1 /* SPDX-License-Identifier: GPL-2.0+ */
2 /*
3  * Read-Copy Update mechanism for mutual exclusion
4  *
5  * Copyright IBM Corporation, 2001
6  *
7  * Author: Dipankar Sarma <dipankar@in.ibm.com>
8  *
9  * Based on the original work by Paul McKenney <paulmck@vnet.ibm.com>
10  * and inputs from Rusty Russell, Andrea Arcangeli and Andi Kleen.
11  * Papers:
12  * http://www.rdrop.com/users/paulmck/paper/rclockpdcsproof.pdf
13  * http://lse.sourceforge.net/locking/rclock_OLS.2001.05.01c.sc.pdf (OLS2001)
14  *
15  * For detailed explanation of Read-Copy Update mechanism see -
16  *		http://lse.sourceforge.net/locking/rcupdate.html
17  *
18  */
19 
20 #ifndef __LINUX_RCUPDATE_H
21 #define __LINUX_RCUPDATE_H
22 
23 #include <linux/types.h>
24 #include <linux/compiler.h>
25 #include <linux/atomic.h>
26 #include <linux/irqflags.h>
27 #include <linux/preempt.h>
28 #include <linux/bottom_half.h>
29 #include <linux/lockdep.h>
30 #include <asm/processor.h>
31 #include <linux/cpumask.h>
32 #include <linux/context_tracking_irq.h>
33 
34 #define ULONG_CMP_GE(a, b)	(ULONG_MAX / 2 >= (a) - (b))
35 #define ULONG_CMP_LT(a, b)	(ULONG_MAX / 2 < (a) - (b))
36 #define ulong2long(a)		(*(long *)(&(a)))
37 #define USHORT_CMP_GE(a, b)	(USHRT_MAX / 2 >= (unsigned short)((a) - (b)))
38 #define USHORT_CMP_LT(a, b)	(USHRT_MAX / 2 < (unsigned short)((a) - (b)))
39 
40 /* Exported common interfaces */
41 void call_rcu(struct rcu_head *head, rcu_callback_t func);
42 void rcu_barrier_tasks(void);
43 void rcu_barrier_tasks_rude(void);
44 void synchronize_rcu(void);
45 
46 struct rcu_gp_oldstate;
47 unsigned long get_completed_synchronize_rcu(void);
48 void get_completed_synchronize_rcu_full(struct rcu_gp_oldstate *rgosp);
49 
50 // Maximum number of unsigned long values corresponding to
51 // not-yet-completed RCU grace periods.
52 #define NUM_ACTIVE_RCU_POLL_OLDSTATE 2
53 
54 /**
55  * same_state_synchronize_rcu - Are two old-state values identical?
56  * @oldstate1: First old-state value.
57  * @oldstate2: Second old-state value.
58  *
59  * The two old-state values must have been obtained from either
60  * get_state_synchronize_rcu(), start_poll_synchronize_rcu(), or
61  * get_completed_synchronize_rcu().  Returns @true if the two values are
62  * identical and @false otherwise.  This allows structures whose lifetimes
63  * are tracked by old-state values to push these values to a list header,
64  * allowing those structures to be slightly smaller.
65  */
same_state_synchronize_rcu(unsigned long oldstate1,unsigned long oldstate2)66 static inline bool same_state_synchronize_rcu(unsigned long oldstate1, unsigned long oldstate2)
67 {
68 	return oldstate1 == oldstate2;
69 }
70 
71 #ifdef CONFIG_PREEMPT_RCU
72 
73 void __rcu_read_lock(void);
74 void __rcu_read_unlock(void);
75 
76 /*
77  * Defined as a macro as it is a very low level header included from
78  * areas that don't even know about current.  This gives the rcu_read_lock()
79  * nesting depth, but makes sense only if CONFIG_PREEMPT_RCU -- in other
80  * types of kernel builds, the rcu_read_lock() nesting depth is unknowable.
81  */
82 #define rcu_preempt_depth() READ_ONCE(current->rcu_read_lock_nesting)
83 
84 #else /* #ifdef CONFIG_PREEMPT_RCU */
85 
86 #ifdef CONFIG_TINY_RCU
87 #define rcu_read_unlock_strict() do { } while (0)
88 #else
89 void rcu_read_unlock_strict(void);
90 #endif
91 
__rcu_read_lock(void)92 static inline void __rcu_read_lock(void)
93 {
94 	preempt_disable();
95 }
96 
__rcu_read_unlock(void)97 static inline void __rcu_read_unlock(void)
98 {
99 	preempt_enable();
100 	if (IS_ENABLED(CONFIG_RCU_STRICT_GRACE_PERIOD))
101 		rcu_read_unlock_strict();
102 }
103 
rcu_preempt_depth(void)104 static inline int rcu_preempt_depth(void)
105 {
106 	return 0;
107 }
108 
109 #endif /* #else #ifdef CONFIG_PREEMPT_RCU */
110 
111 /* Internal to kernel */
112 void rcu_init(void);
113 extern int rcu_scheduler_active;
114 void rcu_sched_clock_irq(int user);
115 void rcu_report_dead(unsigned int cpu);
116 void rcutree_migrate_callbacks(int cpu);
117 
118 #ifdef CONFIG_TASKS_RCU_GENERIC
119 void rcu_init_tasks_generic(void);
120 #else
rcu_init_tasks_generic(void)121 static inline void rcu_init_tasks_generic(void) { }
122 #endif
123 
124 #ifdef CONFIG_RCU_STALL_COMMON
125 void rcu_sysrq_start(void);
126 void rcu_sysrq_end(void);
127 #else /* #ifdef CONFIG_RCU_STALL_COMMON */
rcu_sysrq_start(void)128 static inline void rcu_sysrq_start(void) { }
rcu_sysrq_end(void)129 static inline void rcu_sysrq_end(void) { }
130 #endif /* #else #ifdef CONFIG_RCU_STALL_COMMON */
131 
132 #if defined(CONFIG_NO_HZ_FULL) && (!defined(CONFIG_GENERIC_ENTRY) || !defined(CONFIG_KVM_XFER_TO_GUEST_WORK))
133 void rcu_irq_work_resched(void);
134 #else
rcu_irq_work_resched(void)135 static inline void rcu_irq_work_resched(void) { }
136 #endif
137 
138 #ifdef CONFIG_RCU_NOCB_CPU
139 void rcu_init_nohz(void);
140 int rcu_nocb_cpu_offload(int cpu);
141 int rcu_nocb_cpu_deoffload(int cpu);
142 void rcu_nocb_flush_deferred_wakeup(void);
143 #else /* #ifdef CONFIG_RCU_NOCB_CPU */
rcu_init_nohz(void)144 static inline void rcu_init_nohz(void) { }
rcu_nocb_cpu_offload(int cpu)145 static inline int rcu_nocb_cpu_offload(int cpu) { return -EINVAL; }
rcu_nocb_cpu_deoffload(int cpu)146 static inline int rcu_nocb_cpu_deoffload(int cpu) { return 0; }
rcu_nocb_flush_deferred_wakeup(void)147 static inline void rcu_nocb_flush_deferred_wakeup(void) { }
148 #endif /* #else #ifdef CONFIG_RCU_NOCB_CPU */
149 
150 /**
151  * RCU_NONIDLE - Indicate idle-loop code that needs RCU readers
152  * @a: Code that RCU needs to pay attention to.
