1 /* SPDX-License-Identifier: GPL-2.0 */ 2 #ifndef __LINUX_PREEMPT_H 3 #define __LINUX_PREEMPT_H 4 5 /* 6 * include/linux/preempt.h - macros for accessing and manipulating 7 * preempt_count (used for kernel preemption, interrupt count, etc.) 8 */ 9 10 #include <linux/linkage.h> 11 #include <linux/cleanup.h> 12 #include <linux/types.h> 13 14 /* 15 * We put the hardirq and softirq counter into the preemption 16 * counter. The bitmask has the following meaning: 17 * 18 * - bits 0-7 are the preemption count (max preemption depth: 256) 19 * - bits 8-15 are the softirq count (max # of softirqs: 256) 20 * 21 * The hardirq count could in theory be the same as the number of 22 * interrupts in the system, but we run all interrupt handlers with 23 * interrupts disabled, so we cannot have nesting interrupts. Though 24 * there are a few palaeontologic drivers which reenable interrupts in 25 * the handler, so we need more than one bit here. 26 * 27 * PREEMPT_MASK: 0x000000ff 28 * SOFTIRQ_MASK: 0x0000ff00 29 * HARDIRQ_MASK: 0x000f0000 30 * NMI_MASK: 0x00f00000 31 * PREEMPT_NEED_RESCHED: 0x80000000 32 */ 33 #define PREEMPT_BITS 8 34 #define SOFTIRQ_BITS 8 35 #define HARDIRQ_BITS 4 36 #define NMI_BITS 4 37 38 #define PREEMPT_SHIFT 0 39 #define SOFTIRQ_SHIFT (PREEMPT_SHIFT + PREEMPT_BITS) 40 #define HARDIRQ_SHIFT (SOFTIRQ_SHIFT + SOFTIRQ_BITS) 41 #define NMI_SHIFT (HARDIRQ_SHIFT + HARDIRQ_BITS) 42 43 #define __IRQ_MASK(x) ((1UL << (x))-1) 44 45 #define PREEMPT_MASK (__IRQ_MASK(PREEMPT_BITS) << PREEMPT_SHIFT) 46 #define SOFTIRQ_MASK (__IRQ_MASK(SOFTIRQ_BITS) << SOFTIRQ_SHIFT) 47 #define HARDIRQ_MASK (__IRQ_MASK(HARDIRQ_BITS) << HARDIRQ_SHIFT) 48 #define NMI_MASK (__IRQ_MASK(NMI_BITS) << NMI_SHIFT) 49 50 #define PREEMPT_OFFSET (1UL << PREEMPT_SHIFT) 51 #define SOFTIRQ_OFFSET (1UL << SOFTIRQ_SHIFT) 52 #define HARDIRQ_OFFSET (1UL << HARDIRQ_SHIFT) 53 #define NMI_OFFSET (1UL << NMI_SHIFT) 54 55 #define SOFTIRQ_DISABLE_OFFSET (2 * SOFTIRQ_OFFSET) 56 57 #define PREEMPT_DISABLED (PREEMPT_DISABLE_OFFSET + PREEMPT_ENABLED) 58 59 /* 60 * Disable preemption until the scheduler is running -- use an unconditional 61 * value so that it also works on !PREEMPT_COUNT kernels. 62 * 63 * Reset by start_kernel()->sched_init()->init_idle()->init_idle_preempt_count(). 64 */ 65 #define INIT_PREEMPT_COUNT PREEMPT_OFFSET 66 67 /* 68 * Initial preempt_count value; reflects the preempt_count schedule invariant 69 * which states that during context switches: 70 * 71 * preempt_count() == 2*PREEMPT_DISABLE_OFFSET 72 * 73 * Note: PREEMPT_DISABLE_OFFSET is 0 for !PREEMPT_COUNT kernels. 74 * Note: See finish_task_switch(). 75 */ 76 #define FORK_PREEMPT_COUNT (2*PREEMPT_DISABLE_OFFSET + PREEMPT_ENABLED) 77 78 /* preempt_count() and related functions, depends on PREEMPT_NEED_RESCHED */ 79 #include <asm/preempt.h> 80 81 /** 82 * interrupt_context_level - return interrupt context level 83 * 84 * Returns the current interrupt context level. 85 * 0 - normal context 86 * 1 - softirq context 87 * 2 - hardirq context 88 * 3 - NMI context 89 */ interrupt_context_level(void)90 static __always_inline unsigned char interrupt_context_level(void) 91 { 92 unsigned long pc = preempt_count(); 93 unsigned char level = 0; 94 95 level += !!(pc & (NMI_MASK)); 96 level += !!(pc & (NMI_MASK | HARDIRQ_MASK)); 97 level += !!(pc & (NMI_MASK | HARDIRQ_MASK | SOFTIRQ_OFFSET)); 98 99 return level; 100 } 101 102 /* 103 * These macro definitions avoid redundant invocations of preempt_count() 104 * because such invocations would result in redundant loads given that 105 * preempt_count() is commonly implemented with READ_ONCE(). 