xref: /openbmc/linux/kernel/rcu/tree_exp.h (revision 55b078f0)
1 /*
2  * RCU expedited grace periods
3  *
4  * This program is free software; you can redistribute it and/or modify
5  * it under the terms of the GNU General Public License as published by
6  * the Free Software Foundation; either version 2 of the License, or
7  * (at your option) any later version.
8  *
9  * This program is distributed in the hope that it will be useful,
10  * but WITHOUT ANY WARRANTY; without even the implied warranty of
11  * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
12  * GNU General Public License for more details.
13  *
14  * You should have received a copy of the GNU General Public License
15  * along with this program; if not, you can access it online at
16  * http://www.gnu.org/licenses/gpl-2.0.html.
17  *
18  * Copyright IBM Corporation, 2016
19  *
20  * Authors: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
21  */
22 
23 #include <linux/lockdep.h>
24 
25 /*
26  * Record the start of an expedited grace period.
27  */
28 static void rcu_exp_gp_seq_start(void)
29 {
30 	rcu_seq_start(&rcu_state.expedited_sequence);
31 }
32 
33 /*
34  * Return then value that expedited-grace-period counter will have
35  * at the end of the current grace period.
36  */
37 static __maybe_unused unsigned long rcu_exp_gp_seq_endval(void)
38 {
39 	return rcu_seq_endval(&rcu_state.expedited_sequence);
40 }
41 
42 /*
43  * Record the end of an expedited grace period.
44  */
45 static void rcu_exp_gp_seq_end(void)
46 {
47 	rcu_seq_end(&rcu_state.expedited_sequence);
48 	smp_mb(); /* Ensure that consecutive grace periods serialize. */
49 }
50 
51 /*
52  * Take a snapshot of the expedited-grace-period counter.
53  */
54 static unsigned long rcu_exp_gp_seq_snap(void)
55 {
56 	unsigned long s;
57 
58 	smp_mb(); /* Caller's modifications seen first by other CPUs. */
59 	s = rcu_seq_snap(&rcu_state.expedited_sequence);
60 	trace_rcu_exp_grace_period(rcu_state.name, s, TPS("snap"));
61 	return s;
62 }
63 
64 /*
65  * Given a counter snapshot from rcu_exp_gp_seq_snap(), return true
66  * if a full expedited grace period has elapsed since that snapshot
67  * was taken.
68  */
69 static bool rcu_exp_gp_seq_done(unsigned long s)
70 {
71 	return rcu_seq_done(&rcu_state.expedited_sequence, s);
72 }
73 
74 /*
75  * Reset the ->expmaskinit values in the rcu_node tree to reflect any
76  * recent CPU-online activity.  Note that these masks are not cleared
77  * when CPUs go offline, so they reflect the union of all CPUs that have
78  * ever been online.  This means that this function normally takes its
79  * no-work-to-do fastpath.
80  */
81 static void sync_exp_reset_tree_hotplug(void)
82 {
83 	bool done;
84 	unsigned long flags;
85 	unsigned long mask;
86 	unsigned long oldmask;
87 	int ncpus = smp_load_acquire(&rcu_state.ncpus); /* Order vs. locking. */
88 	struct rcu_node *rnp;
89 	struct rcu_node *rnp_up;
90 
91 	/* If no new CPUs onlined since last time, nothing to do. */
92 	if (likely(ncpus == rcu_state.ncpus_snap))
93 		return;
94 	rcu_state.ncpus_snap = ncpus;
95 
96 	/*
97 	 * Each pass through the following loop propagates newly onlined
98 	 * CPUs for the current rcu_node structure up the rcu_node tree.