153  *
154  * RCU read-side critical sections are forbidden in the inner idle loop,
155  * that is, between the ct_idle_enter() and the ct_idle_exit() -- RCU
156  * will happily ignore any such read-side critical sections.  However,
157  * things like powertop need tracepoints in the inner idle loop.
158  *
159  * This macro provides the way out:  RCU_NONIDLE(do_something_with_RCU())
160  * will tell RCU that it needs to pay attention, invoke its argument
161  * (in this example, calling the do_something_with_RCU() function),
162  * and then tell RCU to go back to ignoring this CPU.  It is permissible
163  * to nest RCU_NONIDLE() wrappers, but not indefinitely (but the limit is
164  * on the order of a million or so, even on 32-bit systems).  It is
165  * not legal to block within RCU_NONIDLE(), nor is it permissible to
166  * transfer control either into or out of RCU_NONIDLE()'s statement.
167  */
168 #define RCU_NONIDLE(a) \
169 	do { \
170 		ct_irq_enter_irqson(); \
171 		do { a; } while (0); \
172 		ct_irq_exit_irqson(); \
173 	} while (0)
174 
175 /*
176  * Note a quasi-voluntary context switch for RCU-tasks's benefit.
177  * This is a macro rather than an inline function to avoid #include hell.
178  */
179 #ifdef CONFIG_TASKS_RCU_GENERIC
180 
181 # ifdef CONFIG_TASKS_RCU
182 # define rcu_tasks_classic_qs(t, preempt)				\
183 	do {								\
184 		if (!(preempt) && READ_ONCE((t)->rcu_tasks_holdout))	\
185 			WRITE_ONCE((t)->rcu_tasks_holdout, false);	\
186 	} while (0)
187 void call_rcu_tasks(struct rcu_head *head, rcu_callback_t func);
188 void synchronize_rcu_tasks(void);
189 # else
190 # define rcu_tasks_classic_qs(t, preempt) do { } while (0)
191 # define call_rcu_tasks call_rcu
192 # define synchronize_rcu_tasks synchronize_rcu
193 # endif
194 
195 # ifdef CONFIG_TASKS_TRACE_RCU
196 // Bits for ->trc_reader_special.b.need_qs field.
197 #define TRC_NEED_QS		0x1  // Task needs a quiescent state.
198 #define TRC_NEED_QS_CHECKED	0x2  // Task has been checked for needing quiescent state.
199 
200 u8 rcu_trc_cmpxchg_need_qs(struct task_struct *t, u8 old, u8 new);
201 void rcu_tasks_trace_qs_blkd(struct task_struct *t);
202 
203 # define rcu_tasks_trace_qs(t)							\
204 	do {									\
205 		int ___rttq_nesting = READ_ONCE((t)->trc_reader_nesting);	\
206 										\
207 		if (likely(!READ_ONCE((t)->trc_reader_special.b.need_qs)) &&	\
208 		    likely(!___rttq_nesting)) {					\
209 			rcu_trc_cmpxchg_need_qs((t), 0,	TRC_NEED_QS_CHECKED);	\
210 		} else if (___rttq_nesting && ___rttq_nesting != INT_MIN &&	\
211 			   !READ_ONCE((t)->trc_reader_special.b.blocked)) {	\
212 			rcu_tasks_trace_qs_blkd(t);				\
213 		}								\
214 	} while (0)
215 # else
216 # define rcu_tasks_trace_qs(t) do { } while (0)
217 # endif
218 
219 #define rcu_tasks_qs(t, preempt)					\
220 do {									\
221 	rcu_tasks_classic_qs((t), (preempt));				\
222 	rcu_tasks_trace_qs(t);						\
223 } while (0)
224 
225 # ifdef CONFIG_TASKS_RUDE_RCU
226 void call_rcu_tasks_rude(struct rcu_head *head, rcu_callback_t func);
227 void synchronize_rcu_tasks_rude(void);
228 # endif
229 
230 #define rcu_note_voluntary_context_switch(t) rcu_tasks_qs(t, false)
231 void exit_tasks_rcu_start(void);
232 void exit_tasks_rcu_finish(void);
233 #else /* #ifdef CONFIG_TASKS_RCU_GENERIC */
234 #define rcu_tasks_classic_qs(t, preempt) do { } while (0)
235 #define rcu_tasks_qs(t, preempt) do { } while (0)
236 #define rcu_note_voluntary_context_switch(t) do { } while (0)
237 #define call_rcu_tasks call_rcu
238 #define synchronize_rcu_tasks synchronize_rcu
exit_tasks_rcu_start(void)239 static inline void exit_tasks_rcu_start(void) { }
exit_tasks_rcu_finish(void)240 static inline void exit_tasks_rcu_finish(void) { }
241 #endif /* #else #ifdef CONFIG_TASKS_RCU_GENERIC */
242 
243 /**
244  * cond_resched_tasks_rcu_qs - Report potential quiescent states to RCU
245  *
246  * This macro resembles cond_resched(), except that it is defined to
247  * report potential quiescent states to RCU-tasks even if the cond_resched()
248  * machinery were to be shut off, as some advocate for PREEMPTION kernels.
249  */
250 #define cond_resched_tasks_rcu_qs() \
251 do { \
252 	rcu_tasks_qs(current, false); \
253 	cond_resched(); \
254 } while (0)
255 
256 /*
257  * Infrastructure to implement the synchronize_() primitives in
258  * TREE_RCU and rcu_barrier_() primitives in TINY_RCU.