106 */ 107 108 #define nmi_count() (preempt_count() & NMI_MASK) 109 #define hardirq_count() (preempt_count() & HARDIRQ_MASK) 110 #ifdef CONFIG_PREEMPT_RT 111 # define softirq_count() (current->softirq_disable_cnt & SOFTIRQ_MASK) 112 # define irq_count() ((preempt_count() & (NMI_MASK | HARDIRQ_MASK)) | softirq_count()) 113 #else 114 # define softirq_count() (preempt_count() & SOFTIRQ_MASK) 115 # define irq_count() (preempt_count() & (NMI_MASK | HARDIRQ_MASK | SOFTIRQ_MASK)) 116 #endif 117 118 /* 119 * Macros to retrieve the current execution context: 120 * 121 * in_nmi() - We're in NMI context 122 * in_hardirq() - We're in hard IRQ context 123 * in_serving_softirq() - We're in softirq context 124 * in_task() - We're in task context 125 */ 126 #define in_nmi() (nmi_count()) 127 #define in_hardirq() (hardirq_count()) 128 #define in_serving_softirq() (softirq_count() & SOFTIRQ_OFFSET) 129 #ifdef CONFIG_PREEMPT_RT 130 # define in_task() (!((preempt_count() & (NMI_MASK | HARDIRQ_MASK)) | in_serving_softirq())) 131 #else 132 # define in_task() (!(preempt_count() & (NMI_MASK | HARDIRQ_MASK | SOFTIRQ_OFFSET))) 133 #endif 134 135 /* 136 * The following macros are deprecated and should not be used in new code: 137 * in_irq() - Obsolete version of in_hardirq() 138 * in_softirq() - We have BH disabled, or are processing softirqs 139 * in_interrupt() - We're in NMI,IRQ,SoftIRQ context or have BH disabled 140 */ 141 #define in_irq() (hardirq_count()) 142 #define in_softirq() (softirq_count()) 143 #define in_interrupt() (irq_count()) 144 145 /* 146 * The preempt_count offset after preempt_disable(); 147 */ 148 #if defined(CONFIG_PREEMPT_COUNT) 149 # define PREEMPT_DISABLE_OFFSET PREEMPT_OFFSET 150 #else 151 # define PREEMPT_DISABLE_OFFSET 0 152 #endif 153 154 /* 155 * The preempt_count offset after spin_lock() 156 */ 157 #if !defined(CONFIG_PREEMPT_RT) 158 #define PREEMPT_LOCK_OFFSET PREEMPT_DISABLE_OFFSET 159 #else 160 /* Locks on RT do not disable preemption */ 161 #define PREEMPT_LOCK_OFFSET 0 162 #endif 163 164 /* 165 * The preempt_count offset needed for things like: 166 * 167 * spin_lock_bh() 168 * 169 * Which need to disable both preemption (CONFIG_PREEMPT_COUNT) and 170 * softirqs, such that unlock sequences of: 171 * 172 * spin_unlock(); 173 * local_bh_enable(); 174 * 175 * Work as expected. 176 */ 177 #define SOFTIRQ_LOCK_OFFSET (SOFTIRQ_DISABLE_OFFSET + PREEMPT_LOCK_OFFSET) 178 179 /* 180 * Are we running in atomic context? WARNING: this macro cannot 181 * always detect atomic context; in particular, it cannot know about 182 * held spinlocks in non-preemptible kernels. Thus it should not be 183 * used in the general case to determine whether sleeping is possible. 184 * Do not use in_atomic() in driver code. 185 */ 186 #define in_atomic() (preempt_count() != 0) 187 188 /* 189 * Check whether we were atomic before we did preempt_disable(): 190 * (used by the scheduler) 191 */ 192 #define in_atomic_preempt_off() (preempt_count() != PREEMPT_DISABLE_OFFSET) 193 194 #if defined(CONFIG_DEBUG_PREEMPT) || defined(CONFIG_TRACE_PREEMPT_TOGGLE) 195 extern void preempt_count_add(int val); 196 extern void preempt_count_sub(int val); 197 #define preempt_count_dec_and_test() \ 198 ({ preempt_count_sub(1); should_resched(0); }) 199 #else 200 #define preempt_count_add(val) __preempt_count_add(val) 201 #define preempt_count_sub(val) __preempt_count_sub(val) 202 #define preempt_count_dec_and_test() __preempt_count_dec_and_test() 203 #endif 204 205 #define __preempt_count_inc() __preempt_count_add(1) 