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 #include "sched.h" 17 18 /* 19 * Bitmask made from a "or" of all commands within enum membarrier_cmd, 20 * except MEMBARRIER_CMD_QUERY. 21 */ 22 #ifdef CONFIG_ARCH_HAS_MEMBARRIER_SYNC_CORE 23 #define MEMBARRIER_PRIVATE_EXPEDITED_SYNC_CORE_BITMASK \ 24 (MEMBARRIER_CMD_PRIVATE_EXPEDITED_SYNC_CORE \ 25 | MEMBARRIER_CMD_REGISTER_PRIVATE_EXPEDITED_SYNC_CORE) 26 #else 27 #define MEMBARRIER_PRIVATE_EXPEDITED_SYNC_CORE_BITMASK 0 28 #endif 29 30 #define MEMBARRIER_CMD_BITMASK \ 31 (MEMBARRIER_CMD_GLOBAL | MEMBARRIER_CMD_GLOBAL_EXPEDITED \ 32 | MEMBARRIER_CMD_REGISTER_GLOBAL_EXPEDITED \ 33 | MEMBARRIER_CMD_PRIVATE_EXPEDITED \ 34 | MEMBARRIER_CMD_REGISTER_PRIVATE_EXPEDITED \ 35 | MEMBARRIER_PRIVATE_EXPEDITED_SYNC_CORE_BITMASK) 36 37 static void ipi_mb(void *info) 38 { 39 smp_mb(); /* IPIs should be serializing but paranoid. */ 40 } 41 42 static int membarrier_global_expedited(void) 43 { 44 int cpu; 45 bool fallback = false; 46 cpumask_var_t tmpmask; 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 82 rcu_read_lock(); 83 p = task_rcu_dereference(&cpu_rq(cpu)->curr); 84 if (p && p->mm && (atomic_read(&p->mm->membarrier_state) & 85 MEMBARRIER_STATE_GLOBAL_EXPEDITED)) { 86 if (!fallback) 87 __cpumask_set_cpu(cpu, tmpmask); 88 else 89 smp_call_function_single(cpu, ipi_mb, NULL, 1); 90 } 91 rcu_read_unlock(); 92 } 93 if (!fallback) { 94 preempt_disable(); 95 smp_call_function_many(tmpmask, ipi_mb, NULL, 1); 96 preempt_enable(); 97 free_cpumask_var(tmpmask); 98 } 99 cpus_read_unlock(); 100 101 /* 102 * Memory barrier on the caller thread _after_ we finished 103 * waiting for the last IPI. Matches memory barriers around 104 * rq->curr modification in scheduler. 105 */ 106 smp_mb(); /* exit from system call is not a mb */ 107 return 0; 108 } 109 110 static int membarrier_private_expedited(int flags) 111 { 112 int cpu; 113 bool fallback = false; 114 cpumask_var_t tmpmask; 115 116 if (flags & MEMBARRIER_FLAG_SYNC_CORE) { 117 if (!IS_ENABLED(CONFIG_ARCH_HAS_MEMBARRIER_SYNC_CORE)) 118 return -EINVAL; 119 if (!(atomic_read(¤t->mm->membarrier_state) & 120 MEMBARRIER_STATE_PRIVATE_EXPEDITED_SYNC_CORE_READY)) 121 return -EPERM; 122 } else { 123 if (!(atomic_read(¤t->mm->membarrier_state) & 124 MEMBARRIER_STATE_PRIVATE_EXPEDITED_READY)) 125 return -EPERM; 126 } 127 128 if (num_online_cpus() == 1) 129 return 0; 130 131 /* 132 * Matches memory barriers around rq->curr modification in 133 * scheduler. 134 */ 135 smp_mb(); /* system call entry is not a mb. */ 136 137 /* 138 * Expedited membarrier commands guarantee that they won't 139 * block, hence the GFP_NOWAIT allocation flag and fallback 140 * implementation. 141 */ 142 if (!zalloc_cpumask_var(&tmpmask, GFP_NOWAIT)) { 143 /* Fallback for OOM. */ 144 fallback = true; 145 } 146 147 cpus_read_lock(); 148 for_each_online_cpu(cpu) { 149 struct task_struct *p; 150 151 /* 152 * Skipping the current CPU is OK even through we can be 153 * migrated at any point. The current CPU, at the point 154 * where we read raw_smp_processor_id(), is ensured to 155 * be in program order with respect to the caller 156 * thread. Therefore, we can skip this CPU from the 157 * iteration. 158 */ 159 if (cpu == raw_smp_processor_id()) 160 continue; 161 rcu_read_lock(); 162 p = task_rcu_dereference(&cpu_rq(cpu)->curr); 163 if (p && p->mm == current->mm) { 164 if (!fallback) 165 __cpumask_set_cpu(cpu, tmpmask); 166 else 167 smp_call_function_single(cpu, ipi_mb, NULL, 1); 168 } 169 rcu_read_unlock(); 170 } 171 if (!fallback) { 172 preempt_disable(); 173 smp_call_function_many(tmpmask, ipi_mb, NULL, 1); 174 preempt_enable(); 175 free_cpumask_var(tmpmask); 176 } 177 cpus_read_unlock(); 178 179 /* 180 * Memory barrier on the caller thread _after_ we finished 181 * waiting for the last IPI. Matches memory barriers around 182 * rq->curr modification in scheduler. 183 */ 184 smp_mb(); /* exit from system call is not a mb */ 185 186 return 0; 187 } 188 189 static int membarrier_register_global_expedited(void) 190 { 191 struct task_struct *p = current; 192 struct mm_struct *mm = p->mm; 193 194 if (atomic_read(&mm->membarrier_state) & 195 MEMBARRIER_STATE_GLOBAL_EXPEDITED_READY) 196 return 0; 197 atomic_or(MEMBARRIER_STATE_GLOBAL_EXPEDITED, &mm->membarrier_state); 198 if (atomic_read(&mm->mm_users) == 1 && get_nr_threads(p) == 1) { 199 /* 200 * For single mm user, single threaded process, we can 201 * simply issue a memory barrier after setting 202 * MEMBARRIER_STATE_GLOBAL_EXPEDITED to guarantee that 203 * no memory access following registration is reordered 204 * before registration. 