259  */
260 
261 #if defined(CONFIG_TREE_RCU)
262 #include <linux/rcutree.h>
263 #elif defined(CONFIG_TINY_RCU)
264 #include <linux/rcutiny.h>
265 #else
266 #error "Unknown RCU implementation specified to kernel configuration"
267 #endif
268 
269 /*
270  * The init_rcu_head_on_stack() and destroy_rcu_head_on_stack() calls
271  * are needed for dynamic initialization and destruction of rcu_head
272  * on the stack, and init_rcu_head()/destroy_rcu_head() are needed for
273  * dynamic initialization and destruction of statically allocated rcu_head
274  * structures.  However, rcu_head structures allocated dynamically in the
275  * heap don't need any initialization.
276  */
277 #ifdef CONFIG_DEBUG_OBJECTS_RCU_HEAD
278 void init_rcu_head(struct rcu_head *head);
279 void destroy_rcu_head(struct rcu_head *head);
280 void init_rcu_head_on_stack(struct rcu_head *head);
281 void destroy_rcu_head_on_stack(struct rcu_head *head);
282 #else /* !CONFIG_DEBUG_OBJECTS_RCU_HEAD */
init_rcu_head(struct rcu_head * head)283 static inline void init_rcu_head(struct rcu_head *head) { }
destroy_rcu_head(struct rcu_head * head)284 static inline void destroy_rcu_head(struct rcu_head *head) { }
init_rcu_head_on_stack(struct rcu_head * head)285 static inline void init_rcu_head_on_stack(struct rcu_head *head) { }
destroy_rcu_head_on_stack(struct rcu_head * head)286 static inline void destroy_rcu_head_on_stack(struct rcu_head *head) { }
287 #endif	/* #else !CONFIG_DEBUG_OBJECTS_RCU_HEAD */
288 
289 #if defined(CONFIG_HOTPLUG_CPU) && defined(CONFIG_PROVE_RCU)
290 bool rcu_lockdep_current_cpu_online(void);
291 #else /* #if defined(CONFIG_HOTPLUG_CPU) && defined(CONFIG_PROVE_RCU) */
rcu_lockdep_current_cpu_online(void)292 static inline bool rcu_lockdep_current_cpu_online(void) { return true; }
293 #endif /* #else #if defined(CONFIG_HOTPLUG_CPU) && defined(CONFIG_PROVE_RCU) */
294 
295 extern struct lockdep_map rcu_lock_map;
296 extern struct lockdep_map rcu_bh_lock_map;
297 extern struct lockdep_map rcu_sched_lock_map;
298 extern struct lockdep_map rcu_callback_map;
299 
300 #ifdef CONFIG_DEBUG_LOCK_ALLOC
301 
rcu_lock_acquire(struct lockdep_map * map)302 static inline void rcu_lock_acquire(struct lockdep_map *map)
303 {
304 	lock_acquire(map, 0, 0, 2, 0, NULL, _THIS_IP_);
305 }
306 
rcu_lock_release(struct lockdep_map * map)307 static inline void rcu_lock_release(struct lockdep_map *map)
308 {
309 	lock_release(map, _THIS_IP_);
310 }
311 
312 int debug_lockdep_rcu_enabled(void);
313 int rcu_read_lock_held(void);
314 int rcu_read_lock_bh_held(void);
315 int rcu_read_lock_sched_held(void);
316 int rcu_read_lock_any_held(void);
317 
318 #else /* #ifdef CONFIG_DEBUG_LOCK_ALLOC */
319 
320 # define rcu_lock_acquire(a)		do { } while (0)
321 # define rcu_lock_release(a)		do { } while (0)
322 
rcu_read_lock_held(void)323 static inline int rcu_read_lock_held(void)
324 {
325 	return 1;
326 }
327 
rcu_read_lock_bh_held(void)328 static inline int rcu_read_lock_bh_held(void)
329 {
330 	return 1;
331 }
332 
rcu_read_lock_sched_held(void)333 static inline int rcu_read_lock_sched_held(void)
334 {
335 	return !preemptible();
336 }
337 
rcu_read_lock_any_held(void)338 static inline int rcu_read_lock_any_held(void)
339 {
340 	return !preemptible();
341 }
342 
343 #endif /* #else #ifdef CONFIG_DEBUG_LOCK_ALLOC */
344 
345 #ifdef CONFIG_PROVE_RCU
346 
347 /**
348  * RCU_LOCKDEP_WARN - emit lockdep splat if specified condition is met
349  * @c: condition to check
350  * @s: informative message
351  */
352 #define RCU_LOCKDEP_WARN(c, s)						\
353 	do {								\
354 		static bool __section(".data.unlikely") __warned;	\
355 		if ((c) && debug_lockdep_rcu_enabled() && !__warned) {	\
356 			__warned = true;				\
357 			lockdep_rcu_suspicious(__FILE__, __LINE__, s);	\
358 		}							\
359 	} while (0)
360 
361 #if defined(CONFIG_PROVE_RCU) && !defined(CONFIG_PREEMPT_RCU)
rcu_preempt_sleep_check(void)362 static inline void rcu_preempt_sleep_check(void)
363 {
364 	RCU_LOCKDEP_WARN(lock_is_held(&rcu_lock_map),
365 			 "Illegal context switch in RCU read-side critical section");
366 }
367 #else /* #ifdef CONFIG_PROVE_RCU */
rcu_preempt_sleep_check(void)368 static inline void rcu_preempt_sleep_check(void) { }
369 #endif /* #else #ifdef CONFIG_PROVE_RCU */
370 
371 #define rcu_sleep_check()						\
372 	do {								\
373 		rcu_preempt_sleep_check();				\
374 		if (!IS_ENABLED(CONFIG_PREEMPT_RT))			\
375 		    RCU_LOCKDEP_WARN(lock_is_held(&rcu_bh_lock_map),	\
376 				 "Illegal context switch in RCU-bh read-side critical section"); \
377 		RCU_LOCKDEP_WARN(lock_is_held(&rcu_sched_lock_map),	\
378 				 "Illegal context switch in RCU-sched read-side critical section"); \
379 	} while (0)
380 
381 #else /* #ifdef CONFIG_PROVE_RCU */
382 
383 #define RCU_LOCKDEP_WARN(c, s) do { } while (0 && (c))
384 #define rcu_sleep_check() do { } while (0)
385 
386 #endif /* #else #ifdef CONFIG_PROVE_RCU */
387 
388 /*
389  * Helper functions for rcu_dereference_check(), rcu_dereference_protected()
390  * and rcu_assign_pointer().  Some of these could be folded into their
391  * callers, but they are left separate in order to ease introduction of
392  * multiple pointers markings to match different RCU implementations
393  * (e.g., __srcu), should this make sense in the future.
