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 * - bits 16-23 are the hardirq disable count (max # of hardirq disable: 256)
21 * - bits 24-27 are the hardirq count (max # of hardirqs: 16)
22 * - bit 28 is the NMI flag (no nesting count, tracked separately)
23 *
24 * The hardirq count could in theory be the same as the number of
25 * interrupts in the system, but we run all interrupt handlers with
26 * interrupts disabled, so we cannot have nesting interrupts. Though
27 * there are a few palaeontologic drivers which reenable interrupts in
28 * the handler, so we need more than one bit here.
29 *
30 * NMI nesting depth is tracked in a separate per-CPU variable
31 * (nmi_nesting) to save bits in preempt_count.
32 *
33 * PREEMPT_MASK: 0x000000ff
34 * SOFTIRQ_MASK: 0x0000ff00
35 * HARDIRQ_DISABLE_MASK: 0x00ff0000
36 * HARDIRQ_MASK: 0x0f000000
37 *
38 * When HAS_SEPARATE_PREEMPT_RESCHED_BITS=y, PREEMPT_NEED_RESCHED is put in a
39 * separate word and that allows 64bit load-store architectures to 'set'
40 * PREEMPT_NEED_RESCHED without messing up the otherwise symmetric
41 * modifications used on preempt_count and still load the whole thing
42 * (single-copy) atomically, without having to resort to full atomic
43 * operations.
44 *
45 * Because of the above, NMI_MASK bits are different depending on
46 * HAS_SEPARATE_PREEMPT_RESCHED_BITS:
47 *
48 * - HAS_SEPARATE_PREEMPT_RESCHED_BITS=n:
49 *
50 * NMI_MASK: 0x10000000
51 * PREEMPT_NEED_RESCHED: 0x80000000
52 *
53 * - HAS_SEPARATE_PREEMPT_RESCHED_BITS=y:
54 * NMI_MASK: 0xf0000000
55 * (PREEMPT_NEED_RESCHED is in a different word)
56 */
57 #define PREEMPT_BITS 8
58 #define SOFTIRQ_BITS 8
59 #define HARDIRQ_DISABLE_BITS 8
60 #define HARDIRQ_BITS 4
61 #define NMI_BITS (1 + 3*IS_ENABLED(CONFIG_HAS_SEPARATE_PREEMPT_RESCHED_BITS))
62
63 #define PREEMPT_SHIFT 0
64 #define SOFTIRQ_SHIFT (PREEMPT_SHIFT + PREEMPT_BITS)
65 #define HARDIRQ_DISABLE_SHIFT (SOFTIRQ_SHIFT + SOFTIRQ_BITS)
66 #define HARDIRQ_SHIFT (HARDIRQ_DISABLE_SHIFT + HARDIRQ_DISABLE_BITS)
67 #define NMI_SHIFT (HARDIRQ_SHIFT + HARDIRQ_BITS)
68
69 #define __IRQ_MASK(x) ((1UL << (x))-1)
70
71 #define PREEMPT_MASK (__IRQ_MASK(PREEMPT_BITS) << PREEMPT_SHIFT)
72 #define SOFTIRQ_MASK (__IRQ_MASK(SOFTIRQ_BITS) << SOFTIRQ_SHIFT)
73 #define HARDIRQ_DISABLE_MASK (__IRQ_MASK(HARDIRQ_DISABLE_BITS) << HARDIRQ_DISABLE_SHIFT)
74 #define HARDIRQ_MASK (__IRQ_MASK(HARDIRQ_BITS) << HARDIRQ_SHIFT)
75 #define NMI_MASK (__IRQ_MASK(NMI_BITS) << NMI_SHIFT)
76
77 #define PREEMPT_OFFSET (1UL << PREEMPT_SHIFT)
78 #define SOFTIRQ_OFFSET (1UL << SOFTIRQ_SHIFT)
79 #define HARDIRQ_DISABLE_OFFSET (1UL << HARDIRQ_DISABLE_SHIFT)
80 #define HARDIRQ_OFFSET (1UL << HARDIRQ_SHIFT)
81 #define NMI_OFFSET (1UL << NMI_SHIFT)
82
83 #define SOFTIRQ_DISABLE_OFFSET (2 * SOFTIRQ_OFFSET)
84
85 #define PREEMPT_DISABLED (PREEMPT_DISABLE_OFFSET + PREEMPT_ENABLED)
86
87 /*
88 * Disable preemption until the scheduler is running -- use an unconditional
89 * value so that it also works on !PREEMPT_COUNT kernels.