205 */ 206 smp_mb(); 207 } else { 208 /* 209 * For multi-mm user threads, we need to ensure all 210 * future scheduler executions will observe the new 211 * thread flag state for this mm. 212 */ 213 synchronize_sched(); 214 } 215 atomic_or(MEMBARRIER_STATE_GLOBAL_EXPEDITED_READY, 216 &mm->membarrier_state); 217 218 return 0; 219 } 220 221 static int membarrier_register_private_expedited(int flags) 222 { 223 struct task_struct *p = current; 224 struct mm_struct *mm = p->mm; 225 int state = MEMBARRIER_STATE_PRIVATE_EXPEDITED_READY; 226 227 if (flags & MEMBARRIER_FLAG_SYNC_CORE) { 228 if (!IS_ENABLED(CONFIG_ARCH_HAS_MEMBARRIER_SYNC_CORE)) 229 return -EINVAL; 230 state = MEMBARRIER_STATE_PRIVATE_EXPEDITED_SYNC_CORE_READY; 231 } 232 233 /* 234 * We need to consider threads belonging to different thread 235 * groups, which use the same mm. (CLONE_VM but not 236 * CLONE_THREAD). 237 */ 238 if (atomic_read(&mm->membarrier_state) & state) 239 return 0; 240 atomic_or(MEMBARRIER_STATE_PRIVATE_EXPEDITED, &mm->membarrier_state); 241 if (flags & MEMBARRIER_FLAG_SYNC_CORE) 242 atomic_or(MEMBARRIER_STATE_PRIVATE_EXPEDITED_SYNC_CORE, 243 &mm->membarrier_state); 244 if (!(atomic_read(&mm->mm_users) == 1 && get_nr_threads(p) == 1)) { 245 /* 246 * Ensure all future scheduler executions will observe the 247 * new thread flag state for this process. 248 */ 249 synchronize_sched(); 250 } 251 atomic_or(state, &mm->membarrier_state); 252 253 return 0; 254 } 255 256 /** 257 * sys_membarrier - issue memory barriers on a set of threads 258 * @cmd: Takes command values defined in enum membarrier_cmd. 259 * @flags: Currently needs to be 0. For future extensions. 260 * 261 * If this system call is not implemented, -ENOSYS is returned. If the 262 * command specified does not exist, not available on the running 263 * kernel, or if the command argument is invalid, this system call 264 * returns -EINVAL. For a given command, with flags argument set to 0, 265 * this system call is guaranteed to always return the same value until 266 * reboot. 267 * 268 * All memory accesses performed in program order from each targeted thread 269 * is guaranteed to be ordered with respect to sys_membarrier(). If we use 270 * the semantic "barrier()" to represent a compiler barrier forcing memory 271 * accesses to be performed in program order across the barrier, and 272 * smp_mb() to represent explicit memory barriers forcing full memory 273 * ordering across the barrier, we have the following ordering table for 274 * each pair of barrier(), sys_membarrier() and smp_mb(): 275 * 276 * The pair ordering is detailed as (O: ordered, X: not ordered): 277 * 278 * barrier() smp_mb() sys_membarrier() 279 * barrier() X X O 280 * smp_mb() X O O 281 * sys_membarrier() O O O 282 */ 283 SYSCALL_DEFINE2(membarrier, int, cmd, int, flags) 284 { 285 if (unlikely(flags)) 286 return -EINVAL; 287 switch (cmd) { 288 case MEMBARRIER_CMD_QUERY: 289 { 290 int cmd_mask = MEMBARRIER_CMD_BITMASK; 291 292 if (tick_nohz_full_enabled()) 293 cmd_mask &= ~MEMBARRIER_CMD_GLOBAL; 294 return cmd_mask; 295 } 296 case MEMBARRIER_CMD_GLOBAL: 297 /* MEMBARRIER_CMD_GLOBAL is not compatible with nohz_full. */ 298 if (tick_nohz_full_enabled()) 299 return -EINVAL; 300 if (num_online_cpus() > 1) 301 synchronize_sched(); 302 return 0; 303 case MEMBARRIER_CMD_GLOBAL_EXPEDITED: 304 return membarrier_global_expedited(); 305 case MEMBARRIER_CMD_REGISTER_GLOBAL_EXPEDITED: 306 return membarrier_register_global_expedited(); 307 case MEMBARRIER_CMD_PRIVATE_EXPEDITED: 308 return membarrier_private_expedited(0); 309 case MEMBARRIER_CMD_REGISTER_PRIVATE_EXPEDITED: 310 return membarrier_register_private_expedited(0); 311 case MEMBARRIER_CMD_PRIVATE_EXPEDITED_SYNC_CORE: 312 return membarrier_private_expedited(MEMBARRIER_FLAG_SYNC_CORE); 313 case MEMBARRIER_CMD_REGISTER_PRIVATE_EXPEDITED_SYNC_CORE: 314 return membarrier_register_private_expedited(MEMBARRIER_FLAG_SYNC_CORE); 315 default: 316 return -EINVAL; 317 } 318 } 319