394  */
395 
396 #ifdef __CHECKER__
397 #define rcu_check_sparse(p, space) \
398 	((void)(((typeof(*p) space *)p) == p))
399 #else /* #ifdef __CHECKER__ */
400 #define rcu_check_sparse(p, space)
401 #endif /* #else #ifdef __CHECKER__ */
402 
403 #define __unrcu_pointer(p, local)					\
404 ({									\
405 	typeof(*p) *local = (typeof(*p) *__force)(p);			\
406 	rcu_check_sparse(p, __rcu);					\
407 	((typeof(*p) __force __kernel *)(local)); 			\
408 })
409 /**
410  * unrcu_pointer - mark a pointer as not being RCU protected
411  * @p: pointer needing to lose its __rcu property
412  *
413  * Converts @p from an __rcu pointer to a __kernel pointer.
414  * This allows an __rcu pointer to be used with xchg() and friends.
415  */
416 #define unrcu_pointer(p) __unrcu_pointer(p, __UNIQUE_ID(rcu))
417 
418 #define __rcu_access_pointer(p, local, space) \
419 ({ \
420 	typeof(*p) *local = (typeof(*p) *__force)READ_ONCE(p); \
421 	rcu_check_sparse(p, space); \
422 	((typeof(*p) __force __kernel *)(local)); \
423 })
424 #define __rcu_dereference_check(p, local, c, space) \
425 ({ \
426 	/* Dependency order vs. p above. */ \
427 	typeof(*p) *local = (typeof(*p) *__force)READ_ONCE(p); \
428 	RCU_LOCKDEP_WARN(!(c), "suspicious rcu_dereference_check() usage"); \
429 	rcu_check_sparse(p, space); \
430 	((typeof(*p) __force __kernel *)(local)); \
431 })
432 #define __rcu_dereference_protected(p, local, c, space) \
433 ({ \
434 	RCU_LOCKDEP_WARN(!(c), "suspicious rcu_dereference_protected() usage"); \
435 	rcu_check_sparse(p, space); \
436 	((typeof(*p) __force __kernel *)(p)); \
437 })
438 #define __rcu_dereference_raw(p, local) \
439 ({ \
440 	/* Dependency order vs. p above. */ \
441 	typeof(p) local = READ_ONCE(p); \
442 	((typeof(*p) __force __kernel *)(local)); \
443 })
444 #define rcu_dereference_raw(p) __rcu_dereference_raw(p, __UNIQUE_ID(rcu))
445 
446 /**
447  * RCU_INITIALIZER() - statically initialize an RCU-protected global variable
448  * @v: The value to statically initialize with.
449  */
450 #define RCU_INITIALIZER(v) (typeof(*(v)) __force __rcu *)(v)
451 
452 /**
453  * rcu_assign_pointer() - assign to RCU-protected pointer
454  * @p: pointer to assign to
455  * @v: value to assign (publish)
456  *
457  * Assigns the specified value to the specified RCU-protected
458  * pointer, ensuring that any concurrent RCU readers will see
459  * any prior initialization.
460  *
461  * Inserts memory barriers on architectures that require them
462  * (which is most of them), and also prevents the compiler from
463  * reordering the code that initializes the structure after the pointer
464  * assignment.  More importantly, this call documents which pointers
465  * will be dereferenced by RCU read-side code.
466  *
467  * In some special cases, you may use RCU_INIT_POINTER() instead
468  * of rcu_assign_pointer().  RCU_INIT_POINTER() is a bit faster due
469  * to the fact that it does not constrain either the CPU or the compiler.
470  * That said, using RCU_INIT_POINTER() when you should have used
471  * rcu_assign_pointer() is a very bad thing that results in
472  * impossible-to-diagnose memory corruption.  So please be careful.
473  * See the RCU_INIT_POINTER() comment header for details.
474  *
475  * Note that rcu_assign_pointer() evaluates each of its arguments only
476  * once, appearances notwithstanding.  One of the "extra" evaluations
477  * is in typeof() and the other visible only to sparse (__CHECKER__),
478  * neither of which actually execute the argument.  As with most cpp
479  * macros, this execute-arguments-only-once property is important, so
480  * please be careful when making changes to rcu_assign_pointer() and the
481  * other macros that it invokes.
482  */
483 #define rcu_assign_pointer(p, v)					      \
484 do {									      \
485 	uintptr_t _r_a_p__v = (uintptr_t)(v);				      \
486 	rcu_check_sparse(p, __rcu);					      \
487 									      \
488 	if (__builtin_constant_p(v) && (_r_a_p__v) == (uintptr_t)NULL)	      \
489 		WRITE_ONCE((p), (typeof(p))(_r_a_p__v));		      \
490 	else								      \
491 		smp_store_release(&p, RCU_INITIALIZER((typeof(p))_r_a_p__v)); \
492 } while (0)
493 
494 /**
495  * rcu_replace_pointer() - replace an RCU pointer, returning its old value
496  * @rcu_ptr: RCU pointer, whose old value is returned
497  * @ptr: regular pointer
498  * @c: the lockdep conditions under which the dereference will take place
499  *
500  * Perform a replacement, where @rcu_ptr is an RCU-annotated
501  * pointer and @c is the lockdep argument that is passed to the
502  * rcu_dereference_protected() call used to read that pointer.  The old
503  * value of @rcu_ptr is returned, and @rcu_ptr is set to @ptr.
504  */
505 #define rcu_replace_pointer(rcu_ptr, ptr, c)				\
506 ({									\
507 	typeof(ptr) __tmp = rcu_dereference_protected((rcu_ptr), (c));	\
508 	rcu_assign_pointer((rcu_ptr), (ptr));				\
509 	__tmp;								\
510 })
511 
512 /**
513  * rcu_access_pointer() - fetch RCU pointer with no dereferencing
514  * @p: The pointer to read
515  *
516  * Return the value of the specified RCU-protected pointer, but omit the
517  * lockdep checks for being in an RCU read-side critical section.  This is
518  * useful when the value of this pointer is accessed, but the pointer is
519  * not dereferenced, for example, when testing an RCU-protected pointer
520  * against NULL.  Although rcu_access_pointer() may also be used in cases
521  * where update-side locks prevent the value of the pointer from changing,
522  * you should instead use rcu_dereference_protected() for this use case.