90 *
91 * Reset by start_kernel()->sched_init()->init_idle()->init_idle_preempt_count().
92 */
93 #define INIT_PREEMPT_COUNT PREEMPT_OFFSET
94
95 /*
96 * Initial preempt_count value; reflects the preempt_count schedule invariant
97 * which states that during context switches:
98 *
99 * preempt_count() == 2*PREEMPT_DISABLE_OFFSET
100 *
101 * Note: PREEMPT_DISABLE_OFFSET is 0 for !PREEMPT_COUNT kernels.
102 * Note: See finish_task_switch().
103 */
104 #define FORK_PREEMPT_COUNT (2*PREEMPT_DISABLE_OFFSET + PREEMPT_ENABLED)
105
106 /* preempt_count() and related functions, depends on PREEMPT_NEED_RESCHED */
107 #include <asm/preempt.h>
108
109 /**
110 * interrupt_context_level - return interrupt context level
111 *
112 * Returns the current interrupt context level.
113 * 0 - normal context
114 * 1 - softirq context
115 * 2 - hardirq context
116 * 3 - NMI context
117 */
interrupt_context_level(void)118 static __always_inline unsigned char interrupt_context_level(void)
119 {
120 unsigned long pc = preempt_count();
121 unsigned char level = 0;
122
123 level += !!(pc & (NMI_MASK));
124 level += !!(pc & (NMI_MASK | HARDIRQ_MASK));
125 level += !!(pc & (NMI_MASK | HARDIRQ_MASK | SOFTIRQ_OFFSET));
126
127 return level;
128 }
129
130 /*
131 * These macro definitions avoid redundant invocations of preempt_count()
132 * because such invocations would result in redundant loads given that
133 * preempt_count() is commonly implemented with READ_ONCE().
134 */
135
136 #define nmi_count() (preempt_count() & NMI_MASK)
137 #define hardirq_count() (preempt_count() & HARDIRQ_MASK)
138 #ifdef CONFIG_PREEMPT_RT
139 # define softirq_count() (current->softirq_disable_cnt & SOFTIRQ_MASK)
140 # define irq_count() ((preempt_count() & (NMI_MASK | HARDIRQ_MASK)) | softirq_count())
141 #else
142 # define softirq_count() (preempt_count() & SOFTIRQ_MASK)
143 # define irq_count() (preempt_count() & (NMI_MASK | HARDIRQ_MASK | SOFTIRQ_MASK))
144 #endif
145
146 /*
147 * Macros to retrieve the current execution context:
148 *
149 * in_nmi() - We're in NMI context
150 * in_hardirq() - We're in hard IRQ context
151 * in_serving_softirq() - We're in softirq context
152 * in_task() - We're in task context
153 */
154 #define in_nmi() (nmi_count())
155 #define in_hardirq() (hardirq_count())
156 #define in_serving_softirq() (softirq_count() & SOFTIRQ_OFFSET)
157 #ifdef CONFIG_PREEMPT_RT
158 # define in_task() (!((preempt_count() & (NMI_MASK | HARDIRQ_MASK)) | in_serving_softirq()))
159 #else
160 # define in_task() (!(preempt_count() & (NMI_MASK | HARDIRQ_MASK | SOFTIRQ_OFFSET)))
161 #endif
162
163 /*
164 * The following macros are deprecated and should not be used in new code:
165 * in_softirq() - We have BH disabled, or are processing softirqs
166 * in_interrupt() - We're in NMI,IRQ,SoftIRQ context or have BH disabled
167 */
168 #define in_softirq() (softirq_count())
169 #define in_interrupt() (irq_count())
170
171 /*
172 * The preempt_count offset after preempt_disable();
173 */
174 #if defined(CONFIG_PREEMPT_COUNT)
175 # define PREEMPT_DISABLE_OFFSET PREEMPT_OFFSET
176 #else
177 # define PREEMPT_DISABLE_OFFSET 0
178 #endif
179
180 /*
181 * The preempt_count offset after spin_lock()
182 */
183 #if !defined(CONFIG_PREEMPT_RT)
184 #define PREEMPT_LOCK_OFFSET PREEMPT_DISABLE_OFFSET
185 #else
186 /* Locks on RT do not disable preemption */
187 #define PREEMPT_LOCK_OFFSET 0
188 #endif
189
190 /*
191 * The preempt_count offset needed for things like:
192 *
193 * spin_lock_bh()
194 *
195 * Which need to disable both preemption (CONFIG_PREEMPT_COUNT) and
196 * softirqs, such that unlock sequences of:
197 *
198 * spin_unlock();
199 * local_bh_enable();
200 *
201 * Work as expected.
