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1/*
2 * Read-Copy Update mechanism for mutual exclusion (tree-based version)
3 * Internal non-public definitions that provide either classic
4 * or preemptible semantics.
5 *
6 * This program is free software; you can redistribute it and/or modify
7 * it under the terms of the GNU General Public License as published by
8 * the Free Software Foundation; either version 2 of the License, or
9 * (at your option) any later version.
10 *
11 * This program is distributed in the hope that it will be useful,
12 * but WITHOUT ANY WARRANTY; without even the implied warranty of
13 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
14 * GNU General Public License for more details.
15 *
16 * You should have received a copy of the GNU General Public License
17 * along with this program; if not, write to the Free Software
18 * Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA 02111-1307, USA.
19 *
20 * Copyright Red Hat, 2009
21 * Copyright IBM Corporation, 2009
22 *
23 * Author: Ingo Molnar <mingo@elte.hu>
24 * Paul E. McKenney <paulmck@linux.vnet.ibm.com>
25 */
26
27#include <linux/delay.h>
28#include <linux/stop_machine.h>
29
30/*
31 * Check the RCU kernel configuration parameters and print informative
32 * messages about anything out of the ordinary. If you like #ifdef, you
33 * will love this function.
34 */
35static void __init rcu_bootup_announce_oddness(void)
36{
37#ifdef CONFIG_RCU_TRACE
38 printk(KERN_INFO "\tRCU debugfs-based tracing is enabled.\n");
39#endif
40#if (defined(CONFIG_64BIT) && CONFIG_RCU_FANOUT != 64) || (!defined(CONFIG_64BIT) && CONFIG_RCU_FANOUT != 32)
41 printk(KERN_INFO "\tCONFIG_RCU_FANOUT set to non-default value of %d\n",
42 CONFIG_RCU_FANOUT);
43#endif
44#ifdef CONFIG_RCU_FANOUT_EXACT
45 printk(KERN_INFO "\tHierarchical RCU autobalancing is disabled.\n");
46#endif
47#ifdef CONFIG_RCU_FAST_NO_HZ
48 printk(KERN_INFO
49 "\tRCU dyntick-idle grace-period acceleration is enabled.\n");
50#endif
51#ifdef CONFIG_PROVE_RCU
52 printk(KERN_INFO "\tRCU lockdep checking is enabled.\n");
53#endif
54#ifdef CONFIG_RCU_TORTURE_TEST_RUNNABLE
55 printk(KERN_INFO "\tRCU torture testing starts during boot.\n");
56#endif
57#if defined(CONFIG_TREE_PREEMPT_RCU) && !defined(CONFIG_RCU_CPU_STALL_VERBOSE)
58 printk(KERN_INFO "\tVerbose stalled-CPUs detection is disabled.\n");
59#endif
60#if NUM_RCU_LVL_4 != 0
61 printk(KERN_INFO "\tExperimental four-level hierarchy is enabled.\n");
62#endif
63}
64
65#ifdef CONFIG_TREE_PREEMPT_RCU
66
67struct rcu_state rcu_preempt_state = RCU_STATE_INITIALIZER(rcu_preempt_state);
68DEFINE_PER_CPU(struct rcu_data, rcu_preempt_data);
69static struct rcu_state *rcu_state = &rcu_preempt_state;
70
71static void rcu_read_unlock_special(struct task_struct *t);
72static int rcu_preempted_readers_exp(struct rcu_node *rnp);
73
74/*
75 * Tell them what RCU they are running.
76 */
77static void __init rcu_bootup_announce(void)
78{
79 printk(KERN_INFO "Preemptible hierarchical RCU implementation.\n");
80 rcu_bootup_announce_oddness();
81}
82
83/*
84 * Return the number of RCU-preempt batches processed thus far
85 * for debug and statistics.
86 */
87long rcu_batches_completed_preempt(void)
88{
89 return rcu_preempt_state.completed;
90}
91EXPORT_SYMBOL_GPL(rcu_batches_completed_preempt);
92
93/*
94 * Return the number of RCU batches processed thus far for debug & stats.
95 */
96long rcu_batches_completed(void)
97{
98 return rcu_batches_completed_preempt();
99}
100EXPORT_SYMBOL_GPL(rcu_batches_completed);
101
102/*
103 * Force a quiescent state for preemptible RCU.
104 */
105void rcu_force_quiescent_state(void)
106{
107 force_quiescent_state(&rcu_preempt_state, 0);
108}
109EXPORT_SYMBOL_GPL(rcu_force_quiescent_state);
110
111/*
112 * Record a preemptible-RCU quiescent state for the specified CPU. Note
113 * that this just means that the task currently running on the CPU is
114 * not in a quiescent state. There might be any number of tasks blocked
115 * while in an RCU read-side critical section.
116 *
117 * Unlike the other rcu_*_qs() functions, callers to this function
118 * must disable irqs in order to protect the assignment to
119 * ->rcu_read_unlock_special.
120 */
121static void rcu_preempt_qs(int cpu)
122{
123 struct rcu_data *rdp = &per_cpu(rcu_preempt_data, cpu);
124
125 rdp->passed_quiesc_completed = rdp->gpnum - 1;
126 barrier();
127 rdp->passed_quiesc = 1;
128 current->rcu_read_unlock_special &= ~RCU_READ_UNLOCK_NEED_QS;
129}
130
131/*
132 * We have entered the scheduler, and the current task might soon be
133 * context-switched away from. If this task is in an RCU read-side
134 * critical section, we will no longer be able to rely on the CPU to
135 * record that fact, so we enqueue the task on the blkd_tasks list.
136 * The task will dequeue itself when it exits the outermost enclosing
137 * RCU read-side critical section. Therefore, the current grace period
138 * cannot be permitted to complete until the blkd_tasks list entries
139 * predating the current grace period drain, in other words, until
140 * rnp->gp_tasks becomes NULL.
141 *
142 * Caller must disable preemption.
143 */
144static void rcu_preempt_note_context_switch(int cpu)
145{
146 struct task_struct *t = current;
147 unsigned long flags;
148 struct rcu_data *rdp;
149 struct rcu_node *rnp;
150
151 if (t->rcu_read_lock_nesting > 0 &&
152 (t->rcu_read_unlock_special & RCU_READ_UNLOCK_BLOCKED) == 0) {
153
154 /* Possibly blocking in an RCU read-side critical section. */
155 rdp = per_cpu_ptr(rcu_preempt_state.rda, cpu);
156 rnp = rdp->mynode;
157 raw_spin_lock_irqsave(&rnp->lock, flags);
158 t->rcu_read_unlock_special |= RCU_READ_UNLOCK_BLOCKED;
159 t->rcu_blocked_node = rnp;
160
161 /*
162 * If this CPU has already checked in, then this task
163 * will hold up the next grace period rather than the
164 * current grace period. Queue the task accordingly.
165 * If the task is queued for the current grace period
166 * (i.e., this CPU has not yet passed through a quiescent
167 * state for the current grace period), then as long
168 * as that task remains queued, the current grace period
169 * cannot end. Note that there is some uncertainty as
170 * to exactly when the current grace period started.
171 * We take a conservative approach, which can result
172 * in unnecessarily waiting on tasks that started very
173 * slightly after the current grace period began. C'est
174 * la vie!!!
175 *
176 * But first, note that the current CPU must still be
177 * on line!
178 */
179 WARN_ON_ONCE((rdp->grpmask & rnp->qsmaskinit) == 0);
180 WARN_ON_ONCE(!list_empty(&t->rcu_node_entry));
181 if ((rnp->qsmask & rdp->grpmask) && rnp->gp_tasks != NULL) {
182 list_add(&t->rcu_node_entry, rnp->gp_tasks->prev);
183 rnp->gp_tasks = &t->rcu_node_entry;
184#ifdef CONFIG_RCU_BOOST
185 if (rnp->boost_tasks != NULL)
186 rnp->boost_tasks = rnp->gp_tasks;
187#endif /* #ifdef CONFIG_RCU_BOOST */
188 } else {
189 list_add(&t->rcu_node_entry, &rnp->blkd_tasks);
190 if (rnp->qsmask & rdp->grpmask)
191 rnp->gp_tasks = &t->rcu_node_entry;
192 }
193 raw_spin_unlock_irqrestore(&rnp->lock, flags);
194 } else if (t->rcu_read_lock_nesting < 0 &&
195 t->rcu_read_unlock_special) {
196
197 /*
198 * Complete exit from RCU read-side critical section on
199 * behalf of preempted instance of __rcu_read_unlock().
200 */
201 rcu_read_unlock_special(t);
202 }
203
204 /*
205 * Either we were not in an RCU read-side critical section to
206 * begin with, or we have now recorded that critical section
207 * globally. Either way, we can now note a quiescent state
208 * for this CPU. Again, if we were in an RCU read-side critical
209 * section, and if that critical section was blocking the current
210 * grace period, then the fact that the task has been enqueued
211 * means that we continue to block the current grace period.
212 */
213 local_irq_save(flags);
214 rcu_preempt_qs(cpu);
215 local_irq_restore(flags);
216}
217
218/*
219 * Tree-preemptible RCU implementation for rcu_read_lock().
220 * Just increment ->rcu_read_lock_nesting, shared state will be updated
221 * if we block.
222 */
223void __rcu_read_lock(void)
224{
225 current->rcu_read_lock_nesting++;
226 barrier(); /* needed if we ever invoke rcu_read_lock in rcutree.c */
227}
228EXPORT_SYMBOL_GPL(__rcu_read_lock);
229
230/*
231 * Check for preempted RCU readers blocking the current grace period
232 * for the specified rcu_node structure. If the caller needs a reliable
233 * answer, it must hold the rcu_node's ->lock.
234 */
235static int rcu_preempt_blocked_readers_cgp(struct rcu_node *rnp)
236{
237 return rnp->gp_tasks != NULL;
238}
239
240/*
241 * Record a quiescent state for all tasks that were previously queued
242 * on the specified rcu_node structure and that were blocking the current
243 * RCU grace period. The caller must hold the specified rnp->lock with
244 * irqs disabled, and this lock is released upon return, but irqs remain
245 * disabled.
