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 | */ |
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 | |
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 | |
433 | static inline void migrate_disable(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 | */ |
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 | #endif /* __LINUX_PREEMPT_H */ |
485 | |