202 */
203 #define SOFTIRQ_LOCK_OFFSET (SOFTIRQ_DISABLE_OFFSET + PREEMPT_LOCK_OFFSET)
204
205 /*
206 * Are we running in atomic context? WARNING: this macro cannot
207 * always detect atomic context; in particular, it cannot know about
208 * held spinlocks in non-preemptible kernels. Thus it should not be
209 * used in the general case to determine whether sleeping is possible.
210 * Do not use in_atomic() in driver code.
211 */
212 #define in_atomic() (preempt_count() != 0)
213
214 /*
215 * Check whether we were atomic before we did preempt_disable():
216 * (used by the scheduler)
217 */
218 #define in_atomic_preempt_off() (preempt_count() != PREEMPT_DISABLE_OFFSET)
219
220 #if defined(CONFIG_DEBUG_PREEMPT) || defined(CONFIG_TRACE_PREEMPT_TOGGLE)
221 extern void preempt_count_add(int val);
222 extern void preempt_count_sub(int val);
223 #define preempt_count_dec_and_test() \
224 ({ preempt_count_sub(1); should_resched(0); })
225 #else
226 #define preempt_count_add(val) __preempt_count_add(val)
227 #define preempt_count_sub(val) __preempt_count_sub(val)
228 #define preempt_count_dec_and_test() __preempt_count_dec_and_test()
229 #endif
230
231 #define __preempt_count_inc() __preempt_count_add(1)
232 #define __preempt_count_dec() __preempt_count_sub(1)
233
234 #define preempt_count_inc() preempt_count_add(1)
235 #define preempt_count_dec() preempt_count_sub(1)
236
237 #ifdef CONFIG_PREEMPT_COUNT
238
239 #define preempt_disable() \
240 do { \
241 preempt_count_inc(); \
242 barrier(); \
243 } while (0)
244
245 #define sched_preempt_enable_no_resched() \
246 do { \
247 barrier(); \
248 preempt_count_dec(); \
249 } while (0)
250
251 #define preempt_enable_no_resched() sched_preempt_enable_no_resched()
252
253 #define preemptible() (preempt_count() == 0 && !irqs_disabled())
254
255 #ifdef CONFIG_PREEMPTION
256 #define preempt_enable() \
257 do { \
258 barrier(); \
259 if (unlikely(preempt_count_dec_and_test())) \
260 __preempt_schedule(); \
261 } while (0)
262
263 #define preempt_enable_notrace() \
264 do { \
265 barrier(); \
266 if (unlikely(__preempt_count_dec_and_test())) \
267 __preempt_schedule_notrace(); \
268 } while (0)
269
270 #define preempt_check_resched() \
271 do { \
272 if (should_resched(0)) \
273 __preempt_schedule(); \
274 } while (0)
275
276 #else /* !CONFIG_PREEMPTION */
277 #define preempt_enable() \
278 do { \
279 barrier(); \
280 preempt_count_dec(); \
281 } while (0)
282
283 #define preempt_enable_notrace() \
284 do { \
285 barrier(); \
286 __preempt_count_dec(); \
287 } while (0)
288
289 #define preempt_check_resched() do { } while (0)
290 #endif /* CONFIG_PREEMPTION */
291
292 #define preempt_disable_notrace() \
293 do { \
294 __preempt_count_inc(); \
295 barrier(); \
296 } while (0)
297
298 #define preempt_enable_no_resched_notrace() \
299 do { \
300 barrier(); \
301 __preempt_count_dec(); \
302 } while (0)
303
304 #else /* !CONFIG_PREEMPT_COUNT */
305
306 /*
307 * Even if we don't have any preemption, we need preempt disable/enable
308 * to be barriers, so that we don't have things like get_user/put_user
309 * that can cause faults and scheduling migrate into our preempt-protected
310 * region.
311 */
312 #define preempt_disable() barrier()
313 #define sched_preempt_enable_no_resched() barrier()
314 #define preempt_enable_no_resched() barrier()
315 #define preempt_enable() barrier()
316 #define preempt_check_resched() do { } while (0)
317
318 #define preempt_disable_notrace() barrier()
319 #define preempt_enable_no_resched_notrace() barrier()
320 #define preempt_enable_notrace() barrier()
321 #define preemptible() 0
322
323 #endif /* CONFIG_PREEMPT_COUNT */
324
325 #ifdef MODULE
326 /*
327 * Modules have no business playing preemption tricks.