99 	 */
100 	rcu_for_each_leaf_node(rnp) {
101 		raw_spin_lock_irqsave_rcu_node(rnp, flags);
102 		if (rnp->expmaskinit == rnp->expmaskinitnext) {
103 			raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
104 			continue;  /* No new CPUs, nothing to do. */
105 		}
106 
107 		/* Update this node's mask, track old value for propagation. */
108 		oldmask = rnp->expmaskinit;
109 		rnp->expmaskinit = rnp->expmaskinitnext;
110 		raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
111 
112 		/* If was already nonzero, nothing to propagate. */
113 		if (oldmask)
114 			continue;
115 
116 		/* Propagate the new CPU up the tree. */
117 		mask = rnp->grpmask;
118 		rnp_up = rnp->parent;
119 		done = false;
120 		while (rnp_up) {
121 			raw_spin_lock_irqsave_rcu_node(rnp_up, flags);
122 			if (rnp_up->expmaskinit)
123 				done = true;
124 			rnp_up->expmaskinit |= mask;
125 			raw_spin_unlock_irqrestore_rcu_node(rnp_up, flags);
126 			if (done)
127 				break;
128 			mask = rnp_up->grpmask;
129 			rnp_up = rnp_up->parent;
130 		}
131 	}
132 }
133 
134 /*
135  * Reset the ->expmask values in the rcu_node tree in preparation for
136  * a new expedited grace period.
137  */
138 static void __maybe_unused sync_exp_reset_tree(void)
139 {
140 	unsigned long flags;
141 	struct rcu_node *rnp;
142 
143 	sync_exp_reset_tree_hotplug();
144 	rcu_for_each_node_breadth_first(rnp) {
145 		raw_spin_lock_irqsave_rcu_node(rnp, flags);
146 		WARN_ON_ONCE(rnp->expmask);
147 		rnp->expmask = rnp->expmaskinit;
148 		raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
149 	}
150 }
151 
152 /*
153  * Return non-zero if there is no RCU expedited grace period in progress
154  * for the specified rcu_node structure, in other words, if all CPUs and
155  * tasks covered by the specified rcu_node structure have done their bit
156  * for the current expedited grace period.  Works only for preemptible
157  * RCU -- other RCU implementation use other means.
158  *
159  * Caller must hold the specificed rcu_node structure's ->lock
160  */
161 static bool sync_rcu_preempt_exp_done(struct rcu_node *rnp)
162 {
163 	raw_lockdep_assert_held_rcu_node(rnp);
164 
165 	return rnp->exp_tasks == NULL &&
166 	       READ_ONCE(rnp->expmask) == 0;
167 }
168 
169 /*
170  * Like sync_rcu_preempt_exp_done(), but this function assumes the caller
171  * doesn't hold the rcu_node's ->lock, and will acquire and release the lock
172  * itself
173  */
174 static bool sync_rcu_preempt_exp_done_unlocked(struct rcu_node *rnp)
175 {
176 	unsigned long flags;
177 	bool ret;
178 
179 	raw_spin_lock_irqsave_rcu_node(rnp, flags);
180 	ret = sync_rcu_preempt_exp_done(rnp);
181 	raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
182 
183 	return ret;
184 }
185 
186 
187 /*
188  * Report the exit from RCU read-side critical section for the last task
189  * that queued itself during or before the current expedited preemptible-RCU
190  * grace period.  This event is reported either to the rcu_node structure on
191  * which the task was queued or to one of that rcu_node structure's ancestors,
192  * recursively up the tree.  (Calm down, calm down, we do the recursion
193  * iteratively!)
194  *
195  * Caller must hold the specified rcu_node structure's ->lock.