246 */
247static void rcu_report_unblock_qs_rnp(struct rcu_node *rnp, unsigned long flags)
248 __releases(rnp->lock)
249{
250 unsigned long mask;
251 struct rcu_node *rnp_p;
252
253 if (rnp->qsmask != 0 || rcu_preempt_blocked_readers_cgp(rnp)) {
254 raw_spin_unlock_irqrestore(&rnp->lock, flags);
255 return; /* Still need more quiescent states! */
256 }
257
258 rnp_p = rnp->parent;
259 if (rnp_p == NULL) {
260 /*
261 * Either there is only one rcu_node in the tree,
262 * or tasks were kicked up to root rcu_node due to
263 * CPUs going offline.
264 */
265 rcu_report_qs_rsp(&rcu_preempt_state, flags);
266 return;
267 }
268
269 /* Report up the rest of the hierarchy. */
270 mask = rnp->grpmask;
271 raw_spin_unlock(&rnp->lock); /* irqs remain disabled. */
272 raw_spin_lock(&rnp_p->lock); /* irqs already disabled. */
273 rcu_report_qs_rnp(mask, &rcu_preempt_state, rnp_p, flags);
274}
275
276/*
277 * Advance a ->blkd_tasks-list pointer to the next entry, instead
278 * returning NULL if at the end of the list.
279 */
280static struct list_head *rcu_next_node_entry(struct task_struct *t,
281 struct rcu_node *rnp)
282{
283 struct list_head *np;
284
285 np = t->rcu_node_entry.next;
286 if (np == &rnp->blkd_tasks)
287 np = NULL;
288 return np;
289}
290
291/*
292 * Handle special cases during rcu_read_unlock(), such as needing to
293 * notify RCU core processing or task having blocked during the RCU
294 * read-side critical section.
295 */
296static noinline void rcu_read_unlock_special(struct task_struct *t)
297{
298 int empty;
299 int empty_exp;
300 unsigned long flags;
301 struct list_head *np;
302 struct rcu_node *rnp;
303 int special;
304
305 /* NMI handlers cannot block and cannot safely manipulate state. */
306 if (in_nmi())
307 return;
308
309 local_irq_save(flags);
310
311 /*
312 * If RCU core is waiting for this CPU to exit critical section,
313 * let it know that we have done so.
314 */
315 special = t->rcu_read_unlock_special;
316 if (special & RCU_READ_UNLOCK_NEED_QS) {
317 rcu_preempt_qs(smp_processor_id());
318 }
319
320 /* Hardware IRQ handlers cannot block. */
321 if (in_irq() || in_serving_softirq()) {
322 local_irq_restore(flags);
323 return;
324 }
325
326 /* Clean up if blocked during RCU read-side critical section. */
327 if (special & RCU_READ_UNLOCK_BLOCKED) {
328 t->rcu_read_unlock_special &= ~RCU_READ_UNLOCK_BLOCKED;
329
330 /*
331 * Remove this task from the list it blocked on. The
332 * task can migrate while we acquire the lock, but at
333 * most one time. So at most two passes through loop.
334 */
335 for (;;) {
336 rnp = t->rcu_blocked_node;
337 raw_spin_lock(&rnp->lock); /* irqs already disabled. */
338 if (rnp == t->rcu_blocked_node)
339 break;
340 raw_spin_unlock(&rnp->lock); /* irqs remain disabled. */
341 }
342 empty = !rcu_preempt_blocked_readers_cgp(rnp);
343 empty_exp = !rcu_preempted_readers_exp(rnp);
344 smp_mb(); /* ensure expedited fastpath sees end of RCU c-s. */
345 np = rcu_next_node_entry(t, rnp);
346 list_del_init(&t->rcu_node_entry);
347 if (&t->rcu_node_entry == rnp->gp_tasks)
348 rnp->gp_tasks = np;
349 if (&t->rcu_node_entry == rnp->exp_tasks)
350 rnp->exp_tasks = np;
351#ifdef CONFIG_RCU_BOOST
352 if (&t->rcu_node_entry == rnp->boost_tasks)
353 rnp->boost_tasks = np;
354 /* Snapshot and clear ->rcu_boosted with rcu_node lock held. */
355 if (t->rcu_boosted) {
356 special |= RCU_READ_UNLOCK_BOOSTED;
357 t->rcu_boosted = 0;
358 }
359#endif /* #ifdef CONFIG_RCU_BOOST */
360 t->rcu_blocked_node = NULL;
361
362 /*
363 * If this was the last task on the current list, and if
364 * we aren't waiting on any CPUs, report the quiescent state.
365 * Note that rcu_report_unblock_qs_rnp() releases rnp->lock.
366 */
367 if (empty)
368 raw_spin_unlock_irqrestore(&rnp->lock, flags);
369 else
370 rcu_report_unblock_qs_rnp(rnp, flags);
371
372#ifdef CONFIG_RCU_BOOST
373 /* Unboost if we were boosted. */
374 if (special & RCU_READ_UNLOCK_BOOSTED) {
375 rt_mutex_unlock(t->rcu_boost_mutex);
376 t->rcu_boost_mutex = NULL;
377 }
378#endif /* #ifdef CONFIG_RCU_BOOST */
379
380 /*
381 * If this was the last task on the expedited lists,
382 * then we need to report up the rcu_node hierarchy.
383 */
384 if (!empty_exp && !rcu_preempted_readers_exp(rnp))
385 rcu_report_exp_rnp(&rcu_preempt_state, rnp);
386 } else {
387 local_irq_restore(flags);
388 }
389}
390
391/*
392 * Tree-preemptible RCU implementation for rcu_read_unlock().
393 * Decrement ->rcu_read_lock_nesting. If the result is zero (outermost
394 * rcu_read_unlock()) and ->rcu_read_unlock_special is non-zero, then
395 * invoke rcu_read_unlock_special() to clean up after a context switch
396 * in an RCU read-side critical section and other special cases.
397 */
398void __rcu_read_unlock(void)
399{
400 struct task_struct *t = current;
401
402 barrier(); /* needed if we ever invoke rcu_read_unlock in rcutree.c */
403 if (t->rcu_read_lock_nesting != 1)
404 --t->rcu_read_lock_nesting;
405 else {
406 t->rcu_read_lock_nesting = INT_MIN;
407 barrier(); /* assign before ->rcu_read_unlock_special load */
408 if (unlikely(ACCESS_ONCE(t->rcu_read_unlock_special)))
409 rcu_read_unlock_special(t);
410 barrier(); /* ->rcu_read_unlock_special load before assign */
411 t->rcu_read_lock_nesting = 0;
412 }
413#ifdef CONFIG_PROVE_LOCKING
414 {
415 int rrln = ACCESS_ONCE(t->rcu_read_lock_nesting);
416
417 WARN_ON_ONCE(rrln < 0 && rrln > INT_MIN / 2);
418 }
419#endif /* #ifdef CONFIG_PROVE_LOCKING */
420}
421EXPORT_SYMBOL_GPL(__rcu_read_unlock);
422
423#ifdef CONFIG_RCU_CPU_STALL_VERBOSE
424
425/*
426 * Dump detailed information for all tasks blocking the current RCU
427 * grace period on the specified rcu_node structure.
428 */
429static void rcu_print_detail_task_stall_rnp(struct rcu_node *rnp)
430{
431 unsigned long flags;
432 struct task_struct *t;
433
434 if (!rcu_preempt_blocked_readers_cgp(rnp))
435 return;
436 raw_spin_lock_irqsave(&rnp->lock, flags);
437 t = list_entry(rnp->gp_tasks,
438 struct task_struct, rcu_node_entry);
439 list_for_each_entry_continue(t, &rnp->blkd_tasks, rcu_node_entry)
440 sched_show_task(t);
441 raw_spin_unlock_irqrestore(&rnp->lock, flags);
442}
443
444/*
445 * Dump detailed information for all tasks blocking the current RCU
446 * grace period.
447 */
448static void rcu_print_detail_task_stall(struct rcu_state *rsp)
449{
450 struct rcu_node *rnp = rcu_get_root(rsp);
451
452 rcu_print_detail_task_stall_rnp(rnp);
453 rcu_for_each_leaf_node(rsp, rnp)
454 rcu_print_detail_task_stall_rnp(rnp);
455}
456
457#else /* #ifdef CONFIG_RCU_CPU_STALL_VERBOSE */
458
459static void rcu_print_detail_task_stall(struct rcu_state *rsp)
460{
461}
462
463#endif /* #else #ifdef CONFIG_RCU_CPU_STALL_VERBOSE */
464
465/*
466 * Scan the current list of tasks blocked within RCU read-side critical
467 * sections, printing out the tid of each.
468 */
469static void rcu_print_task_stall(struct rcu_node *rnp)
470{
471 struct task_struct *t;
472
473 if (!rcu_preempt_blocked_readers_cgp(rnp))
474 return;
475 t = list_entry(rnp->gp_tasks,
476 struct task_struct, rcu_node_entry);
477 list_for_each_entry_continue(t, &rnp->blkd_tasks, rcu_node_entry)
478 printk(" P%d", t->pid);
479}
480
481/*
482 * Suppress preemptible RCU's CPU stall warnings by pushing the
483 * time of the next stall-warning message comfortably far into the
484 * future.
485 */
486static void rcu_preempt_stall_reset(void)
487{
488 rcu_preempt_state.jiffies_stall = jiffies + ULONG_MAX / 2;
489}
490
491/*
492 * Check that the list of blocked tasks for the newly completed grace
493 * period is in fact empty. It is a serious bug to complete a grace
494 * period that still has RCU readers blocked! This function must be
495 * invoked -before- updating this rnp's ->gpnum, and the rnp's ->lock
496 * must be held by the caller.
497 *
498 * Also, if there are blocked tasks on the list, they automatically
499 * block the newly created grace period, so set up ->gp_tasks accordingly.
