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/list.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(&notifier->link);
364 	notifier->ops = ops;
365 }
366 
367 #endif
368 
369 #ifdef CONFIG_SMP
370 
371 /*
372  * Migrate-Disable and why it is undesired.
373  *
374  * When a preempted task becomes elegible to run under the ideal model (IOW it
375  * becomes one of the M highest priority tasks), it might still have to wait
376  * for the preemptee's migrate_disable() section to complete. Thereby suffering
377  * a reduction in bandwidth in the exact duration of the migrate_disable()
378  * section.
379  *
380  * Per this argument, the change from preempt_disable() to migrate_disable()
381  * gets us:
382  *
383  * - a higher priority tasks gains reduced wake-up latency; with preempt_disable()
384  *   it would have had to wait for the lower priority task.
385  *
386  * - a lower priority tasks; which under preempt_disable() could've instantly
387  *   migrated away when another CPU becomes available, is now constrained
388  *   by the ability to push the higher priority task away, which might itself be
389  *   in a migrate_disable() section, reducing it's available bandwidth.
390  *
391  * IOW it trades latency / moves the interference term, but it stays in the
392  * system, and as long as it remains unbounded, the system is not fully
393  * deterministic.
394  *
395  *
396  * The reason we have it anyway.
397  *
398  * PREEMPT_RT breaks a number of assumptions traditionally held. By forcing a
399  * number of primitives into becoming preemptible, they would also allow
400  * migration. This turns out to break a bunch of per-cpu usage. To this end,
401  * all these primitives employ migirate_disable() to restore this implicit
402  * assumption.
403  *
404  * This is a 'temporary' work-around at best. The correct solution is getting
405  * rid of the above assumptions and reworking the code to employ explicit
406  * per-cpu locking or short preempt-disable regions.
407  *
408  * The end goal must be to get rid of migrate_disable(), alternatively we need
409  * a schedulability theory that does not depend on abritrary migration.
410  *
411  *
412  * Notes on the implementation.
413  *
414  * The implementation is particularly tricky since existing code patterns
415  * dictate neither migrate_disable() nor migrate_enable() is allowed to block.
416  * This means that it cannot use cpus_read_lock() to serialize against hotplug,
417  * nor can it easily migrate itself into a pending affinity mask change on
418  * migrate_enable().
419  *
420  *
421  * Note: even non-work-conserving schedulers like semi-partitioned depends on
422  *       migration, so migrate_disable() is not only a problem for
423  *       work-conserving schedulers.
424  *
425  */
426 extern void migrate_disable(void);
427 extern void migrate_enable(void);
428 
429 #else
430 
migrate_disable(void)431 static inline void migrate_disable(void) { }
migrate_enable(void)432 static inline void migrate_enable(void) { }
433 
434 #endif /* CONFIG_SMP */
435 
436 /**
437  * preempt_disable_nested - Disable preemption inside a normally preempt disabled section
438  *
439  * Use for code which requires preemption protection inside a critical
440  * section which has preemption disabled implicitly on non-PREEMPT_RT
441  * enabled kernels, by e.g.:
442  *  - holding a spinlock/rwlock
443  *  - soft interrupt context
444  *  - regular interrupt handlers
445  *
446  * On PREEMPT_RT enabled kernels spinlock/rwlock held sections, soft
447  * interrupt context and regular interrupt handlers are preemptible and
448  * only prevent migration. preempt_disable_nested() ensures that preemption
449  * is disabled for cases which require CPU local serialization even on
450  * PREEMPT_RT. For non-PREEMPT_RT kernels this is a NOP.
451  *
452  * The use cases are code sequences which are not serialized by a
453  * particular lock instance, e.g.:
454  *  - seqcount write side critical sections where the seqcount is not
455  *    associated to a particular lock and therefore the automatic
456  *    protection mechanism does not work. This prevents a live lock
457  *    against a preempting high priority reader.
458  *  - RMW per CPU variable updates like vmstat.
459  */
460 /* Macro to avoid header recursion hell vs. lockdep */
461 #define preempt_disable_nested()				\
462 do {								\
463 	if (IS_ENABLED(CONFIG_PREEMPT_RT))			\
464 		preempt_disable();				\
465 	else							\
466 		lockdep_assert_preemption_disabled();		\
467 } while (0)
468 
469 /**
470  * preempt_enable_nested - Undo the effect of preempt_disable_nested()
471  */
preempt_enable_nested(void)472 static __always_inline void preempt_enable_nested(void)
473 {
474 	if (IS_ENABLED(CONFIG_PREEMPT_RT))
475 		preempt_enable();
476 }
477 
478 DEFINE_LOCK_GUARD_0(preempt, preempt_disable(), preempt_enable())
479 DEFINE_LOCK_GUARD_0(preempt_notrace, preempt_disable_notrace(), preempt_enable_notrace())
480 DEFINE_LOCK_GUARD_0(migrate, migrate_disable(), migrate_enable())
481 
482 #endif /* __LINUX_PREEMPT_H */
483