196  */
197 static void __rcu_report_exp_rnp(struct rcu_node *rnp,
198 				 bool wake, unsigned long flags)
199 	__releases(rnp->lock)
200 {
201 	unsigned long mask;
202 
203 	for (;;) {
204 		if (!sync_rcu_preempt_exp_done(rnp)) {
205 			if (!rnp->expmask)
206 				rcu_initiate_boost(rnp, flags);
207 			else
208 				raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
209 			break;
210 		}
211 		if (rnp->parent == NULL) {
212 			raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
213 			if (wake) {
214 				smp_mb(); /* EGP done before wake_up(). */
215 				swake_up_one(&rcu_state.expedited_wq);
216 			}
217 			break;
218 		}
219 		mask = rnp->grpmask;
220 		raw_spin_unlock_rcu_node(rnp); /* irqs remain disabled */
221 		rnp = rnp->parent;
222 		raw_spin_lock_rcu_node(rnp); /* irqs already disabled */
223 		WARN_ON_ONCE(!(rnp->expmask & mask));
224 		rnp->expmask &= ~mask;
225 	}
226 }
227 
228 /*
229  * Report expedited quiescent state for specified node.  This is a
230  * lock-acquisition wrapper function for __rcu_report_exp_rnp().
231  */
232 static void __maybe_unused rcu_report_exp_rnp(struct rcu_node *rnp, bool wake)
233 {
234 	unsigned long flags;
235 
236 	raw_spin_lock_irqsave_rcu_node(rnp, flags);
237 	__rcu_report_exp_rnp(rnp, wake, flags);
238 }
239 
240 /*
241  * Report expedited quiescent state for multiple CPUs, all covered by the
242  * specified leaf rcu_node structure.
243  */
244 static void rcu_report_exp_cpu_mult(struct rcu_node *rnp,
245 				    unsigned long mask, bool wake)
246 {
247 	unsigned long flags;
248 
249 	raw_spin_lock_irqsave_rcu_node(rnp, flags);
250 	if (!(rnp->expmask & mask)) {
251 		raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
252 		return;
253 	}
254 	rnp->expmask &= ~mask;
255 	__rcu_report_exp_rnp(rnp, wake, flags); /* Releases rnp->lock. */
256 }
257 
258 /*
259  * Report expedited quiescent state for specified rcu_data (CPU).
260  */
261 static void rcu_report_exp_rdp(struct rcu_data *rdp)
262 {
263 	WRITE_ONCE(rdp->deferred_qs, false);
264 	rcu_report_exp_cpu_mult(rdp->mynode, rdp->grpmask, true);
265 }
266 
267 /* Common code for work-done checking. */
268 static bool sync_exp_work_done(unsigned long s)
269 {
270 	if (rcu_exp_gp_seq_done(s)) {
271 		trace_rcu_exp_grace_period(rcu_state.name, s, TPS("done"));
272 		/* Ensure test happens before caller kfree(). */
273 		smp_mb__before_atomic(); /* ^^^ */
274 		return true;
275 	}
276 	return false;
277 }
278 
279 /*
280  * Funnel-lock acquisition for expedited grace periods.  Returns true
281  * if some other task completed an expedited grace period that this task
282  * can piggy-back on, and with no mutex held.  Otherwise, returns false
283  * with the mutex held, indicating that the caller must actually do the
284  * expedited grace period.
285  */
286 static bool exp_funnel_lock(unsigned long s)
287 {
288 	struct rcu_data *rdp = per_cpu_ptr(&rcu_data, raw_smp_processor_id());
289 	struct rcu_node *rnp = rdp->mynode;
290 	struct rcu_node *rnp_root = rcu_get_root();
291 
292 	/* Low-contention fastpath. */
293 	if (ULONG_CMP_LT(READ_ONCE(rnp->exp_seq_rq), s) &&
294 	    (rnp == rnp_root ||
295 	     ULONG_CMP_LT(READ_ONCE(rnp_root->exp_seq_rq), s)) &&
296 	    mutex_trylock(&rcu_state.exp_mutex))
297 		goto fastpath;
298 
299 	/*
300 	 * Each pass through the following loop works its way up
301 	 * the rcu_node tree, returning if others have done the work or
302 	 * otherwise falls through to acquire ->exp_mutex.  The mapping
303 	 * from CPU to rcu_node structure can be inexact, as it is just
304 	 * promoting locality and is not strictly needed for correctness.