523  * Within an RCU read-side critical section, there is little reason to
524  * use rcu_access_pointer().
525  *
526  * It is usually best to test the rcu_access_pointer() return value
527  * directly in order to avoid accidental dereferences being introduced
528  * by later inattentive changes.  In other words, assigning the
529  * rcu_access_pointer() return value to a local variable results in an
530  * accident waiting to happen.
531  *
532  * It is also permissible to use rcu_access_pointer() when read-side
533  * access to the pointer was removed at least one grace period ago, as is
534  * the case in the context of the RCU callback that is freeing up the data,
535  * or after a synchronize_rcu() returns.  This can be useful when tearing
536  * down multi-linked structures after a grace period has elapsed.  However,
537  * rcu_dereference_protected() is normally preferred for this use case.
538  */
539 #define rcu_access_pointer(p) __rcu_access_pointer((p), __UNIQUE_ID(rcu), __rcu)
540 
541 /**
542  * rcu_dereference_check() - rcu_dereference with debug checking
543  * @p: The pointer to read, prior to dereferencing
544  * @c: The conditions under which the dereference will take place
545  *
546  * Do an rcu_dereference(), but check that the conditions under which the
547  * dereference will take place are correct.  Typically the conditions
548  * indicate the various locking conditions that should be held at that
549  * point.  The check should return true if the conditions are satisfied.
550  * An implicit check for being in an RCU read-side critical section
551  * (rcu_read_lock()) is included.
552  *
553  * For example:
554  *
555  *	bar = rcu_dereference_check(foo->bar, lockdep_is_held(&foo->lock));
556  *
557  * could be used to indicate to lockdep that foo->bar may only be dereferenced
558  * if either rcu_read_lock() is held, or that the lock required to replace
559  * the bar struct at foo->bar is held.
560  *
561  * Note that the list of conditions may also include indications of when a lock
562  * need not be held, for example during initialisation or destruction of the
563  * target struct:
564  *
565  *	bar = rcu_dereference_check(foo->bar, lockdep_is_held(&foo->lock) ||
566  *					      atomic_read(&foo->usage) == 0);
567  *
568  * Inserts memory barriers on architectures that require them
569  * (currently only the Alpha), prevents the compiler from refetching
570  * (and from merging fetches), and, more importantly, documents exactly
571  * which pointers are protected by RCU and checks that the pointer is
572  * annotated as __rcu.
573  */
574 #define rcu_dereference_check(p, c) \
575 	__rcu_dereference_check((p), __UNIQUE_ID(rcu), \
576 				(c) || rcu_read_lock_held(), __rcu)
577 
578 /**
579  * rcu_dereference_bh_check() - rcu_dereference_bh with debug checking
580  * @p: The pointer to read, prior to dereferencing
581  * @c: The conditions under which the dereference will take place
582  *
583  * This is the RCU-bh counterpart to rcu_dereference_check().  However,
584  * please note that starting in v5.0 kernels, vanilla RCU grace periods
585  * wait for local_bh_disable() regions of code in addition to regions of
586  * code demarked by rcu_read_lock() and rcu_read_unlock().  This means
587  * that synchronize_rcu(), call_rcu, and friends all take not only
588  * rcu_read_lock() but also rcu_read_lock_bh() into account.
589  */
590 #define rcu_dereference_bh_check(p, c) \
591 	__rcu_dereference_check((p), __UNIQUE_ID(rcu), \
592 				(c) || rcu_read_lock_bh_held(), __rcu)
593 
594 /**
595  * rcu_dereference_sched_check() - rcu_dereference_sched with debug checking
596  * @p: The pointer to read, prior to dereferencing
597  * @c: The conditions under which the dereference will take place
598  *
599  * This is the RCU-sched counterpart to rcu_dereference_check().
600  * However, please note that starting in v5.0 kernels, vanilla RCU grace
601  * periods wait for preempt_disable() regions of code in addition to
602  * regions of code demarked by rcu_read_lock() and rcu_read_unlock().
603  * This means that synchronize_rcu(), call_rcu, and friends all take not
604  * only rcu_read_lock() but also rcu_read_lock_sched() into account.
605  */
606 #define rcu_dereference_sched_check(p, c) \
607 	__rcu_dereference_check((p), __UNIQUE_ID(rcu), \
608 				(c) || rcu_read_lock_sched_held(), \
609 				__rcu)
610 
611 /*
612  * The tracing infrastructure traces RCU (we want that), but unfortunately
613  * some of the RCU checks causes tracing to lock up the system.
614  *
615  * The no-tracing version of rcu_dereference_raw() must not call
616  * rcu_read_lock_held().
617  */
618 #define rcu_dereference_raw_check(p) \
619 	__rcu_dereference_check((p), __UNIQUE_ID(rcu), 1, __rcu)
620 
621 /**
622  * rcu_dereference_protected() - fetch RCU pointer when updates prevented
623  * @p: The pointer to read, prior to dereferencing
624  * @c: The conditions under which the dereference will take place
625  *
626  * Return the value of the specified RCU-protected pointer, but omit
627  * the READ_ONCE().  This is useful in cases where update-side locks
628  * prevent the value of the pointer from changing.  Please note that this
629  * primitive does *not* prevent the compiler from repeating this reference
630  * or combining it with other references, so it should not be used without
631  * protection of appropriate locks.
632  *
633  * This function is only for update-side use.  Using this function
634  * when protected only by rcu_read_lock() will result in infrequent
635  * but very ugly failures.
636  */
637 #define rcu_dereference_protected(p, c) \
638 	__rcu_dereference_protected((p), __UNIQUE_ID(rcu), (c), __rcu)
639 
640 
641 /**
642  * rcu_dereference() - fetch RCU-protected pointer for dereferencing
643  * @p: The pointer to read, prior to dereferencing
644  *
645  * This is a simple wrapper around rcu_dereference_check().
646  */
647 #define rcu_dereference(p) rcu_dereference_check(p, 0)
648 
649 /**
650  * rcu_dereference_bh() - fetch an RCU-bh-protected pointer for dereferencing
651  * @p: The pointer to read, prior to dereferencing
652  *
653  * Makes rcu_dereference_check() do the dirty work.