500 */
501static void rcu_preempt_check_blocked_tasks(struct rcu_node *rnp)
502{
503 WARN_ON_ONCE(rcu_preempt_blocked_readers_cgp(rnp));
504 if (!list_empty(&rnp->blkd_tasks))
505 rnp->gp_tasks = rnp->blkd_tasks.next;
506 WARN_ON_ONCE(rnp->qsmask);
507}
508
509#ifdef CONFIG_HOTPLUG_CPU
510
511/*
512 * Handle tasklist migration for case in which all CPUs covered by the
513 * specified rcu_node have gone offline. Move them up to the root
514 * rcu_node. The reason for not just moving them to the immediate
515 * parent is to remove the need for rcu_read_unlock_special() to
516 * make more than two attempts to acquire the target rcu_node's lock.
517 * Returns true if there were tasks blocking the current RCU grace
518 * period.
519 *
520 * Returns 1 if there was previously a task blocking the current grace
521 * period on the specified rcu_node structure.
522 *
523 * The caller must hold rnp->lock with irqs disabled.
524 */
525static int rcu_preempt_offline_tasks(struct rcu_state *rsp,
526 struct rcu_node *rnp,
527 struct rcu_data *rdp)
528{
529 struct list_head *lp;
530 struct list_head *lp_root;
531 int retval = 0;
532 struct rcu_node *rnp_root = rcu_get_root(rsp);
533 struct task_struct *t;
534
535 if (rnp == rnp_root) {
536 WARN_ONCE(1, "Last CPU thought to be offlined?");
537 return 0; /* Shouldn't happen: at least one CPU online. */
538 }
539
540 /* If we are on an internal node, complain bitterly. */
541 WARN_ON_ONCE(rnp != rdp->mynode);
542
543 /*
544 * Move tasks up to root rcu_node. Don't try to get fancy for
545 * this corner-case operation -- just put this node's tasks
546 * at the head of the root node's list, and update the root node's
547 * ->gp_tasks and ->exp_tasks pointers to those of this node's,
548 * if non-NULL. This might result in waiting for more tasks than
549 * absolutely necessary, but this is a good performance/complexity
550 * tradeoff.
551 */
552 if (rcu_preempt_blocked_readers_cgp(rnp))
553 retval |= RCU_OFL_TASKS_NORM_GP;
554 if (rcu_preempted_readers_exp(rnp))
555 retval |= RCU_OFL_TASKS_EXP_GP;
556 lp = &rnp->blkd_tasks;
557 lp_root = &rnp_root->blkd_tasks;
558 while (!list_empty(lp)) {
559 t = list_entry(lp->next, typeof(*t), rcu_node_entry);
560 raw_spin_lock(&rnp_root->lock); /* irqs already disabled */
561 list_del(&t->rcu_node_entry);
562 t->rcu_blocked_node = rnp_root;
563 list_add(&t->rcu_node_entry, lp_root);
564 if (&t->rcu_node_entry == rnp->gp_tasks)
565 rnp_root->gp_tasks = rnp->gp_tasks;
566 if (&t->rcu_node_entry == rnp->exp_tasks)
567 rnp_root->exp_tasks = rnp->exp_tasks;
568#ifdef CONFIG_RCU_BOOST
569 if (&t->rcu_node_entry == rnp->boost_tasks)
570 rnp_root->boost_tasks = rnp->boost_tasks;
571#endif /* #ifdef CONFIG_RCU_BOOST */
572 raw_spin_unlock(&rnp_root->lock); /* irqs still disabled */
573 }
574
575#ifdef CONFIG_RCU_BOOST
576 /* In case root is being boosted and leaf is not. */
577 raw_spin_lock(&rnp_root->lock); /* irqs already disabled */
578 if (rnp_root->boost_tasks != NULL &&
579 rnp_root->boost_tasks != rnp_root->gp_tasks)
580 rnp_root->boost_tasks = rnp_root->gp_tasks;
581 raw_spin_unlock(&rnp_root->lock); /* irqs still disabled */
582#endif /* #ifdef CONFIG_RCU_BOOST */
583
584 rnp->gp_tasks = NULL;
585 rnp->exp_tasks = NULL;
586 return retval;
587}
588
589/*
590 * Do CPU-offline processing for preemptible RCU.
591 */
592static void rcu_preempt_offline_cpu(int cpu)
593{
594 __rcu_offline_cpu(cpu, &rcu_preempt_state);
595}
596
597#endif /* #ifdef CONFIG_HOTPLUG_CPU */
598
599/*
600 * Check for a quiescent state from the current CPU. When a task blocks,
601 * the task is recorded in the corresponding CPU's rcu_node structure,
602 * which is checked elsewhere.
603 *
604 * Caller must disable hard irqs.
605 */
606static void rcu_preempt_check_callbacks(int cpu)
607{
608 struct task_struct *t = current;
609
610 if (t->rcu_read_lock_nesting == 0) {
611 rcu_preempt_qs(cpu);
612 return;
613 }
614 if (t->rcu_read_lock_nesting > 0 &&
615 per_cpu(rcu_preempt_data, cpu).qs_pending)
616 t->rcu_read_unlock_special |= RCU_READ_UNLOCK_NEED_QS;
617}
618
619/*
620 * Process callbacks for preemptible RCU.
621 */
622static void rcu_preempt_process_callbacks(void)
623{
624 __rcu_process_callbacks(&rcu_preempt_state,
625 &__get_cpu_var(rcu_preempt_data));
626}
627
628#ifdef CONFIG_RCU_BOOST
629
630static void rcu_preempt_do_callbacks(void)
631{
632 rcu_do_batch(&rcu_preempt_state, &__get_cpu_var(rcu_preempt_data));
633}
634
635#endif /* #ifdef CONFIG_RCU_BOOST */
636
637/*
638 * Queue a preemptible-RCU callback for invocation after a grace period.
639 */
640void call_rcu(struct rcu_head *head, void (*func)(struct rcu_head *rcu))
641{
642 __call_rcu(head, func, &rcu_preempt_state);
643}
644EXPORT_SYMBOL_GPL(call_rcu);
645
646/**
647 * synchronize_rcu - wait until a grace period has elapsed.
648 *
649 * Control will return to the caller some time after a full grace
650 * period has elapsed, in other words after all currently executing RCU
651 * read-side critical sections have completed. Note, however, that
652 * upon return from synchronize_rcu(), the caller might well be executing
653 * concurrently with new RCU read-side critical sections that began while
654 * synchronize_rcu() was waiting. RCU read-side critical sections are
655 * delimited by rcu_read_lock() and rcu_read_unlock(), and may be nested.
656 */
657void synchronize_rcu(void)
658{
659 struct rcu_synchronize rcu;
660
661 if (!rcu_scheduler_active)
662 return;
663
664 init_rcu_head_on_stack(&rcu.head);
665 init_completion(&rcu.completion);
666 /* Will wake me after RCU finished. */
667 call_rcu(&rcu.head, wakeme_after_rcu);
668 /* Wait for it. */
669 wait_for_completion(&rcu.completion);
670 destroy_rcu_head_on_stack(&rcu.head);
671}
672EXPORT_SYMBOL_GPL(synchronize_rcu);
673
674static DECLARE_WAIT_QUEUE_HEAD(sync_rcu_preempt_exp_wq);
675static long sync_rcu_preempt_exp_count;
676static DEFINE_MUTEX(sync_rcu_preempt_exp_mutex);
677
678/*
679 * Return non-zero if there are any tasks in RCU read-side critical
680 * sections blocking the current preemptible-RCU expedited grace period.
681 * If there is no preemptible-RCU expedited grace period currently in
682 * progress, returns zero unconditionally.
683 */
684static int rcu_preempted_readers_exp(struct rcu_node *rnp)
685{
686 return rnp->exp_tasks != NULL;
687}
688
689/*
690 * return non-zero if there is no RCU expedited grace period in progress
691 * for the specified rcu_node structure, in other words, if all CPUs and
692 * tasks covered by the specified rcu_node structure have done their bit
693 * for the current expedited grace period. Works only for preemptible
694 * RCU -- other RCU implementation use other means.
695 *
696 * Caller must hold sync_rcu_preempt_exp_mutex.
697 */
698static int sync_rcu_preempt_exp_done(struct rcu_node *rnp)
699{
700 return !rcu_preempted_readers_exp(rnp) &&
701 ACCESS_ONCE(rnp->expmask) == 0;
702}
703
704/*
705 * Report the exit from RCU read-side critical section for the last task
706 * that queued itself during or before the current expedited preemptible-RCU
707 * grace period. This event is reported either to the rcu_node structure on
708 * which the task was queued or to one of that rcu_node structure's ancestors,
709 * recursively up the tree. (Calm down, calm down, we do the recursion
710 * iteratively!)
711 *
712 * Caller must hold sync_rcu_preempt_exp_mutex.
713 */
714static void rcu_report_exp_rnp(struct rcu_state *rsp, struct rcu_node *rnp)
715{
716 unsigned long flags;
717 unsigned long mask;
718
719 raw_spin_lock_irqsave(&rnp->lock, flags);
720 for (;;) {
721 if (!sync_rcu_preempt_exp_done(rnp)) {
722 raw_spin_unlock_irqrestore(&rnp->lock, flags);
723 break;
724 }
725 if (rnp->parent == NULL) {
726 raw_spin_unlock_irqrestore(&rnp->lock, flags);
727 wake_up(&sync_rcu_preempt_exp_wq);
728 break;
729 }
730 mask = rnp->grpmask;
731 raw_spin_unlock(&rnp->lock); /* irqs remain disabled */
732 rnp = rnp->parent;
733 raw_spin_lock(&rnp->lock); /* irqs already disabled */
734 rnp->expmask &= ~mask;
735 }
736}
737
738/*
739 * Snapshot the tasks blocking the newly started preemptible-RCU expedited
740 * grace period for the specified rcu_node structure. If there are no such
741 * tasks, report it up the rcu_node hierarchy.
742 *
743 * Caller must hold sync_rcu_preempt_exp_mutex and rsp->onofflock.