305 	 */
306 	for (; rnp != NULL; rnp = rnp->parent) {
307 		if (sync_exp_work_done(s))
308 			return true;
309 
310 		/* Work not done, either wait here or go up. */
311 		spin_lock(&rnp->exp_lock);
312 		if (ULONG_CMP_GE(rnp->exp_seq_rq, s)) {
313 
314 			/* Someone else doing GP, so wait for them. */
315 			spin_unlock(&rnp->exp_lock);
316 			trace_rcu_exp_funnel_lock(rcu_state.name, rnp->level,
317 						  rnp->grplo, rnp->grphi,
318 						  TPS("wait"));
319 			wait_event(rnp->exp_wq[rcu_seq_ctr(s) & 0x3],
320 				   sync_exp_work_done(s));
321 			return true;
322 		}
323 		rnp->exp_seq_rq = s; /* Followers can wait on us. */
324 		spin_unlock(&rnp->exp_lock);
325 		trace_rcu_exp_funnel_lock(rcu_state.name, rnp->level,
326 					  rnp->grplo, rnp->grphi, TPS("nxtlvl"));
327 	}
328 	mutex_lock(&rcu_state.exp_mutex);
329 fastpath:
330 	if (sync_exp_work_done(s)) {
331 		mutex_unlock(&rcu_state.exp_mutex);
332 		return true;
333 	}
334 	rcu_exp_gp_seq_start();
335 	trace_rcu_exp_grace_period(rcu_state.name, s, TPS("start"));
336 	return false;
337 }
338 
339 /*
340  * Select the CPUs within the specified rcu_node that the upcoming
341  * expedited grace period needs to wait for.
342  */
343 static void sync_rcu_exp_select_node_cpus(struct work_struct *wp)
344 {
345 	int cpu;
346 	unsigned long flags;
347 	smp_call_func_t func;
348 	unsigned long mask_ofl_test;
349 	unsigned long mask_ofl_ipi;
350 	int ret;
351 	struct rcu_exp_work *rewp =
352 		container_of(wp, struct rcu_exp_work, rew_work);
353 	struct rcu_node *rnp = container_of(rewp, struct rcu_node, rew);
354 
355 	func = rewp->rew_func;
356 	raw_spin_lock_irqsave_rcu_node(rnp, flags);
357 
358 	/* Each pass checks a CPU for identity, offline, and idle. */
359 	mask_ofl_test = 0;
360 	for_each_leaf_node_cpu_mask(rnp, cpu, rnp->expmask) {
361 		unsigned long mask = leaf_node_cpu_bit(rnp, cpu);
362 		struct rcu_data *rdp = per_cpu_ptr(&rcu_data, cpu);
363 		int snap;
364 
365 		if (raw_smp_processor_id() == cpu ||
366 		    !(rnp->qsmaskinitnext & mask)) {
367 			mask_ofl_test |= mask;
368 		} else {
369 			snap = rcu_dynticks_snap(rdp);
370 			if (rcu_dynticks_in_eqs(snap))
371 				mask_ofl_test |= mask;
372 			else
373 				rdp->exp_dynticks_snap = snap;
374 		}
375 	}
376 	mask_ofl_ipi = rnp->expmask & ~mask_ofl_test;
377 
378 	/*
379 	 * Need to wait for any blocked tasks as well.	Note that
380 	 * additional blocking tasks will also block the expedited GP
381 	 * until such time as the ->expmask bits are cleared.