654  */
655 #define rcu_dereference_bh(p) rcu_dereference_bh_check(p, 0)
656 
657 /**
658  * rcu_dereference_sched() - fetch RCU-sched-protected pointer for dereferencing
659  * @p: The pointer to read, prior to dereferencing
660  *
661  * Makes rcu_dereference_check() do the dirty work.
662  */
663 #define rcu_dereference_sched(p) rcu_dereference_sched_check(p, 0)
664 
665 /**
666  * rcu_pointer_handoff() - Hand off a pointer from RCU to other mechanism
667  * @p: The pointer to hand off
668  *
669  * This is simply an identity function, but it documents where a pointer
670  * is handed off from RCU to some other synchronization mechanism, for
671  * example, reference counting or locking.  In C11, it would map to
672  * kill_dependency().  It could be used as follows::
673  *
674  *	rcu_read_lock();
675  *	p = rcu_dereference(gp);
676  *	long_lived = is_long_lived(p);
677  *	if (long_lived) {
678  *		if (!atomic_inc_not_zero(p->refcnt))
679  *			long_lived = false;
680  *		else
681  *			p = rcu_pointer_handoff(p);
682  *	}
683  *	rcu_read_unlock();
684  */
685 #define rcu_pointer_handoff(p) (p)
686 
687 /**
688  * rcu_read_lock() - mark the beginning of an RCU read-side critical section
689  *
690  * When synchronize_rcu() is invoked on one CPU while other CPUs
691  * are within RCU read-side critical sections, then the
692  * synchronize_rcu() is guaranteed to block until after all the other
693  * CPUs exit their critical sections.  Similarly, if call_rcu() is invoked
694  * on one CPU while other CPUs are within RCU read-side critical
695  * sections, invocation of the corresponding RCU callback is deferred
696  * until after the all the other CPUs exit their critical sections.
697  *
698  * In v5.0 and later kernels, synchronize_rcu() and call_rcu() also
699  * wait for regions of code with preemption disabled, including regions of
700  * code with interrupts or softirqs disabled.  In pre-v5.0 kernels, which
701  * define synchronize_sched(), only code enclosed within rcu_read_lock()
702  * and rcu_read_unlock() are guaranteed to be waited for.
703  *
704  * Note, however, that RCU callbacks are permitted to run concurrently
705  * with new RCU read-side critical sections.  One way that this can happen
706  * is via the following sequence of events: (1) CPU 0 enters an RCU
707  * read-side critical section, (2) CPU 1 invokes call_rcu() to register
708  * an RCU callback, (3) CPU 0 exits the RCU read-side critical section,
709  * (4) CPU 2 enters a RCU read-side critical section, (5) the RCU
710  * callback is invoked.  This is legal, because the RCU read-side critical
711  * section that was running concurrently with the call_rcu() (and which
712  * therefore might be referencing something that the corresponding RCU
713  * callback would free up) has completed before the corresponding
714  * RCU callback is invoked.
715  *
716  * RCU read-side critical sections may be nested.  Any deferred actions
717  * will be deferred until the outermost RCU read-side critical section
718  * completes.
719  *
720  * You can avoid reading and understanding the next paragraph by
721  * following this rule: don't put anything in an rcu_read_lock() RCU
722  * read-side critical section that would block in a !PREEMPTION kernel.
723  * But if you want the full story, read on!
724  *
725  * In non-preemptible RCU implementations (pure TREE_RCU and TINY_RCU),
726  * it is illegal to block while in an RCU read-side critical section.
727  * In preemptible RCU implementations (PREEMPT_RCU) in CONFIG_PREEMPTION
728  * kernel builds, RCU read-side critical sections may be preempted,
729  * but explicit blocking is illegal.  Finally, in preemptible RCU
730  * implementations in real-time (with -rt patchset) kernel builds, RCU
731  * read-side critical sections may be preempted and they may also block, but
732  * only when acquiring spinlocks that are subject to priority inheritance.
733  */
rcu_read_lock(void)734 static __always_inline void rcu_read_lock(void)
735 {
736 	__rcu_read_lock();
737 	__acquire(RCU);
738 	rcu_lock_acquire(&rcu_lock_map);
739 	RCU_LOCKDEP_WARN(!rcu_is_watching(),
740 			 "rcu_read_lock() used illegally while idle");
741 }
742 
743 /*
744  * So where is rcu_write_lock()?  It does not exist, as there is no
745  * way for writers to lock out RCU readers.  This is a feature, not
746  * a bug -- this property is what provides RCU's performance benefits.
747  * Of course, writers must coordinate with each other.  The normal
748  * spinlock primitives work well for this, but any other technique may be
749  * used as well.  RCU does not care how the writers keep out of each
750  * others' way, as long as they do so.
751  */
752 
753 /**
754  * rcu_read_unlock() - marks the end of an RCU read-side critical section.
755  *
756  * In almost all situations, rcu_read_unlock() is immune from deadlock.
757  * In recent kernels that have consolidated synchronize_sched() and
758  * synchronize_rcu_bh() into synchronize_rcu(), this deadlock immunity
759  * also extends to the scheduler's runqueue and priority-inheritance
760  * spinlocks, courtesy of the quiescent-state deferral that is carried
761  * out when rcu_read_unlock() is invoked with interrupts disabled.
762  *
763  * See rcu_read_lock() for more information.
764  */
rcu_read_unlock(void)765 static inline void rcu_read_unlock(void)
766 {
767 	RCU_LOCKDEP_WARN(!rcu_is_watching(),
768 			 "rcu_read_unlock() used illegally while idle");
769 	__release(RCU);
770 	__rcu_read_unlock();
771 	rcu_lock_release(&rcu_lock_map); /* Keep acq info for rls diags. */
772 }
773 
774 /**
775  * rcu_read_lock_bh() - mark the beginning of an RCU-bh critical section
776  *
777  * This is equivalent to rcu_read_lock(), but also disables softirqs.
778  * Note that anything else that disables softirqs can also serve as an RCU
779  * read-side critical section.  However, please note that this equivalence
780  * applies only to v5.0 and later.  Before v5.0, rcu_read_lock() and
781  * rcu_read_lock_bh() were unrelated.
782  *
783  * Note that rcu_read_lock_bh() and the matching rcu_read_unlock_bh()
784  * must occur in the same context, for example, it is illegal to invoke
785  * rcu_read_unlock_bh() from one task if the matching rcu_read_lock_bh()
786  * was invoked from some other task.