744 */
745static void
746sync_rcu_preempt_exp_init(struct rcu_state *rsp, struct rcu_node *rnp)
747{
748 unsigned long flags;
749 int must_wait = 0;
750
751 raw_spin_lock_irqsave(&rnp->lock, flags);
752 if (list_empty(&rnp->blkd_tasks))
753 raw_spin_unlock_irqrestore(&rnp->lock, flags);
754 else {
755 rnp->exp_tasks = rnp->blkd_tasks.next;
756 rcu_initiate_boost(rnp, flags); /* releases rnp->lock */
757 must_wait = 1;
758 }
759 if (!must_wait)
760 rcu_report_exp_rnp(rsp, rnp);
761}
762
763/*
764 * Wait for an rcu-preempt grace period, but expedite it. The basic idea
765 * is to invoke synchronize_sched_expedited() to push all the tasks to
766 * the ->blkd_tasks lists and wait for this list to drain.
767 */
768void synchronize_rcu_expedited(void)
769{
770 unsigned long flags;
771 struct rcu_node *rnp;
772 struct rcu_state *rsp = &rcu_preempt_state;
773 long snap;
774 int trycount = 0;
775
776 smp_mb(); /* Caller's modifications seen first by other CPUs. */
777 snap = ACCESS_ONCE(sync_rcu_preempt_exp_count) + 1;
778 smp_mb(); /* Above access cannot bleed into critical section. */
779
780 /*
781 * Acquire lock, falling back to synchronize_rcu() if too many
782 * lock-acquisition failures. Of course, if someone does the
783 * expedited grace period for us, just leave.
784 */
785 while (!mutex_trylock(&sync_rcu_preempt_exp_mutex)) {
786 if (trycount++ < 10)
787 udelay(trycount * num_online_cpus());
788 else {
789 synchronize_rcu();
790 return;
791 }
792 if ((ACCESS_ONCE(sync_rcu_preempt_exp_count) - snap) > 0)
793 goto mb_ret; /* Others did our work for us. */
794 }
795 if ((ACCESS_ONCE(sync_rcu_preempt_exp_count) - snap) > 0)
796 goto unlock_mb_ret; /* Others did our work for us. */
797
798 /* force all RCU readers onto ->blkd_tasks lists. */
799 synchronize_sched_expedited();
800
801 raw_spin_lock_irqsave(&rsp->onofflock, flags);
802
803 /* Initialize ->expmask for all non-leaf rcu_node structures. */
804 rcu_for_each_nonleaf_node_breadth_first(rsp, rnp) {
805 raw_spin_lock(&rnp->lock); /* irqs already disabled. */
806 rnp->expmask = rnp->qsmaskinit;
807 raw_spin_unlock(&rnp->lock); /* irqs remain disabled. */
808 }
809
810 /* Snapshot current state of ->blkd_tasks lists. */
811 rcu_for_each_leaf_node(rsp, rnp)
812 sync_rcu_preempt_exp_init(rsp, rnp);
813 if (NUM_RCU_NODES > 1)
814 sync_rcu_preempt_exp_init(rsp, rcu_get_root(rsp));
815
816 raw_spin_unlock_irqrestore(&rsp->onofflock, flags);
817
818 /* Wait for snapshotted ->blkd_tasks lists to drain. */
819 rnp = rcu_get_root(rsp);
820 wait_event(sync_rcu_preempt_exp_wq,
821 sync_rcu_preempt_exp_done(rnp));
822
823 /* Clean up and exit. */
824 smp_mb(); /* ensure expedited GP seen before counter increment. */
825 ACCESS_ONCE(sync_rcu_preempt_exp_count)++;
826unlock_mb_ret:
827 mutex_unlock(&sync_rcu_preempt_exp_mutex);
828mb_ret:
829 smp_mb(); /* ensure subsequent action seen after grace period. */
830}
831EXPORT_SYMBOL_GPL(synchronize_rcu_expedited);
832
833/*
834 * Check to see if there is any immediate preemptible-RCU-related work
835 * to be done.
836 */
837static int rcu_preempt_pending(int cpu)
838{
839 return __rcu_pending(&rcu_preempt_state,
840 &per_cpu(rcu_preempt_data, cpu));
841}
842
843/*
844 * Does preemptible RCU need the CPU to stay out of dynticks mode?
845 */
846static int rcu_preempt_needs_cpu(int cpu)
847{
848 return !!per_cpu(rcu_preempt_data, cpu).nxtlist;
849}
850
851/**
852 * rcu_barrier - Wait until all in-flight call_rcu() callbacks complete.
853 */
854void rcu_barrier(void)
855{
856 _rcu_barrier(&rcu_preempt_state, call_rcu);
857}
858EXPORT_SYMBOL_GPL(rcu_barrier);
859
860/*
861 * Initialize preemptible RCU's per-CPU data.
862 */
863static void __cpuinit rcu_preempt_init_percpu_data(int cpu)
864{
865 rcu_init_percpu_data(cpu, &rcu_preempt_state, 1);
866}
867
868/*
869 * Move preemptible RCU's callbacks from dying CPU to other online CPU.
870 */
871static void rcu_preempt_send_cbs_to_online(void)
872{
873 rcu_send_cbs_to_online(&rcu_preempt_state);
874}
875
876/*
877 * Initialize preemptible RCU's state structures.
878 */
879static void __init __rcu_init_preempt(void)
880{
881 rcu_init_one(&rcu_preempt_state, &rcu_preempt_data);
882}
883
884/*
885 * Check for a task exiting while in a preemptible-RCU read-side
886 * critical section, clean up if so. No need to issue warnings,
887 * as debug_check_no_locks_held() already does this if lockdep
888 * is enabled.
889 */
890void exit_rcu(void)
891{
892 struct task_struct *t = current;
893
894 if (t->rcu_read_lock_nesting == 0)
895 return;
896 t->rcu_read_lock_nesting = 1;
897 __rcu_read_unlock();
898}
899
900#else /* #ifdef CONFIG_TREE_PREEMPT_RCU */
901
902static struct rcu_state *rcu_state = &rcu_sched_state;
903
904/*
905 * Tell them what RCU they are running.
906 */
907static void __init rcu_bootup_announce(void)
908{
909 printk(KERN_INFO "Hierarchical RCU implementation.\n");
910 rcu_bootup_announce_oddness();
911}
912
913/*
914 * Return the number of RCU batches processed thus far for debug & stats.
915 */
916long rcu_batches_completed(void)
917{
918 return rcu_batches_completed_sched();
919}
920EXPORT_SYMBOL_GPL(rcu_batches_completed);
921
922/*
923 * Force a quiescent state for RCU, which, because there is no preemptible
924 * RCU, becomes the same as rcu-sched.
925 */
926void rcu_force_quiescent_state(void)
927{
928 rcu_sched_force_quiescent_state();
929}
930EXPORT_SYMBOL_GPL(rcu_force_quiescent_state);
931
932/*
933 * Because preemptible RCU does not exist, we never have to check for
934 * CPUs being in quiescent states.
935 */
936static void rcu_preempt_note_context_switch(int cpu)
937{
938}
939
940/*
941 * Because preemptible RCU does not exist, there are never any preempted
942 * RCU readers.
943 */
944static int rcu_preempt_blocked_readers_cgp(struct rcu_node *rnp)
945{
946 return 0;
947}
948
949#ifdef CONFIG_HOTPLUG_CPU
950
951/* Because preemptible RCU does not exist, no quieting of tasks. */
952static void rcu_report_unblock_qs_rnp(struct rcu_node *rnp, unsigned long flags)
953{
954 raw_spin_unlock_irqrestore(&rnp->lock, flags);
955}
956
957#endif /* #ifdef CONFIG_HOTPLUG_CPU */
958
959/*
960 * Because preemptible RCU does not exist, we never have to check for
961 * tasks blocked within RCU read-side critical sections.
962 */
963static void rcu_print_detail_task_stall(struct rcu_state *rsp)
964{
965}
966
967/*
968 * Because preemptible RCU does not exist, we never have to check for
969 * tasks blocked within RCU read-side critical sections.
970 */
971static void rcu_print_task_stall(struct rcu_node *rnp)
972{
973}
974
975/*
976 * Because preemptible RCU does not exist, there is no need to suppress
977 * its CPU stall warnings.
978 */
979static void rcu_preempt_stall_reset(void)
980{
981}
982
983/*
984 * Because there is no preemptible RCU, there can be no readers blocked,
985 * so there is no need to check for blocked tasks. So check only for
986 * bogus qsmask values.
987 */
988static void rcu_preempt_check_blocked_tasks(struct rcu_node *rnp)
989{
990 WARN_ON_ONCE(rnp->qsmask);
991}
992
993#ifdef CONFIG_HOTPLUG_CPU
994
995/*
996 * Because preemptible RCU does not exist, it never needs to migrate
997 * tasks that were blocked within RCU read-side critical sections, and
998 * such non-existent tasks cannot possibly have been blocking the current
999 * grace period.
1000 */
1001static int rcu_preempt_offline_tasks(struct rcu_state *rsp,
1002 struct rcu_node *rnp,
1003 struct rcu_data *rdp)
1004{
1005 return 0;
1006}
1007
1008/*
1009 * Because preemptible RCU does not exist, it never needs CPU-offline
1010 * processing.
1011 */
1012static void rcu_preempt_offline_cpu(int cpu)
1013{
1014}
1015
1016#endif /* #ifdef CONFIG_HOTPLUG_CPU */
1017
1018/*
1019 * Because preemptible RCU does not exist, it never has any callbacks
1020 * to check.
1021 */
1022static void rcu_preempt_check_callbacks(int cpu)
1023{
1024}
1025
1026/*
1027 * Because preemptible RCU does not exist, it never has any callbacks
1028 * to process.
1029 */
1030static void rcu_preempt_process_callbacks(void)
1031{
1032}
1033
1034/*
1035 * Wait for an rcu-preempt grace period, but make it happen quickly.
1036 * But because preemptible RCU does not exist, map to rcu-sched.