328 */
329 #undef sched_preempt_enable_no_resched
330 #undef preempt_enable_no_resched
331 #undef preempt_enable_no_resched_notrace
332 #undef preempt_check_resched
333 #endif
334
335 #define preempt_set_need_resched() \
336 do { \
337 set_preempt_need_resched(); \
338 } while (0)
339 #define preempt_fold_need_resched() \
340 do { \
341 if (tif_need_resched()) \
342 set_preempt_need_resched(); \
343 } while (0)
344
345 #ifdef CONFIG_PREEMPT_NOTIFIERS
346
347 struct preempt_notifier;
348 struct task_struct;
349
350 /**
351 * preempt_ops - notifiers called when a task is preempted and rescheduled
352 * @sched_in: we're about to be rescheduled:
353 * notifier: struct preempt_notifier for the task being scheduled
354 * cpu: cpu we're scheduled on
355 * @sched_out: we've just been preempted
356 * notifier: struct preempt_notifier for the task being preempted
357 * next: the task that's kicking us out
358 *
359 * Please note that sched_in and out are called under different
360 * contexts. sched_out is called with rq lock held and irq disabled
361 * while sched_in is called without rq lock and irq enabled. This
362 * difference is intentional and depended upon by its users.
363 */
364 struct preempt_ops {
365 void (*sched_in)(struct preempt_notifier *notifier, int cpu);
366 void (*sched_out)(struct preempt_notifier *notifier,
367 struct task_struct *next);
368 };
369
370 /**
371 * preempt_notifier - key for installing preemption notifiers
372 * @link: internal use
373 * @ops: defines the notifier functions to be called
374 *
375 * Usually used in conjunction with container_of().
376 */
377 struct preempt_notifier {
378 struct hlist_node link;
379 struct preempt_ops *ops;
380 };
381
382 void preempt_notifier_inc(void);
383 void preempt_notifier_dec(void);
384 void preempt_notifier_register(struct preempt_notifier *notifier);
385 void preempt_notifier_unregister(struct preempt_notifier *notifier);
386
preempt_notifier_init(struct preempt_notifier * notifier,struct preempt_ops * ops)387 static inline void preempt_notifier_init(struct preempt_notifier *notifier,
388 struct preempt_ops *ops)
389 {
390 /* INIT_HLIST_NODE() open coded, to avoid dependency on list.h */
391 notifier->link.next = NULL;
392 notifier->link.pprev = NULL;
393 notifier->ops = ops;
394 }
395
396 #endif
397
398 /*
399 * Migrate-Disable and why it is undesired.
400 *
401 * When a preempted task becomes eligible to run under the ideal model (IOW it
402 * becomes one of the M highest priority tasks), it might still have to wait
403 * for the preemptee's migrate_disable() section to complete. Thereby suffering
404 * a reduction in bandwidth in the exact duration of the migrate_disable()
405 * section.
406 *
407 * Per this argument, the change from preempt_disable() to migrate_disable()
408 * gets us:
409 *
410 * - a higher priority tasks gains reduced wake-up latency; with preempt_disable()
411 * it would have had to wait for the lower priority task.
412 *
413 * - a lower priority tasks; which under preempt_disable() could've instantly
414 * migrated away when another CPU becomes available, is now constrained
415 * by the ability to push the higher priority task away, which might itself be
416 * in a migrate_disable() section, reducing its available bandwidth.
417 *
418 * IOW it trades latency / moves the interference term, but it stays in the
419 * system, and as long as it remains unbounded, the system is not fully
420 * deterministic.
421 *
422 *
423 * The reason we have it anyway.
424 *
425 * PREEMPT_RT breaks a number of assumptions traditionally held. By forcing a
426 * number of primitives into becoming preemptible, they would also allow
427 * migration. This turns out to break a bunch of per-cpu usage. To this end,
428 * all these primitives employ migrate_disable() to restore this implicit
429 * assumption.
430 *
431 * This is a 'temporary' work-around at best. The correct solution is getting
432 * rid of the above assumptions and reworking the code to employ explicit
433 * per-cpu locking or short preempt-disable regions.
434 *
435 * The end goal must be to get rid of migrate_disable(), alternatively we need
436 * a schedulability theory that does not depend on arbitrary migration.