787  */
rcu_read_lock_bh(void)788 static inline void rcu_read_lock_bh(void)
789 {
790 	local_bh_disable();
791 	__acquire(RCU_BH);
792 	rcu_lock_acquire(&rcu_bh_lock_map);
793 	RCU_LOCKDEP_WARN(!rcu_is_watching(),
794 			 "rcu_read_lock_bh() used illegally while idle");
795 }
796 
797 /**
798  * rcu_read_unlock_bh() - marks the end of a softirq-only RCU critical section
799  *
800  * See rcu_read_lock_bh() for more information.
801  */
rcu_read_unlock_bh(void)802 static inline void rcu_read_unlock_bh(void)
803 {
804 	RCU_LOCKDEP_WARN(!rcu_is_watching(),
805 			 "rcu_read_unlock_bh() used illegally while idle");
806 	rcu_lock_release(&rcu_bh_lock_map);
807 	__release(RCU_BH);
808 	local_bh_enable();
809 }
810 
811 /**
812  * rcu_read_lock_sched() - mark the beginning of a RCU-sched critical section
813  *
814  * This is equivalent to rcu_read_lock(), but also disables preemption.
815  * Read-side critical sections can also be introduced by anything else that
816  * disables preemption, including local_irq_disable() and friends.  However,
817  * please note that the equivalence to rcu_read_lock() applies only to
818  * v5.0 and later.  Before v5.0, rcu_read_lock() and rcu_read_lock_sched()
819  * were unrelated.
820  *
821  * Note that rcu_read_lock_sched() and the matching rcu_read_unlock_sched()
822  * must occur in the same context, for example, it is illegal to invoke
823  * rcu_read_unlock_sched() from process context if the matching
824  * rcu_read_lock_sched() was invoked from an NMI handler.
825  */
rcu_read_lock_sched(void)826 static inline void rcu_read_lock_sched(void)
827 {
828 	preempt_disable();
829 	__acquire(RCU_SCHED);
830 	rcu_lock_acquire(&rcu_sched_lock_map);
831 	RCU_LOCKDEP_WARN(!rcu_is_watching(),
832 			 "rcu_read_lock_sched() used illegally while idle");
833 }
834 
835 /* Used by lockdep and tracing: cannot be traced, cannot call lockdep. */
rcu_read_lock_sched_notrace(void)836 static inline notrace void rcu_read_lock_sched_notrace(void)
837 {
838 	preempt_disable_notrace();
839 	__acquire(RCU_SCHED);
840 }
841 
842 /**
843  * rcu_read_unlock_sched() - marks the end of a RCU-classic critical section
844  *
845  * See rcu_read_lock_sched() for more information.
846  */
rcu_read_unlock_sched(void)847 static inline void rcu_read_unlock_sched(void)
848 {
849 	RCU_LOCKDEP_WARN(!rcu_is_watching(),
850 			 "rcu_read_unlock_sched() used illegally while idle");
851 	rcu_lock_release(&rcu_sched_lock_map);
852 	__release(RCU_SCHED);
853 	preempt_enable();
854 }
855 
856 /* Used by lockdep and tracing: cannot be traced, cannot call lockdep. */
rcu_read_unlock_sched_notrace(void)857 static inline notrace void rcu_read_unlock_sched_notrace(void)
858 {
859 	__release(RCU_SCHED);
860 	preempt_enable_notrace();
861 }
862 
863 /**
864  * RCU_INIT_POINTER() - initialize an RCU protected pointer
865  * @p: The pointer to be initialized.
866  * @v: The value to initialized the pointer to.
867  *
868  * Initialize an RCU-protected pointer in special cases where readers
869  * do not need ordering constraints on the CPU or the compiler.  These
870  * special cases are:
871  *
872  * 1.	This use of RCU_INIT_POINTER() is NULLing out the pointer *or*
873  * 2.	The caller has taken whatever steps are required to prevent
874  *	RCU readers from concurrently accessing this pointer *or*
875  * 3.	The referenced data structure has already been exposed to
876  *	readers either at compile time or via rcu_assign_pointer() *and*
877  *
878  *	a.	You have not made *any* reader-visible changes to
879  *		this structure since then *or*
880  *	b.	It is OK for readers accessing this structure from its
881  *		new location to see the old state of the structure.  (For
882  *		example, the changes were to statistical counters or to
883  *		other state where exact synchronization is not required.)
884  *
885  * Failure to follow these rules governing use of RCU_INIT_POINTER() will
886  * result in impossible-to-diagnose memory corruption.  As in the structures
887  * will look OK in crash dumps, but any concurrent RCU readers might
888  * see pre-initialized values of the referenced data structure.  So
889  * please be very careful how you use RCU_INIT_POINTER()!!!
890  *
891  * If you are creating an RCU-protected linked structure that is accessed
892  * by a single external-to-structure RCU-protected pointer, then you may
893  * use RCU_INIT_POINTER() to initialize the internal RCU-protected
894  * pointers, but you must use rcu_assign_pointer() to initialize the
895  * external-to-structure pointer *after* you have completely initialized
896  * the reader-accessible portions of the linked structure.
897  *
898  * Note that unlike rcu_assign_pointer(), RCU_INIT_POINTER() provides no
899  * ordering guarantees for either the CPU or the compiler.
900  */
901 #define RCU_INIT_POINTER(p, v) \
902 	do { \
903 		rcu_check_sparse(p, __rcu); \
904 		WRITE_ONCE(p, RCU_INITIALIZER(v)); \
905 	} while (0)
906 
907 /**
908  * RCU_POINTER_INITIALIZER() - statically initialize an RCU protected pointer
909  * @p: The pointer to be initialized.
910  * @v: The value to initialized the pointer to.
911  *
912  * GCC-style initialization for an RCU-protected pointer in a structure field.
913  */
914 #define RCU_POINTER_INITIALIZER(p, v) \
915 		.p = RCU_INITIALIZER(v)
916 
917 /*
918  * Does the specified offset indicate that the corresponding rcu_head
919  * structure can be handled by kvfree_rcu()?
920  */
921 #define __is_kvfree_rcu_offset(offset) ((offset) < 4096)
922 
923 /**
924  * kfree_rcu() - kfree an object after a grace period.
925  * @ptr: pointer to kfree for both single- and double-argument invocations.
926  * @rhf: the name of the struct rcu_head within the type of @ptr,
927  *       but only for double-argument invocations.