1037 */
1038void synchronize_rcu_expedited(void)
1039{
1040 synchronize_sched_expedited();
1041}
1042EXPORT_SYMBOL_GPL(synchronize_rcu_expedited);
1043
1044#ifdef CONFIG_HOTPLUG_CPU
1045
1046/*
1047 * Because preemptible RCU does not exist, there is never any need to
1048 * report on tasks preempted in RCU read-side critical sections during
1049 * expedited RCU grace periods.
1050 */
1051static void rcu_report_exp_rnp(struct rcu_state *rsp, struct rcu_node *rnp)
1052{
1053 return;
1054}
1055
1056#endif /* #ifdef CONFIG_HOTPLUG_CPU */
1057
1058/*
1059 * Because preemptible RCU does not exist, it never has any work to do.
1060 */
1061static int rcu_preempt_pending(int cpu)
1062{
1063 return 0;
1064}
1065
1066/*
1067 * Because preemptible RCU does not exist, it never needs any CPU.
1068 */
1069static int rcu_preempt_needs_cpu(int cpu)
1070{
1071 return 0;
1072}
1073
1074/*
1075 * Because preemptible RCU does not exist, rcu_barrier() is just
1076 * another name for rcu_barrier_sched().
1077 */
1078void rcu_barrier(void)
1079{
1080 rcu_barrier_sched();
1081}
1082EXPORT_SYMBOL_GPL(rcu_barrier);
1083
1084/*
1085 * Because preemptible RCU does not exist, there is no per-CPU
1086 * data to initialize.
1087 */
1088static void __cpuinit rcu_preempt_init_percpu_data(int cpu)
1089{
1090}
1091
1092/*
1093 * Because there is no preemptible RCU, there are no callbacks to move.
1094 */
1095static void rcu_preempt_send_cbs_to_online(void)
1096{
1097}
1098
1099/*
1100 * Because preemptible RCU does not exist, it need not be initialized.
1101 */
1102static void __init __rcu_init_preempt(void)
1103{
1104}
1105
1106#endif /* #else #ifdef CONFIG_TREE_PREEMPT_RCU */
1107
1108#ifdef CONFIG_RCU_BOOST
1109
1110#include "rtmutex_common.h"
1111
1112#ifdef CONFIG_RCU_TRACE
1113
1114static void rcu_initiate_boost_trace(struct rcu_node *rnp)
1115{
1116 if (list_empty(&rnp->blkd_tasks))
1117 rnp->n_balk_blkd_tasks++;
1118 else if (rnp->exp_tasks == NULL && rnp->gp_tasks == NULL)
1119 rnp->n_balk_exp_gp_tasks++;
1120 else if (rnp->gp_tasks != NULL && rnp->boost_tasks != NULL)
1121 rnp->n_balk_boost_tasks++;
1122 else if (rnp->gp_tasks != NULL && rnp->qsmask != 0)
1123 rnp->n_balk_notblocked++;
1124 else if (rnp->gp_tasks != NULL &&
1125 ULONG_CMP_LT(jiffies, rnp->boost_time))
1126 rnp->n_balk_notyet++;
1127 else
1128 rnp->n_balk_nos++;
1129}
1130
1131#else /* #ifdef CONFIG_RCU_TRACE */
1132
1133static void rcu_initiate_boost_trace(struct rcu_node *rnp)
1134{
1135}
1136
1137#endif /* #else #ifdef CONFIG_RCU_TRACE */
1138
1139/*
1140 * Carry out RCU priority boosting on the task indicated by ->exp_tasks
1141 * or ->boost_tasks, advancing the pointer to the next task in the
1142 * ->blkd_tasks list.
1143 *
1144 * Note that irqs must be enabled: boosting the task can block.
1145 * Returns 1 if there are more tasks needing to be boosted.
1146 */
1147static int rcu_boost(struct rcu_node *rnp)
1148{
1149 unsigned long flags;
1150 struct rt_mutex mtx;
1151 struct task_struct *t;
1152 struct list_head *tb;
1153
1154 if (rnp->exp_tasks == NULL && rnp->boost_tasks == NULL)
1155 return 0; /* Nothing left to boost. */
1156
1157 raw_spin_lock_irqsave(&rnp->lock, flags);
1158
1159 /*
1160 * Recheck under the lock: all tasks in need of boosting
1161 * might exit their RCU read-side critical sections on their own.
1162 */
1163 if (rnp->exp_tasks == NULL && rnp->boost_tasks == NULL) {
1164 raw_spin_unlock_irqrestore(&rnp->lock, flags);
1165 return 0;
1166 }
1167
1168 /*
1169 * Preferentially boost tasks blocking expedited grace periods.
1170 * This cannot starve the normal grace periods because a second
1171 * expedited grace period must boost all blocked tasks, including
1172 * those blocking the pre-existing normal grace period.
1173 */
1174 if (rnp->exp_tasks != NULL) {
1175 tb = rnp->exp_tasks;
1176 rnp->n_exp_boosts++;
1177 } else {
1178 tb = rnp->boost_tasks;
1179 rnp->n_normal_boosts++;
1180 }
1181 rnp->n_tasks_boosted++;
1182
1183 /*
1184 * We boost task t by manufacturing an rt_mutex that appears to
1185 * be held by task t. We leave a pointer to that rt_mutex where
1186 * task t can find it, and task t will release the mutex when it
1187 * exits its outermost RCU read-side critical section. Then
1188 * simply acquiring this artificial rt_mutex will boost task
1189 * t's priority. (Thanks to tglx for suggesting this approach!)
1190 *
1191 * Note that task t must acquire rnp->lock to remove itself from
1192 * the ->blkd_tasks list, which it will do from exit() if from
1193 * nowhere else. We therefore are guaranteed that task t will
1194 * stay around at least until we drop rnp->lock. Note that
1195 * rnp->lock also resolves races between our priority boosting
1196 * and task t's exiting its outermost RCU read-side critical
1197 * section.
1198 */
1199 t = container_of(tb, struct task_struct, rcu_node_entry);
1200 rt_mutex_init_proxy_locked(&mtx, t);
1201 t->rcu_boost_mutex = &mtx;
1202 t->rcu_boosted = 1;
1203 raw_spin_unlock_irqrestore(&rnp->lock, flags);
1204 rt_mutex_lock(&mtx); /* Side effect: boosts task t's priority. */
1205 rt_mutex_unlock(&mtx); /* Keep lockdep happy. */
1206
1207 return rnp->exp_tasks != NULL || rnp->boost_tasks != NULL;
1208}
1209
1210/*
1211 * Timer handler to initiate waking up of boost kthreads that
1212 * have yielded the CPU due to excessive numbers of tasks to
1213 * boost. We wake up the per-rcu_node kthread, which in turn
1214 * will wake up the booster kthread.
1215 */
1216static void rcu_boost_kthread_timer(unsigned long arg)
1217{
1218 invoke_rcu_node_kthread((struct rcu_node *)arg);
1219}
1220
1221/*
1222 * Priority-boosting kthread. One per leaf rcu_node and one for the
1223 * root rcu_node.
1224 */
1225static int rcu_boost_kthread(void *arg)
1226{
1227 struct rcu_node *rnp = (struct rcu_node *)arg;
1228 int spincnt = 0;
1229 int more2boost;
1230
1231 for (;;) {
1232 rnp->boost_kthread_status = RCU_KTHREAD_WAITING;
1233 rcu_wait(rnp->boost_tasks || rnp->exp_tasks);
1234 rnp->boost_kthread_status = RCU_KTHREAD_RUNNING;
1235 more2boost = rcu_boost(rnp);
1236 if (more2boost)
1237 spincnt++;
1238 else
1239 spincnt = 0;
1240 if (spincnt > 10) {
1241 rcu_yield(rcu_boost_kthread_timer, (unsigned long)rnp);
1242 spincnt = 0;
1243 }
1244 }
1245 /* NOTREACHED */
1246 return 0;
1247}
1248
1249/*
1250 * Check to see if it is time to start boosting RCU readers that are
1251 * blocking the current grace period, and, if so, tell the per-rcu_node
1252 * kthread to start boosting them. If there is an expedited grace
1253 * period in progress, it is always time to boost.
1254 *
1255 * The caller must hold rnp->lock, which this function releases,
1256 * but irqs remain disabled. The ->boost_kthread_task is immortal,
1257 * so we don't need to worry about it going away.
1258 */
1259static void rcu_initiate_boost(struct rcu_node *rnp, unsigned long flags)
1260{
1261 struct task_struct *t;
1262
1263 if (!rcu_preempt_blocked_readers_cgp(rnp) && rnp->exp_tasks == NULL) {
1264 rnp->n_balk_exp_gp_tasks++;
1265 raw_spin_unlock_irqrestore(&rnp->lock, flags);
1266 return;
1267 }
1268 if (rnp->exp_tasks != NULL ||
1269 (rnp->gp_tasks != NULL &&
1270 rnp->boost_tasks == NULL &&
1271 rnp->qsmask == 0 &&
1272 ULONG_CMP_GE(jiffies, rnp->boost_time))) {
1273 if (rnp->exp_tasks == NULL)
1274 rnp->boost_tasks = rnp->gp_tasks;
1275 raw_spin_unlock_irqrestore(&rnp->lock, flags);
1276 t = rnp->boost_kthread_task;
1277 if (t != NULL)
1278 wake_up_process(t);
1279 } else {
1280 rcu_initiate_boost_trace(rnp);
1281 raw_spin_unlock_irqrestore(&rnp->lock, flags);
1282 }
1283}
1284
1285/*
1286 * Wake up the per-CPU kthread to invoke RCU callbacks.
1287 */
1288static void invoke_rcu_callbacks_kthread(void)
1289{
1290 unsigned long flags;
1291
1292 local_irq_save(flags);
1293 __this_cpu_write(rcu_cpu_has_work, 1);
1294 if (__this_cpu_read(rcu_cpu_kthread_task) == NULL) {
1295 local_irq_restore(flags);
1296 return;
1297 }
1298 wake_up_process(__this_cpu_read(rcu_cpu_kthread_task));
1299 local_irq_restore(flags);
1300}
1301
1302/*
1303 * Set the affinity of the boost kthread. The CPU-hotplug locks are
1304 * held, so no one should be messing with the existence of the boost
1305 * kthread.