437 *
438 *
439 * Notes on the implementation.
440 *
441 * The implementation is particularly tricky since existing code patterns
442 * dictate neither migrate_disable() nor migrate_enable() is allowed to block.
443 * This means that it cannot use cpus_read_lock() to serialize against hotplug,
444 * nor can it easily migrate itself into a pending affinity mask change on
445 * migrate_enable().
446 *
447 *
448 * Note: even non-work-conserving schedulers like semi-partitioned depends on
449 * migration, so migrate_disable() is not only a problem for
450 * work-conserving schedulers.
451 *
452 */
453
454 /**
455 * preempt_disable_nested - Disable preemption inside a normally preempt disabled section
456 *
457 * Use for code which requires preemption protection inside a critical
458 * section which has preemption disabled implicitly on non-PREEMPT_RT
459 * enabled kernels, by e.g.:
460 * - holding a spinlock/rwlock
461 * - soft interrupt context
462 * - regular interrupt handlers
463 *
464 * On PREEMPT_RT enabled kernels spinlock/rwlock held sections, soft
465 * interrupt context and regular interrupt handlers are preemptible and
466 * only prevent migration. preempt_disable_nested() ensures that preemption
467 * is disabled for cases which require CPU local serialization even on
468 * PREEMPT_RT. For non-PREEMPT_RT kernels this is a NOP.
469 *
470 * The use cases are code sequences which are not serialized by a
471 * particular lock instance, e.g.:
472 * - seqcount write side critical sections where the seqcount is not
473 * associated to a particular lock and therefore the automatic
474 * protection mechanism does not work. This prevents a live lock
475 * against a preempting high priority reader.
476 * - RMW per CPU variable updates like vmstat.
477 */
478 /* Macro to avoid header recursion hell vs. lockdep */
479 #define preempt_disable_nested() \
480 do { \
481 if (IS_ENABLED(CONFIG_PREEMPT_RT)) \
482 preempt_disable(); \
483 else \
484 lockdep_assert_preemption_disabled(); \
485 } while (0)
486
487 /**
488 * preempt_enable_nested - Undo the effect of preempt_disable_nested()
489 */
preempt_enable_nested(void)490 static __always_inline void preempt_enable_nested(void)
491 {
492 if (IS_ENABLED(CONFIG_PREEMPT_RT))
493 preempt_enable();
494 }
495
496 DEFINE_LOCK_GUARD_0(preempt, preempt_disable(), preempt_enable())
497 DEFINE_LOCK_GUARD_0(preempt_notrace, preempt_disable_notrace(), preempt_enable_notrace())
498
499 #ifdef CONFIG_PREEMPT_DYNAMIC
500
501 extern bool preempt_model_none(void);
502 extern bool preempt_model_voluntary(void);
503 extern bool preempt_model_full(void);
504 extern bool preempt_model_lazy(void);
505
506 #else
507
508 static inline bool preempt_model_none(void)
509 {
510 return IS_ENABLED(CONFIG_PREEMPT_NONE);
511 }
512 static inline bool preempt_model_voluntary(void)
513 {
514 return IS_ENABLED(CONFIG_PREEMPT_VOLUNTARY);
515 }
516 static inline bool preempt_model_full(void)
517 {
518 return IS_ENABLED(CONFIG_PREEMPT);
519 }
520
521 static inline bool preempt_model_lazy(void)
522 {
523 return IS_ENABLED(CONFIG_PREEMPT_LAZY);
524 }
525
526 #endif
527
preempt_model_rt(void)528 static inline bool preempt_model_rt(void)
529 {
530 return IS_ENABLED(CONFIG_PREEMPT_RT);
531 }
532
533 extern const char *preempt_model_str(void);
534
535 /*
536 * Does the preemption model allow non-cooperative preemption?
537 *
538 * For !CONFIG_PREEMPT_DYNAMIC kernels this is an exact match with
539 * CONFIG_PREEMPTION; for CONFIG_PREEMPT_DYNAMIC this doesn't work as the
540 * kernel is *built* with CONFIG_PREEMPTION=y but may run with e.g. the
541 * PREEMPT_NONE model.
542 */
preempt_model_preemptible(void)543 static inline bool preempt_model_preemptible(void)
544 {
545 return preempt_model_full() || preempt_model_lazy() || preempt_model_rt();
546 }
547
548 #endif /* __LINUX_PREEMPT_H */
549