206 #define __preempt_count_dec() __preempt_count_sub(1) 207 208 #define preempt_count_inc() preempt_count_add(1) 209 #define preempt_count_dec() preempt_count_sub(1) 210 211 #ifdef CONFIG_PREEMPT_COUNT 212 213 #define preempt_disable() \ 214 do { \ 215 preempt_count_inc(); \ 216 barrier(); \ 217 } while (0) 218 219 #define sched_preempt_enable_no_resched() \ 220 do { \ 221 barrier(); \ 222 preempt_count_dec(); \ 223 } while (0) 224 225 #define preempt_enable_no_resched() sched_preempt_enable_no_resched() 226 227 #define preemptible() (preempt_count() == 0 && !irqs_disabled()) 228 229 #ifdef CONFIG_PREEMPTION 230 #define preempt_enable() \ 231 do { \ 232 barrier(); \ 233 if (unlikely(preempt_count_dec_and_test())) \ 234 __preempt_schedule(); \ 235 } while (0) 236 237 #define preempt_enable_notrace() \ 238 do { \ 239 barrier(); \ 240 if (unlikely(__preempt_count_dec_and_test())) \ 241 __preempt_schedule_notrace(); \ 242 } while (0) 243 244 #define preempt_check_resched() \ 245 do { \ 246 if (should_resched(0)) \ 247 __preempt_schedule(); \ 248 } while (0) 249 250 #else /* !CONFIG_PREEMPTION */ 251 #define preempt_enable() \ 252 do { \ 253 barrier(); \ 254 preempt_count_dec(); \ 255 } while (0) 256 257 #define preempt_enable_notrace() \ 258 do { \ 259 barrier(); \ 260 __preempt_count_dec(); \ 261 } while (0) 262 263 #define preempt_check_resched() do { } while (0) 264 #endif /* CONFIG_PREEMPTION */ 265 266 #define preempt_disable_notrace() \ 267 do { \ 268 __preempt_count_inc(); \ 269 barrier(); \ 270 } while (0) 271 272 #define preempt_enable_no_resched_notrace() \ 273 do { \ 274 barrier(); \ 275 __preempt_count_dec(); \ 276 } while (0) 277 278 #else /* !CONFIG_PREEMPT_COUNT */ 279 280 /* 281 * Even if we don't have any preemption, we need preempt disable/enable 282 * to be barriers, so that we don't have things like get_user/put_user 283 * that can cause faults and scheduling migrate into our preempt-protected 284 * region. 285 */ 286 #define preempt_disable() barrier() 287 #define sched_preempt_enable_no_resched() barrier() 288 #define preempt_enable_no_resched() barrier() 289 #define preempt_enable() barrier() 290 #define preempt_check_resched() do { } while (0) 291 292 #define preempt_disable_notrace() barrier() 293 #define preempt_enable_no_resched_notrace() barrier() 294 #define preempt_enable_notrace() barrier() 295 #define preemptible() 0 296 297 #endif /* CONFIG_PREEMPT_COUNT */ 298 299 #ifdef MODULE 300 /* 301 * Modules have no business playing preemption tricks. 302 */ 303 #undef sched_preempt_enable_no_resched 304 #undef preempt_enable_no_resched 305 #undef preempt_enable_no_resched_notrace 306 #undef preempt_check_resched 307 #endif 308 309 #define preempt_set_need_resched() \ 310 do { \ 311 set_preempt_need_resched(); \ 312 } while (0) 313 #define preempt_fold_need_resched() \ 314 do { \ 315 if (tif_need_resched()) \ 316 set_preempt_need_resched(); \ 317 } while (0) 318 319 #ifdef CONFIG_PREEMPT_NOTIFIERS 320 321 struct preempt_notifier; 322 323 /** 324 * preempt_ops - notifiers called when a task is preempted and rescheduled 325 * @sched_in: we're about to be rescheduled: 326 * notifier: struct preempt_notifier for the task being scheduled 327 * cpu: cpu we're scheduled on 328 * @sched_out: we've just been preempted 329 * notifier: struct preempt_notifier for the task being preempted 330 * next: the task that's kicking us out 331 * 332 * Please note that sched_in and out are called under different 333 * contexts. sched_out is called with rq lock held and irq disabled 334 * while sched_in is called without rq lock and irq enabled. This 335 * difference is intentional and depended upon by its users. 