1306 */
1307static void rcu_boost_kthread_setaffinity(struct rcu_node *rnp,
1308 cpumask_var_t cm)
1309{
1310 struct task_struct *t;
1311
1312 t = rnp->boost_kthread_task;
1313 if (t != NULL)
1314 set_cpus_allowed_ptr(rnp->boost_kthread_task, cm);
1315}
1316
1317#define RCU_BOOST_DELAY_JIFFIES DIV_ROUND_UP(CONFIG_RCU_BOOST_DELAY * HZ, 1000)
1318
1319/*
1320 * Do priority-boost accounting for the start of a new grace period.
1321 */
1322static void rcu_preempt_boost_start_gp(struct rcu_node *rnp)
1323{
1324 rnp->boost_time = jiffies + RCU_BOOST_DELAY_JIFFIES;
1325}
1326
1327/*
1328 * Create an RCU-boost kthread for the specified node if one does not
1329 * already exist. We only create this kthread for preemptible RCU.
1330 * Returns zero if all is well, a negated errno otherwise.
1331 */
1332static int __cpuinit rcu_spawn_one_boost_kthread(struct rcu_state *rsp,
1333 struct rcu_node *rnp,
1334 int rnp_index)
1335{
1336 unsigned long flags;
1337 struct sched_param sp;
1338 struct task_struct *t;
1339
1340 if (&rcu_preempt_state != rsp)
1341 return 0;
1342 rsp->boost = 1;
1343 if (rnp->boost_kthread_task != NULL)
1344 return 0;
1345 t = kthread_create(rcu_boost_kthread, (void *)rnp,
1346 "rcub%d", rnp_index);
1347 if (IS_ERR(t))
1348 return PTR_ERR(t);
1349 raw_spin_lock_irqsave(&rnp->lock, flags);
1350 rnp->boost_kthread_task = t;
1351 raw_spin_unlock_irqrestore(&rnp->lock, flags);
1352 sp.sched_priority = RCU_KTHREAD_PRIO;
1353 sched_setscheduler_nocheck(t, SCHED_FIFO, &sp);
1354 wake_up_process(t); /* get to TASK_INTERRUPTIBLE quickly. */
1355 return 0;
1356}
1357
1358#ifdef CONFIG_HOTPLUG_CPU
1359
1360/*
1361 * Stop the RCU's per-CPU kthread when its CPU goes offline,.
1362 */
1363static void rcu_stop_cpu_kthread(int cpu)
1364{
1365 struct task_struct *t;
1366
1367 /* Stop the CPU's kthread. */
1368 t = per_cpu(rcu_cpu_kthread_task, cpu);
1369 if (t != NULL) {
1370 per_cpu(rcu_cpu_kthread_task, cpu) = NULL;
1371 kthread_stop(t);
1372 }
1373}
1374
1375#endif /* #ifdef CONFIG_HOTPLUG_CPU */
1376
1377static void rcu_kthread_do_work(void)
1378{
1379 rcu_do_batch(&rcu_sched_state, &__get_cpu_var(rcu_sched_data));
1380 rcu_do_batch(&rcu_bh_state, &__get_cpu_var(rcu_bh_data));
1381 rcu_preempt_do_callbacks();
1382}
1383
1384/*
1385 * Wake up the specified per-rcu_node-structure kthread.
1386 * Because the per-rcu_node kthreads are immortal, we don't need
1387 * to do anything to keep them alive.
1388 */
1389static void invoke_rcu_node_kthread(struct rcu_node *rnp)
1390{
1391 struct task_struct *t;
1392
1393 t = rnp->node_kthread_task;
1394 if (t != NULL)
1395 wake_up_process(t);
1396}
1397
1398/*
1399 * Set the specified CPU's kthread to run RT or not, as specified by
1400 * the to_rt argument. The CPU-hotplug locks are held, so the task
1401 * is not going away.
1402 */
1403static void rcu_cpu_kthread_setrt(int cpu, int to_rt)
1404{
1405 int policy;
1406 struct sched_param sp;
1407 struct task_struct *t;
1408
1409 t = per_cpu(rcu_cpu_kthread_task, cpu);
1410 if (t == NULL)
1411 return;
1412 if (to_rt) {
1413 policy = SCHED_FIFO;
1414 sp.sched_priority = RCU_KTHREAD_PRIO;
1415 } else {
1416 policy = SCHED_NORMAL;
1417 sp.sched_priority = 0;
1418 }
1419 sched_setscheduler_nocheck(t, policy, &sp);
1420}
1421
1422/*
1423 * Timer handler to initiate the waking up of per-CPU kthreads that
1424 * have yielded the CPU due to excess numbers of RCU callbacks.
1425 * We wake up the per-rcu_node kthread, which in turn will wake up
1426 * the booster kthread.
1427 */
1428static void rcu_cpu_kthread_timer(unsigned long arg)
1429{
1430 struct rcu_data *rdp = per_cpu_ptr(rcu_state->rda, arg);
1431 struct rcu_node *rnp = rdp->mynode;
1432
1433 atomic_or(rdp->grpmask, &rnp->wakemask);
1434 invoke_rcu_node_kthread(rnp);
1435}
1436
1437/*
1438 * Drop to non-real-time priority and yield, but only after posting a
1439 * timer that will cause us to regain our real-time priority if we
1440 * remain preempted. Either way, we restore our real-time priority
1441 * before returning.
1442 */
1443static void rcu_yield(void (*f)(unsigned long), unsigned long arg)
1444{
1445 struct sched_param sp;
1446 struct timer_list yield_timer;
1447
1448 setup_timer_on_stack(&yield_timer, f, arg);
1449 mod_timer(&yield_timer, jiffies + 2);
1450 sp.sched_priority = 0;
1451 sched_setscheduler_nocheck(current, SCHED_NORMAL, &sp);
1452 set_user_nice(current, 19);
1453 schedule();
1454 sp.sched_priority = RCU_KTHREAD_PRIO;
1455 sched_setscheduler_nocheck(current, SCHED_FIFO, &sp);
1456 del_timer(&yield_timer);
1457}
1458
1459/*
1460 * Handle cases where the rcu_cpu_kthread() ends up on the wrong CPU.
1461 * This can happen while the corresponding CPU is either coming online
1462 * or going offline. We cannot wait until the CPU is fully online
1463 * before starting the kthread, because the various notifier functions
1464 * can wait for RCU grace periods. So we park rcu_cpu_kthread() until
1465 * the corresponding CPU is online.
1466 *
1467 * Return 1 if the kthread needs to stop, 0 otherwise.
1468 *
1469 * Caller must disable bh. This function can momentarily enable it.
1470 */
1471static int rcu_cpu_kthread_should_stop(int cpu)
1472{
1473 while (cpu_is_offline(cpu) ||
1474 !cpumask_equal(¤t->cpus_allowed, cpumask_of(cpu)) ||
1475 smp_processor_id() != cpu) {
1476 if (kthread_should_stop())
1477 return 1;
1478 per_cpu(rcu_cpu_kthread_status, cpu) = RCU_KTHREAD_OFFCPU;
1479 per_cpu(rcu_cpu_kthread_cpu, cpu) = raw_smp_processor_id();
1480 local_bh_enable();
1481 schedule_timeout_uninterruptible(1);
1482 if (!cpumask_equal(¤t->cpus_allowed, cpumask_of(cpu)))
1483 set_cpus_allowed_ptr(current, cpumask_of(cpu));
1484 local_bh_disable();
1485 }
1486 per_cpu(rcu_cpu_kthread_cpu, cpu) = cpu;
1487 return 0;
1488}
1489
1490/*
1491 * Per-CPU kernel thread that invokes RCU callbacks. This replaces the
1492 * earlier RCU softirq.
1493 */
1494static int rcu_cpu_kthread(void *arg)
1495{
1496 int cpu = (int)(long)arg;
1497 unsigned long flags;
1498 int spincnt = 0;
1499 unsigned int *statusp = &per_cpu(rcu_cpu_kthread_status, cpu);
1500 char work;
1501 char *workp = &per_cpu(rcu_cpu_has_work, cpu);
1502
1503 for (;;) {
1504 *statusp = RCU_KTHREAD_WAITING;
1505 rcu_wait(*workp != 0 || kthread_should_stop());
1506 local_bh_disable();
1507 if (rcu_cpu_kthread_should_stop(cpu)) {
1508 local_bh_enable();
1509 break;
1510 }
1511 *statusp = RCU_KTHREAD_RUNNING;
1512 per_cpu(rcu_cpu_kthread_loops, cpu)++;
1513 local_irq_save(flags);
1514 work = *workp;
1515 *workp = 0;
1516 local_irq_restore(flags);
1517 if (work)
1518 rcu_kthread_do_work();
1519 local_bh_enable();
1520 if (*workp != 0)
1521 spincnt++;
1522 else
1523 spincnt = 0;
1524 if (spincnt > 10) {
1525 *statusp = RCU_KTHREAD_YIELDING;
1526 rcu_yield(rcu_cpu_kthread_timer, (unsigned long)cpu);
1527 spincnt = 0;
1528 }
1529 }
1530 *statusp = RCU_KTHREAD_STOPPED;
1531 return 0;
1532}
1533
1534/*
1535 * Spawn a per-CPU kthread, setting up affinity and priority.
1536 * Because the CPU hotplug lock is held, no other CPU will be attempting
1537 * to manipulate rcu_cpu_kthread_task. There might be another CPU
1538 * attempting to access it during boot, but the locking in kthread_bind()
1539 * will enforce sufficient ordering.
1540 *
1541 * Please note that we cannot simply refuse to wake up the per-CPU
1542 * kthread because kthreads are created in TASK_UNINTERRUPTIBLE state,
1543 * which can result in softlockup complaints if the task ends up being
1544 * idle for more than a couple of minutes.
