1 /* 2 * Copyright (C) 2010-2017 Mathieu Desnoyers <mathieu.desnoyers@efficios.com> 3 * 4 * membarrier system call 5 * 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. 10 * 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. 15 */ 16 17 #include <linux/syscalls.h> 18 #include <linux/membarrier.h> 19 #include <linux/tick.h> 20 #include <linux/cpumask.h> 21 #include <linux/atomic.h> 22 23 #include "sched.h" /* for cpu_rq(). */ 24 25 /* 26 * Bitmask made from a "or" of all commands within enum membarrier_cmd, 27 * except MEMBARRIER_CMD_QUERY. 28 */ 29 #define MEMBARRIER_CMD_BITMASK \ 30 (MEMBARRIER_CMD_SHARED | MEMBARRIER_CMD_PRIVATE_EXPEDITED \ 31 | MEMBARRIER_CMD_REGISTER_PRIVATE_EXPEDITED) 32 33 static void ipi_mb(void *info) 34 { 35 smp_mb(); /* IPIs should be serializing but paranoid. */ 36 } 37 38 static int membarrier_private_expedited(void) 39 { 40 int cpu; 41 bool fallback = false; 42 cpumask_var_t tmpmask; 43 44 if (!(atomic_read(¤t->mm->membarrier_state) 45 & MEMBARRIER_STATE_PRIVATE_EXPEDITED_READY)) 46 return -EPERM; 47 48 if (num_online_cpus() == 1) 49 return 0; 50 51 /* 52 * Matches memory barriers around rq->curr modification in 53 * scheduler. 54 */ 55 smp_mb(); /* system call entry is not a mb. */ 56 57 /* 58 * Expedited membarrier commands guarantee that they won't 59 * block, hence the GFP_NOWAIT allocation flag and fallback 60 * implementation. 61 */ 62 if (!zalloc_cpumask_var(&tmpmask, GFP_NOWAIT)) { 63 /* Fallback for OOM. */ 64 fallback = true; 65 } 66 67 cpus_read_lock(); 68 for_each_online_cpu(cpu) { 69 struct task_struct *p; 70 71 /* 72 * Skipping the current CPU is OK even through we can be 73 * migrated at any point. The current CPU, at the point 74 * where we read raw_smp_processor_id(), is ensured to 75 * be in program order with respect to the caller 76 * thread. Therefore, we can skip this CPU from the 77 * iteration. 78 */ 79 if (cpu == raw_smp_processor_id()) 80 continue; 81 rcu_read_lock(); 82 p = task_rcu_dereference(&cpu_rq(cpu)->curr); 83 if (p && p->mm == current->mm) { 84 if (!fallback) 85 __cpumask_set_cpu(cpu, tmpmask); 86 else 87 smp_call_function_single(cpu, ipi_mb, NULL, 1); 88 } 89 rcu_read_unlock(); 90 } 91 if (!fallback) { 92 preempt_disable(); 93 smp_call_function_many(tmpmask, ipi_mb, NULL, 1); 94 preempt_enable(); 95 free_cpumask_var(tmpmask); 96 } 97 cpus_read_unlock(); 98 99 /* 100 * Memory barrier on the caller thread _after_ we finished 101 * waiting for the last IPI. Matches memory barriers around 102 * rq->curr modification in scheduler. 103 */ 104 smp_mb(); /* exit from system call is not a mb */ 105 return 0; 106 } 107 108 static void membarrier_register_private_expedited(void) 109 { 110 struct task_struct *p = current; 111 struct mm_struct *mm = p->mm; 112 113 /* 114 * We need to consider threads belonging to different thread 115 * groups, which use the same mm. (CLONE_VM but not 116 * CLONE_THREAD). 117 */ 118 if (atomic_read(&mm->membarrier_state) 119 & MEMBARRIER_STATE_PRIVATE_EXPEDITED_READY) 120 return; 121 atomic_or(MEMBARRIER_STATE_PRIVATE_EXPEDITED_READY, 122 &mm->membarrier_state); 123 } 124 125 /** 126 * sys_membarrier - issue memory barriers on a set of threads 127 * @cmd: Takes command values defined in enum membarrier_cmd. 128 * @flags: Currently needs to be 0. For future extensions. 129 * 130 * If this system call is not implemented, -ENOSYS is returned. If the 131 * command specified does not exist, not available on the running 132 * kernel, or if the command argument is invalid, this system call 133 * returns -EINVAL. For a given command, with flags argument set to 0, 134 * this system call is guaranteed to always return the same value until 135 * reboot. 136 * 137 * All memory accesses performed in program order from each targeted thread 138 * is guaranteed to be ordered with respect to sys_membarrier(). If we use 139 * the semantic "barrier()" to represent a compiler barrier forcing memory 140 * accesses to be performed in program order across the barrier, and 141 * smp_mb() to represent explicit memory barriers forcing full memory 142 * ordering across the barrier, we have the following ordering table for 143 * each pair of barrier(), sys_membarrier() and smp_mb(): 144 * 145 * The pair ordering is detailed as (O: ordered, X: not ordered): 146 * 147 * barrier() smp_mb() sys_membarrier() 148 * barrier() X X O 149 * smp_mb() X O O 150 * sys_membarrier() O O O 151 */ 152 SYSCALL_DEFINE2(membarrier, int, cmd, int, flags) 153 { 154 if (unlikely(flags)) 155 return -EINVAL; 156 switch (cmd) { 157 case MEMBARRIER_CMD_QUERY: 158 { 159 int cmd_mask = MEMBARRIER_CMD_BITMASK; 160 161 if (tick_nohz_full_enabled()) 162 cmd_mask &= ~MEMBARRIER_CMD_SHARED; 163 return cmd_mask; 164 } 165 case MEMBARRIER_CMD_SHARED: 166 /* MEMBARRIER_CMD_SHARED is not compatible with nohz_full. */ 167 if (tick_nohz_full_enabled()) 168 return -EINVAL; 169 if (num_online_cpus() > 1) 170 synchronize_sched(); 171 return 0; 172 case MEMBARRIER_CMD_PRIVATE_EXPEDITED: 173 return membarrier_private_expedited(); 174 case MEMBARRIER_CMD_REGISTER_PRIVATE_EXPEDITED: 175 membarrier_register_private_expedited(); 176 return 0; 177 default: 178 return -EINVAL; 179 } 180 } 181