336 */ 337 struct preempt_ops { 338 void (*sched_in)(struct preempt_notifier *notifier, int cpu); 339 void (*sched_out)(struct preempt_notifier *notifier, 340 struct task_struct *next); 341 }; 342 343 /** 344 * preempt_notifier - key for installing preemption notifiers 345 * @link: internal use 346 * @ops: defines the notifier functions to be called 347 * 348 * Usually used in conjunction with container_of(). 349 */ 350 struct preempt_notifier { 351 struct hlist_node link; 352 struct preempt_ops *ops; 353 }; 354 355 void preempt_notifier_inc(void); 356 void preempt_notifier_dec(void); 357 void preempt_notifier_register(struct preempt_notifier *notifier); 358 void preempt_notifier_unregister(struct preempt_notifier *notifier); 359 preempt_notifier_init(struct preempt_notifier * notifier,struct preempt_ops * ops)360 static inline void preempt_notifier_init(struct preempt_notifier *notifier, 361 struct preempt_ops *ops) 362 { 363 /* INIT_HLIST_NODE() open coded, to avoid dependency on list.h */ 364 notifier->link.next = NULL; 365 notifier->link.pprev = NULL; 366 notifier->ops = ops; 367 } 368 369 #endif 370 371 #ifdef CONFIG_SMP 372 373 /* 374 * Migrate-Disable and why it is undesired. 375 * 376 * When a preempted task becomes elegible to run under the ideal model (IOW it 377 * becomes one of the M highest priority tasks), it might still have to wait 378 * for the preemptee's migrate_disable() section to complete. Thereby suffering 379 * a reduction in bandwidth in the exact duration of the migrate_disable() 380 * section. 381 * 382 * Per this argument, the change from preempt_disable() to migrate_disable() 383 * gets us: 384 * 385 * - a higher priority tasks gains reduced wake-up latency; with preempt_disable() 386 * it would have had to wait for the lower priority task. 387 * 388 * - a lower priority tasks; which under preempt_disable() could've instantly 389 * migrated away when another CPU becomes available, is now constrained 390 * by the ability to push the higher priority task away, which might itself be 391 * in a migrate_disable() section, reducing it's available bandwidth. 392 * 393 * IOW it trades latency / moves the interference term, but it stays in the 394 * system, and as long as it remains unbounded, the system is not fully 395 * deterministic. 396 * 397 * 398 * The reason we have it anyway. 399 * 400 * PREEMPT_RT breaks a number of assumptions traditionally held. By forcing a 401 * number of primitives into becoming preemptible, they would also allow 402 * migration. This turns out to break a bunch of per-cpu usage. To this end, 403 * all these primitives employ migirate_disable() to restore this implicit 404 * assumption. 405 * 406 * This is a 'temporary' work-around at best. The correct solution is getting 407 * rid of the above assumptions and reworking the code to employ explicit 408 * per-cpu locking or short preempt-disable regions. 409 * 410 * The end goal must be to get rid of migrate_disable(), alternatively we need 411 * a schedulability theory that does not depend on abritrary migration. 412 * 413 * 414 * Notes on the implementation. 415 * 416 * The implementation is particularly tricky since existing code patterns 417 * dictate neither migrate_disable() nor migrate_enable() is allowed to block. 418 * This means that it cannot use cpus_read_lock() to serialize against hotplug, 419 * nor can it easily migrate itself into a pending affinity mask change on 420 * migrate_enable(). 