1545 *
1546 * However, please note also that we cannot bind the per-CPU kthread to its
1547 * CPU until that CPU is fully online. We also cannot wait until the
1548 * CPU is fully online before we create its per-CPU kthread, as this would
1549 * deadlock the system when CPU notifiers tried waiting for grace
1550 * periods. So we bind the per-CPU kthread to its CPU only if the CPU
1551 * is online. If its CPU is not yet fully online, then the code in
1552 * rcu_cpu_kthread() will wait until it is fully online, and then do
1553 * the binding.
1554 */
1555static int __cpuinit rcu_spawn_one_cpu_kthread(int cpu)
1556{
1557 struct sched_param sp;
1558 struct task_struct *t;
1559
1560 if (!rcu_scheduler_fully_active ||
1561 per_cpu(rcu_cpu_kthread_task, cpu) != NULL)
1562 return 0;
1563 t = kthread_create(rcu_cpu_kthread, (void *)(long)cpu, "rcuc%d", cpu);
1564 if (IS_ERR(t))
1565 return PTR_ERR(t);
1566 if (cpu_online(cpu))
1567 kthread_bind(t, cpu);
1568 per_cpu(rcu_cpu_kthread_cpu, cpu) = cpu;
1569 WARN_ON_ONCE(per_cpu(rcu_cpu_kthread_task, cpu) != NULL);
1570 sp.sched_priority = RCU_KTHREAD_PRIO;
1571 sched_setscheduler_nocheck(t, SCHED_FIFO, &sp);
1572 per_cpu(rcu_cpu_kthread_task, cpu) = t;
1573 wake_up_process(t); /* Get to TASK_INTERRUPTIBLE quickly. */
1574 return 0;
1575}
1576
1577/*
1578 * Per-rcu_node kthread, which is in charge of waking up the per-CPU
1579 * kthreads when needed. We ignore requests to wake up kthreads
1580 * for offline CPUs, which is OK because force_quiescent_state()
1581 * takes care of this case.
1582 */
1583static int rcu_node_kthread(void *arg)
1584{
1585 int cpu;
1586 unsigned long flags;
1587 unsigned long mask;
1588 struct rcu_node *rnp = (struct rcu_node *)arg;
1589 struct sched_param sp;
1590 struct task_struct *t;
1591
1592 for (;;) {
1593 rnp->node_kthread_status = RCU_KTHREAD_WAITING;
1594 rcu_wait(atomic_read(&rnp->wakemask) != 0);
1595 rnp->node_kthread_status = RCU_KTHREAD_RUNNING;
1596 raw_spin_lock_irqsave(&rnp->lock, flags);
1597 mask = atomic_xchg(&rnp->wakemask, 0);
1598 rcu_initiate_boost(rnp, flags); /* releases rnp->lock. */
1599 for (cpu = rnp->grplo; cpu <= rnp->grphi; cpu++, mask >>= 1) {
1600 if ((mask & 0x1) == 0)
1601 continue;
1602 preempt_disable();
1603 t = per_cpu(rcu_cpu_kthread_task, cpu);
1604 if (!cpu_online(cpu) || t == NULL) {
1605 preempt_enable();
1606 continue;
1607 }
1608 per_cpu(rcu_cpu_has_work, cpu) = 1;
1609 sp.sched_priority = RCU_KTHREAD_PRIO;
1610 sched_setscheduler_nocheck(t, SCHED_FIFO, &sp);
1611 preempt_enable();
1612 }
1613 }
1614 /* NOTREACHED */
1615 rnp->node_kthread_status = RCU_KTHREAD_STOPPED;
1616 return 0;
1617}
1618
1619/*
1620 * Set the per-rcu_node kthread's affinity to cover all CPUs that are
1621 * served by the rcu_node in question. The CPU hotplug lock is still
1622 * held, so the value of rnp->qsmaskinit will be stable.
1623 *
1624 * We don't include outgoingcpu in the affinity set, use -1 if there is
1625 * no outgoing CPU. If there are no CPUs left in the affinity set,
1626 * this function allows the kthread to execute on any CPU.
1627 */
1628static void rcu_node_kthread_setaffinity(struct rcu_node *rnp, int outgoingcpu)
1629{
1630 cpumask_var_t cm;
1631 int cpu;
1632 unsigned long mask = rnp->qsmaskinit;
1633
1634 if (rnp->node_kthread_task == NULL)
1635 return;
1636 if (!alloc_cpumask_var(&cm, GFP_KERNEL))
1637 return;
1638 cpumask_clear(cm);
1639 for (cpu = rnp->grplo; cpu <= rnp->grphi; cpu++, mask >>= 1)
1640 if ((mask & 0x1) && cpu != outgoingcpu)
1641 cpumask_set_cpu(cpu, cm);
1642 if (cpumask_weight(cm) == 0) {
1643 cpumask_setall(cm);
1644 for (cpu = rnp->grplo; cpu <= rnp->grphi; cpu++)
1645 cpumask_clear_cpu(cpu, cm);
1646 WARN_ON_ONCE(cpumask_weight(cm) == 0);
1647 }
1648 set_cpus_allowed_ptr(rnp->node_kthread_task, cm);
1649 rcu_boost_kthread_setaffinity(rnp, cm);
1650 free_cpumask_var(cm);
1651}
1652
1653/*
1654 * Spawn a per-rcu_node kthread, setting priority and affinity.
1655 * Called during boot before online/offline can happen, or, if
1656 * during runtime, with the main CPU-hotplug locks held. So only
1657 * one of these can be executing at a time.
1658 */
1659static int __cpuinit rcu_spawn_one_node_kthread(struct rcu_state *rsp,
1660 struct rcu_node *rnp)
1661{
1662 unsigned long flags;
1663 int rnp_index = rnp - &rsp->node[0];
1664 struct sched_param sp;
1665 struct task_struct *t;
1666
1667 if (!rcu_scheduler_fully_active ||
1668 rnp->qsmaskinit == 0)
1669 return 0;
1670 if (rnp->node_kthread_task == NULL) {
1671 t = kthread_create(rcu_node_kthread, (void *)rnp,
1672 "rcun%d", rnp_index);
1673 if (IS_ERR(t))
1674 return PTR_ERR(t);
1675 raw_spin_lock_irqsave(&rnp->lock, flags);
1676 rnp->node_kthread_task = t;
1677 raw_spin_unlock_irqrestore(&rnp->lock, flags);
1678 sp.sched_priority = 99;
1679 sched_setscheduler_nocheck(t, SCHED_FIFO, &sp);
1680 wake_up_process(t); /* get to TASK_INTERRUPTIBLE quickly. */
1681 }
1682 return rcu_spawn_one_boost_kthread(rsp, rnp, rnp_index);
1683}
1684
1685/*
1686 * Spawn all kthreads -- called as soon as the scheduler is running.
1687 */
1688static int __init rcu_spawn_kthreads(void)
1689{
1690 int cpu;
1691 struct rcu_node *rnp;
1692
1693 rcu_scheduler_fully_active = 1;
1694 for_each_possible_cpu(cpu) {
1695 per_cpu(rcu_cpu_has_work, cpu) = 0;
1696 if (cpu_online(cpu))
1697 (void)rcu_spawn_one_cpu_kthread(cpu);
1698 }
1699 rnp = rcu_get_root(rcu_state);
1700 (void)rcu_spawn_one_node_kthread(rcu_state, rnp);
1701 if (NUM_RCU_NODES > 1) {
1702 rcu_for_each_leaf_node(rcu_state, rnp)
1703 (void)rcu_spawn_one_node_kthread(rcu_state, rnp);
1704 }
1705 return 0;
1706}
1707early_initcall(rcu_spawn_kthreads);
1708
1709static void __cpuinit rcu_prepare_kthreads(int cpu)
1710{
1711 struct rcu_data *rdp = per_cpu_ptr(rcu_state->rda, cpu);
1712 struct rcu_node *rnp = rdp->mynode;
1713
1714 /* Fire up the incoming CPU's kthread and leaf rcu_node kthread. */
1715 if (rcu_scheduler_fully_active) {
1716 (void)rcu_spawn_one_cpu_kthread(cpu);
1717 if (rnp->node_kthread_task == NULL)
1718 (void)rcu_spawn_one_node_kthread(rcu_state, rnp);
1719 }
1720}
1721
1722#else /* #ifdef CONFIG_RCU_BOOST */
1723
1724static void rcu_initiate_boost(struct rcu_node *rnp, unsigned long flags)
1725{
1726 raw_spin_unlock_irqrestore(&rnp->lock, flags);
1727}
1728
1729static void invoke_rcu_callbacks_kthread(void)
1730{
1731 WARN_ON_ONCE(1);
1732}
1733
1734static void rcu_preempt_boost_start_gp(struct rcu_node *rnp)
1735{
1736}
1737
1738#ifdef CONFIG_HOTPLUG_CPU
1739
1740static void rcu_stop_cpu_kthread(int cpu)
1741{
1742}
1743
1744#endif /* #ifdef CONFIG_HOTPLUG_CPU */
1745
1746static void rcu_node_kthread_setaffinity(struct rcu_node *rnp, int outgoingcpu)
1747{
1748}
1749
1750static void rcu_cpu_kthread_setrt(int cpu, int to_rt)
1751{
1752}
1753
1754static int __init rcu_scheduler_really_started(void)
1755{
1756 rcu_scheduler_fully_active = 1;
1757 return 0;
1758}
1759early_initcall(rcu_scheduler_really_started);
1760
1761static void __cpuinit rcu_prepare_kthreads(int cpu)
1762{
1763}
1764
1765#endif /* #else #ifdef CONFIG_RCU_BOOST */
1766
1767#ifndef CONFIG_SMP
1768
1769void synchronize_sched_expedited(void)
1770{
1771 cond_resched();
1772}
1773EXPORT_SYMBOL_GPL(synchronize_sched_expedited);
1774
1775#else /* #ifndef CONFIG_SMP */
1776
1777static atomic_t sync_sched_expedited_started = ATOMIC_INIT(0);
1778static atomic_t sync_sched_expedited_done = ATOMIC_INIT(0);
1779
1780static int synchronize_sched_expedited_cpu_stop(void *data)
1781{
1782 /*
1783 * There must be a full memory barrier on each affected CPU
1784 * between the time that try_stop_cpus() is called and the
1785 * time that it returns.