928  *
929  * Many rcu callbacks functions just call kfree() on the base structure.
930  * These functions are trivial, but their size adds up, and furthermore
931  * when they are used in a kernel module, that module must invoke the
932  * high-latency rcu_barrier() function at module-unload time.
933  *
934  * The kfree_rcu() function handles this issue.  Rather than encoding a
935  * function address in the embedded rcu_head structure, kfree_rcu() instead
936  * encodes the offset of the rcu_head structure within the base structure.
937  * Because the functions are not allowed in the low-order 4096 bytes of
938  * kernel virtual memory, offsets up to 4095 bytes can be accommodated.
939  * If the offset is larger than 4095 bytes, a compile-time error will
940  * be generated in kvfree_rcu_arg_2(). If this error is triggered, you can
941  * either fall back to use of call_rcu() or rearrange the structure to
942  * position the rcu_head structure into the first 4096 bytes.
943  *
944  * Note that the allowable offset might decrease in the future, for example,
945  * to allow something like kmem_cache_free_rcu().
946  *
947  * The BUILD_BUG_ON check must not involve any function calls, hence the
948  * checks are done in macros here.
949  */
950 #define kfree_rcu(ptr, rhf...) kvfree_rcu(ptr, ## rhf)
951 
952 /**
953  * kvfree_rcu() - kvfree an object after a grace period.
954  *
955  * This macro consists of one or two arguments and it is
956  * based on whether an object is head-less or not. If it
957  * has a head then a semantic stays the same as it used
958  * to be before:
959  *
960  *     kvfree_rcu(ptr, rhf);
961  *
962  * where @ptr is a pointer to kvfree(), @rhf is the name
963  * of the rcu_head structure within the type of @ptr.
964  *
965  * When it comes to head-less variant, only one argument
966  * is passed and that is just a pointer which has to be
967  * freed after a grace period. Therefore the semantic is
968  *
969  *     kvfree_rcu(ptr);
970  *
971  * where @ptr is the pointer to be freed by kvfree().
972  *
973  * Please note, head-less way of freeing is permitted to
974  * use from a context that has to follow might_sleep()
975  * annotation. Otherwise, please switch and embed the
976  * rcu_head structure within the type of @ptr.
977  */
978 #define kvfree_rcu(...) KVFREE_GET_MACRO(__VA_ARGS__,		\
979 	kvfree_rcu_arg_2, kvfree_rcu_arg_1)(__VA_ARGS__)
980 
981 #define KVFREE_GET_MACRO(_1, _2, NAME, ...) NAME
982 #define kvfree_rcu_arg_2(ptr, rhf)					\
983 do {									\
984 	typeof (ptr) ___p = (ptr);					\
985 									\
986 	if (___p) {									\
987 		BUILD_BUG_ON(!__is_kvfree_rcu_offset(offsetof(typeof(*(ptr)), rhf)));	\
988 		kvfree_call_rcu(&((___p)->rhf), (rcu_callback_t)(unsigned long)		\
989 			(offsetof(typeof(*(ptr)), rhf)));				\
990 	}										\
991 } while (0)
992 
993 #define kvfree_rcu_arg_1(ptr)					\
994 do {								\
995 	typeof(ptr) ___p = (ptr);				\
996 								\
997 	if (___p)						\
998 		kvfree_call_rcu(NULL, (rcu_callback_t) (___p));	\
999 } while (0)
1000 
1001 /*
1002  * Place this after a lock-acquisition primitive to guarantee that
1003  * an UNLOCK+LOCK pair acts as a full barrier.  This guarantee applies
1004  * if the UNLOCK and LOCK are executed by the same CPU or if the
1005  * UNLOCK and LOCK operate on the same lock variable.
1006  */
1007 #ifdef CONFIG_ARCH_WEAK_RELEASE_ACQUIRE
1008 #define smp_mb__after_unlock_lock()	smp_mb()  /* Full ordering for lock. */
1009 #else /* #ifdef CONFIG_ARCH_WEAK_RELEASE_ACQUIRE */
1010 #define smp_mb__after_unlock_lock()	do { } while (0)
1011 #endif /* #else #ifdef CONFIG_ARCH_WEAK_RELEASE_ACQUIRE */
1012 
1013 
1014 /* Has the specified rcu_head structure been handed to call_rcu()? */
1015 
1016 /**
1017  * rcu_head_init - Initialize rcu_head for rcu_head_after_call_rcu()
1018  * @rhp: The rcu_head structure to initialize.
1019  *
1020  * If you intend to invoke rcu_head_after_call_rcu() to test whether a
1021  * given rcu_head structure has already been passed to call_rcu(), then
1022  * you must also invoke this rcu_head_init() function on it just after
1023  * allocating that structure.  Calls to this function must not race with
1024  * calls to call_rcu(), rcu_head_after_call_rcu(), or callback invocation.
1025  */
rcu_head_init(struct rcu_head * rhp)1026 static inline void rcu_head_init(struct rcu_head *rhp)
1027 {
1028 	rhp->func = (rcu_callback_t)~0L;
1029 }
1030 
1031 /**
1032  * rcu_head_after_call_rcu() - Has this rcu_head been passed to call_rcu()?
1033  * @rhp: The rcu_head structure to test.
1034  * @f: The function passed to call_rcu() along with @rhp.
1035  *
1036  * Returns @true if the @rhp has been passed to call_rcu() with @func,
1037  * and @false otherwise.  Emits a warning in any other case, including
1038  * the case where @rhp has already been invoked after a grace period.
1039  * Calls to this function must not race with callback invocation.  One way
1040  * to avoid such races is to enclose the call to rcu_head_after_call_rcu()
1041  * in an RCU read-side critical section that includes a read-side fetch
1042  * of the pointer to the structure containing @rhp.
1043  */
1044 static inline bool
rcu_head_after_call_rcu(struct rcu_head * rhp,rcu_callback_t f)1045 rcu_head_after_call_rcu(struct rcu_head *rhp, rcu_callback_t f)
1046 {
1047 	rcu_callback_t func = READ_ONCE(rhp->func);
1048 
1049 	if (func == f)
1050 		return true;
1051 	WARN_ON_ONCE(func != (rcu_callback_t)~0L);
1052 	return false;
1053 }
1054 
1055 /* kernel/ksysfs.c definitions */
1056 extern int rcu_expedited;
1057 extern int rcu_normal;
1058 
1059 #endif /* __LINUX_RCUPDATE_H */
1060