421 * 422 * 423 * Note: even non-work-conserving schedulers like semi-partitioned depends on 424 * migration, so migrate_disable() is not only a problem for 425 * work-conserving schedulers. 426 * 427 */ 428 extern void migrate_disable(void); 429 extern void migrate_enable(void); 430 431 #else 432 migrate_disable(void)433 static inline void migrate_disable(void) { } migrate_enable(void)434 static inline void migrate_enable(void) { } 435 436 #endif /* CONFIG_SMP */ 437 438 /** 439 * preempt_disable_nested - Disable preemption inside a normally preempt disabled section 440 * 441 * Use for code which requires preemption protection inside a critical 442 * section which has preemption disabled implicitly on non-PREEMPT_RT 443 * enabled kernels, by e.g.: 444 * - holding a spinlock/rwlock 445 * - soft interrupt context 446 * - regular interrupt handlers 447 * 448 * On PREEMPT_RT enabled kernels spinlock/rwlock held sections, soft 449 * interrupt context and regular interrupt handlers are preemptible and 450 * only prevent migration. preempt_disable_nested() ensures that preemption 451 * is disabled for cases which require CPU local serialization even on 452 * PREEMPT_RT. For non-PREEMPT_RT kernels this is a NOP. 453 * 454 * The use cases are code sequences which are not serialized by a 455 * particular lock instance, e.g.: 456 * - seqcount write side critical sections where the seqcount is not 457 * associated to a particular lock and therefore the automatic 458 * protection mechanism does not work. This prevents a live lock 459 * against a preempting high priority reader. 460 * - RMW per CPU variable updates like vmstat. 461 */ 462 /* Macro to avoid header recursion hell vs. lockdep */ 463 #define preempt_disable_nested() \ 464 do { \ 465 if (IS_ENABLED(CONFIG_PREEMPT_RT)) \ 466 preempt_disable(); \ 467 else \ 468 lockdep_assert_preemption_disabled(); \ 469 } while (0) 470 471 /** 472 * preempt_enable_nested - Undo the effect of preempt_disable_nested() 473 */ preempt_enable_nested(void)474 static __always_inline void preempt_enable_nested(void) 475 { 476 if (IS_ENABLED(CONFIG_PREEMPT_RT)) 477 preempt_enable(); 478 } 479 480 DEFINE_LOCK_GUARD_0(preempt, preempt_disable(), preempt_enable()) 481 DEFINE_LOCK_GUARD_0(preempt_notrace, preempt_disable_notrace(), preempt_enable_notrace()) 482 DEFINE_LOCK_GUARD_0(migrate, migrate_disable(), migrate_enable()) 483 484 #ifdef CONFIG_PREEMPT_DYNAMIC 485 486 extern bool preempt_model_none(void); 487 extern bool preempt_model_voluntary(void); 488 extern bool preempt_model_full(void); 489 490 #else 491 492 static inline bool preempt_model_none(void) 493 { 494 return IS_ENABLED(CONFIG_PREEMPT_NONE); 495 } 496 static inline bool preempt_model_voluntary(void) 497 { 498 return IS_ENABLED(CONFIG_PREEMPT_VOLUNTARY); 499 } 500 static inline bool preempt_model_full(void) 501 { 502 return IS_ENABLED(CONFIG_PREEMPT); 503 } 504 505 #endif 506 preempt_model_rt(void)507 static inline bool preempt_model_rt(void) 508 { 509 return IS_ENABLED(CONFIG_PREEMPT_RT); 510 } 511 512 /* 513 * Does the preemption model allow non-cooperative preemption? 514 * 515 * For !CONFIG_PREEMPT_DYNAMIC kernels this is an exact match with 516 * CONFIG_PREEMPTION; for CONFIG_PREEMPT_DYNAMIC this doesn't work as the 517 * kernel is *built* with CONFIG_PREEMPTION=y but may run with e.g. the 518 * PREEMPT_NONE model. 519 */ preempt_model_preemptible(void)520 static inline bool preempt_model_preemptible(void) 521 { 522 return preempt_model_full() || preempt_model_rt(); 523 } 524 525 #endif /* __LINUX_PREEMPT_H */ 526