1786 *
1787 * In the current initial implementation of cpu_stop, the
1788 * above condition is already met when the control reaches
1789 * this point and the following smp_mb() is not strictly
1790 * necessary. Do smp_mb() anyway for documentation and
1791 * robustness against future implementation changes.
1792 */
1793 smp_mb(); /* See above comment block. */
1794 return 0;
1795}
1796
1797/*
1798 * Wait for an rcu-sched grace period to elapse, but use "big hammer"
1799 * approach to force grace period to end quickly. This consumes
1800 * significant time on all CPUs, and is thus not recommended for
1801 * any sort of common-case code.
1802 *
1803 * Note that it is illegal to call this function while holding any
1804 * lock that is acquired by a CPU-hotplug notifier. Failing to
1805 * observe this restriction will result in deadlock.
1806 *
1807 * This implementation can be thought of as an application of ticket
1808 * locking to RCU, with sync_sched_expedited_started and
1809 * sync_sched_expedited_done taking on the roles of the halves
1810 * of the ticket-lock word. Each task atomically increments
1811 * sync_sched_expedited_started upon entry, snapshotting the old value,
1812 * then attempts to stop all the CPUs. If this succeeds, then each
1813 * CPU will have executed a context switch, resulting in an RCU-sched
1814 * grace period. We are then done, so we use atomic_cmpxchg() to
1815 * update sync_sched_expedited_done to match our snapshot -- but
1816 * only if someone else has not already advanced past our snapshot.
1817 *
1818 * On the other hand, if try_stop_cpus() fails, we check the value
1819 * of sync_sched_expedited_done. If it has advanced past our
1820 * initial snapshot, then someone else must have forced a grace period
1821 * some time after we took our snapshot. In this case, our work is
1822 * done for us, and we can simply return. Otherwise, we try again,
1823 * but keep our initial snapshot for purposes of checking for someone
1824 * doing our work for us.
1825 *
1826 * If we fail too many times in a row, we fall back to synchronize_sched().
1827 */
1828void synchronize_sched_expedited(void)
1829{
1830 int firstsnap, s, snap, trycount = 0;
1831
1832 /* Note that atomic_inc_return() implies full memory barrier. */
1833 firstsnap = snap = atomic_inc_return(&sync_sched_expedited_started);
1834 get_online_cpus();
1835
1836 /*
1837 * Each pass through the following loop attempts to force a
1838 * context switch on each CPU.
1839 */
1840 while (try_stop_cpus(cpu_online_mask,
1841 synchronize_sched_expedited_cpu_stop,
1842 NULL) == -EAGAIN) {
1843 put_online_cpus();
1844
1845 /* No joy, try again later. Or just synchronize_sched(). */
1846 if (trycount++ < 10)
1847 udelay(trycount * num_online_cpus());
1848 else {
1849 synchronize_sched();
1850 return;
1851 }
1852
1853 /* Check to see if someone else did our work for us. */
1854 s = atomic_read(&sync_sched_expedited_done);
1855 if (UINT_CMP_GE((unsigned)s, (unsigned)firstsnap)) {
1856 smp_mb(); /* ensure test happens before caller kfree */
1857 return;
1858 }
1859
1860 /*
1861 * Refetching sync_sched_expedited_started allows later
1862 * callers to piggyback on our grace period. We subtract
1863 * 1 to get the same token that the last incrementer got.
1864 * We retry after they started, so our grace period works
1865 * for them, and they started after our first try, so their
1866 * grace period works for us.
1867 */
1868 get_online_cpus();
1869 snap = atomic_read(&sync_sched_expedited_started) - 1;
1870 smp_mb(); /* ensure read is before try_stop_cpus(). */
1871 }
1872
1873 /*
1874 * Everyone up to our most recent fetch is covered by our grace
1875 * period. Update the counter, but only if our work is still
1876 * relevant -- which it won't be if someone who started later
1877 * than we did beat us to the punch.
1878 */
1879 do {
1880 s = atomic_read(&sync_sched_expedited_done);
1881 if (UINT_CMP_GE((unsigned)s, (unsigned)snap)) {
1882 smp_mb(); /* ensure test happens before caller kfree */
1883 break;
1884 }
1885 } while (atomic_cmpxchg(&sync_sched_expedited_done, s, snap) != s);
1886
1887 put_online_cpus();
1888}
1889EXPORT_SYMBOL_GPL(synchronize_sched_expedited);
1890
1891#endif /* #else #ifndef CONFIG_SMP */
1892
1893#if !defined(CONFIG_RCU_FAST_NO_HZ)
1894
1895/*
1896 * Check to see if any future RCU-related work will need to be done
1897 * by the current CPU, even if none need be done immediately, returning
1898 * 1 if so. This function is part of the RCU implementation; it is -not-
1899 * an exported member of the RCU API.
1900 *
1901 * Because we have preemptible RCU, just check whether this CPU needs
1902 * any flavor of RCU. Do not chew up lots of CPU cycles with preemption
1903 * disabled in a most-likely vain attempt to cause RCU not to need this CPU.
1904 */
1905int rcu_needs_cpu(int cpu)
1906{
1907 return rcu_needs_cpu_quick_check(cpu);
1908}
1909
1910/*
1911 * Check to see if we need to continue a callback-flush operations to
1912 * allow the last CPU to enter dyntick-idle mode. But fast dyntick-idle
1913 * entry is not configured, so we never do need to.
1914 */
1915static void rcu_needs_cpu_flush(void)
1916{
1917}
1918
1919#else /* #if !defined(CONFIG_RCU_FAST_NO_HZ) */
1920
1921#define RCU_NEEDS_CPU_FLUSHES 5
1922static DEFINE_PER_CPU(int, rcu_dyntick_drain);
1923static DEFINE_PER_CPU(unsigned long, rcu_dyntick_holdoff);
1924
1925/*
1926 * Check to see if any future RCU-related work will need to be done
1927 * by the current CPU, even if none need be done immediately, returning
1928 * 1 if so. This function is part of the RCU implementation; it is -not-
1929 * an exported member of the RCU API.
1930 *
1931 * Because we are not supporting preemptible RCU, attempt to accelerate
1932 * any current grace periods so that RCU no longer needs this CPU, but
1933 * only if all other CPUs are already in dynticks-idle mode. This will
1934 * allow the CPU cores to be powered down immediately, as opposed to after
1935 * waiting many milliseconds for grace periods to elapse.
1936 *
1937 * Because it is not legal to invoke rcu_process_callbacks() with irqs
1938 * disabled, we do one pass of force_quiescent_state(), then do a
1939 * invoke_rcu_core() to cause rcu_process_callbacks() to be invoked
1940 * later. The per-cpu rcu_dyntick_drain variable controls the sequencing.
1941 */
1942int rcu_needs_cpu(int cpu)
1943{
1944 int c = 0;
1945 int snap;
1946 int thatcpu;
1947
1948 /* Check for being in the holdoff period. */
1949 if (per_cpu(rcu_dyntick_holdoff, cpu) == jiffies)
1950 return rcu_needs_cpu_quick_check(cpu);
1951
1952 /* Don't bother unless we are the last non-dyntick-idle CPU. */
1953 for_each_online_cpu(thatcpu) {
1954 if (thatcpu == cpu)
1955 continue;
1956 snap = atomic_add_return(0, &per_cpu(rcu_dynticks,
1957 thatcpu).dynticks);
1958 smp_mb(); /* Order sampling of snap with end of grace period. */
1959 if ((snap & 0x1) != 0) {
1960 per_cpu(rcu_dyntick_drain, cpu) = 0;
1961 per_cpu(rcu_dyntick_holdoff, cpu) = jiffies - 1;
1962 return rcu_needs_cpu_quick_check(cpu);
1963 }
1964 }
1965
1966 /* Check and update the rcu_dyntick_drain sequencing. */
1967 if (per_cpu(rcu_dyntick_drain, cpu) <= 0) {
1968 /* First time through, initialize the counter. */
1969 per_cpu(rcu_dyntick_drain, cpu) = RCU_NEEDS_CPU_FLUSHES;
1970 } else if (--per_cpu(rcu_dyntick_drain, cpu) <= 0) {
1971 /* We have hit the limit, so time to give up. */
1972 per_cpu(rcu_dyntick_holdoff, cpu) = jiffies;
1973 return rcu_needs_cpu_quick_check(cpu);
1974 }
1975
1976 /* Do one step pushing remaining RCU callbacks through. */
1977 if (per_cpu(rcu_sched_data, cpu).nxtlist) {
1978 rcu_sched_qs(cpu);
1979 force_quiescent_state(&rcu_sched_state, 0);
1980 c = c || per_cpu(rcu_sched_data, cpu).nxtlist;
1981 }
1982 if (per_cpu(rcu_bh_data, cpu).nxtlist) {
1983 rcu_bh_qs(cpu);
1984 force_quiescent_state(&rcu_bh_state, 0);
1985 c = c || per_cpu(rcu_bh_data, cpu).nxtlist;
1986 }
1987
1988 /* If RCU callbacks are still pending, RCU still needs this CPU. */
1989 if (c)
1990 invoke_rcu_core();
1991 return c;
1992}
1993
1994/*
1995 * Check to see if we need to continue a callback-flush operations to
1996 * allow the last CPU to enter dyntick-idle mode.
1997 */
1998static void rcu_needs_cpu_flush(void)
1999{
2000 int cpu = smp_processor_id();
2001 unsigned long flags;
2002
2003 if (per_cpu(rcu_dyntick_drain, cpu) <= 0)
2004 return;
2005 local_irq_save(flags);
2006 (void)rcu_needs_cpu(cpu);
2007 local_irq_restore(flags);
2008}
2009
2010#endif /* #else #if !defined(CONFIG_RCU_FAST_NO_HZ) */