip this CPU from the
73 if (cpu == raw_smp_processor_id())
76 p = task_rcu_dereference(&cpu_rq(cpu)->curr);
77 if (p && p->mm == current->mm) {
79 __cpumask_set_cpu(cpu, tmpmask);
81 smp_call_function_single(cpu, ipi_mb, NULL, 1);
86 smp_call_function_many(tmpmask, ipi_mb, NULL, 1);
87 free_cpumask_var(tmpmask);
92 * Memory barrier on the caller thread _after_ we finished
93 * waiting for the last IPI. Matches memory barriers around
94 * rq->curr modification in scheduler.
96 smp_mb(); /* exit from system call is not a mb */
100 * sys_membarrier - issue memory barriers on a set of threads
101 * @cmd: Takes command values defined in enum membarrier_cmd.
102 * @flags: Currently needs to be 0. For future extensions.
104 * If this system call is not implemented, -ENOSYS is returned. If the
105 * command specified does not exist, not available on the running
106 * kernel, or if the command argument is invalid, this system call
107 * returns -EINVAL. For a given command, with flags argument set to 0,
108 * this system call is guaranteed to always return the same value until
111 * All memory accesses performed in program order from each targeted thread
112 * is guaranteed to be ordered with respect to sys_membarrier(). If we use
113 * the semantic "barrier()" to represent a compiler barrier forcing memory
114 * accesses to be performed in program order across the barrier, and
115 * smp_mb() to represent explicit memory barriers forcing full memory
116 * ordering across the barrier, we have the following ordering table for
117 * each pair of barrier(), sys_membarrier() and smp_mb():
119 * The pair ordering is detailed as (O: ordered, X: not ordered):
121 * barrier() smp_mb() sys_membarrier()
124 * sys_membarrier() O O O
126 SYSCALL_DEFINE2(membarrier, int, cmd, int, flags)
131 case MEMBARRIER_CMD_QUERY:
133 int cmd_mask = MEMBARRIER_CMD_BITMASK;
135 if (tick_nohz_full_enabled())
136 cmd_mask &= ~MEMBARRIER_CMD_SHARED;
139 case MEMBARRIER_CMD_SHARED:
140 /* MEMBARRIER_CMD_SHARED is not compatible with nohz_full. */
141 if (tick_nohz_full_enabled())
143 if (num_online_cpus() > 1)
146 case MEMBARRIER_CMD_PRIVATE_EXPEDITED:
147 membarrier_private_expedited();