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 #endif /* __LINUX_PREEMPT_H */
485