382 	 */
383 	if (rcu_preempt_has_tasks(rnp))
384 		rnp->exp_tasks = rnp->blkd_tasks.next;
385 	raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
386 
387 	/* IPI the remaining CPUs for expedited quiescent state. */
388 	for_each_leaf_node_cpu_mask(rnp, cpu, rnp->expmask) {
389 		unsigned long mask = leaf_node_cpu_bit(rnp, cpu);
390 		struct rcu_data *rdp = per_cpu_ptr(&rcu_data, cpu);
391 
392 		if (!(mask_ofl_ipi & mask))
393 			continue;
394 retry_ipi:
395 		if (rcu_dynticks_in_eqs_since(rdp, rdp->exp_dynticks_snap)) {
396 			mask_ofl_test |= mask;
397 			continue;
398 		}
399 		ret = smp_call_function_single(cpu, func, NULL, 0);
400 		if (!ret) {
401 			mask_ofl_ipi &= ~mask;
402 			continue;
403 		}
404 		/* Failed, raced with CPU hotplug operation. */
405 		raw_spin_lock_irqsave_rcu_node(rnp, flags);
406 		if ((rnp->qsmaskinitnext & mask) &&
407 		    (rnp->expmask & mask)) {
408 			/* Online, so delay for a bit and try again. */
409 			raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
410 			trace_rcu_exp_grace_period(rcu_state.name, rcu_exp_gp_seq_endval(), TPS("selectofl"));
411 			schedule_timeout_uninterruptible(1);
412 			goto retry_ipi;
413 		}
414 		/* CPU really is offline, so we can ignore it. */
415 		if (!(rnp->expmask & mask))
416 			mask_ofl_ipi &= ~mask;
417 		raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
418 	}
419 	/* Report quiescent states for those that went offline. */
420 	mask_ofl_test |= mask_ofl_ipi;
421 	if (mask_ofl_test)
422 		rcu_report_exp_cpu_mult(rnp, mask_ofl_test, false);
423 }
424 
425 /*
426  * Select the nodes that the upcoming expedited grace period needs
427  * to wait for.
428  */
429 static void sync_rcu_exp_select_cpus(smp_call_func_t func)
430 {
431 	int cpu;
432 	struct rcu_node *rnp;
433 
434 	trace_rcu_exp_grace_period(rcu_state.name, rcu_exp_gp_seq_endval(), TPS("reset"));
435 	sync_exp_reset_tree();
436 	trace_rcu_exp_grace_period(rcu_state.name, rcu_exp_gp_seq_endval(), TPS("select"));
437 
438 	/* Schedule work for each leaf rcu_node structure. */
439 	rcu_for_each_leaf_node(rnp) {
440 		rnp->exp_need_flush = false;
441 		if (!READ_ONCE(rnp->expmask))
442 			continue; /* Avoid early boot non-existent wq. */
443 		rnp->rew.rew_func = func;
444 		if (!READ_ONCE(rcu_par_gp_wq) ||
445 		    rcu_scheduler_active != RCU_SCHEDULER_RUNNING ||
446 		    rcu_is_last_leaf_node(rnp)) {
447 			/* No workqueues yet or last leaf, do direct call. */
448 			sync_rcu_exp_select_node_cpus(&rnp->rew.rew_work);
449 			continue;
450 		}
451 		INIT_WORK(&rnp->rew.rew_work, sync_rcu_exp_select_node_cpus);
452 		preempt_disable();
453 		cpu = cpumask_next(rnp->grplo - 1, cpu_online_mask);
454 		/* If all offline, queue the work on an unbound CPU. */
455 		if (unlikely(cpu > rnp->grphi))
456 			cpu = WORK_CPU_UNBOUND;
457 		queue_work_on(cpu, rcu_par_gp_wq, &rnp->rew.rew_work);
458 		preempt_enable();
459 		rnp->exp_need_flush = true;
460 	}
461 
462 	/* Wait for workqueue jobs (if any) to complete. */
463 	rcu_for_each_leaf_node(rnp)
464 		if (rnp->exp_need_flush)
465 			flush_work(&rnp->rew.rew_work);
466 }
467 
468 static void synchronize_sched_expedited_wait(void)
469 {
470 	int cpu;
471 	unsigned long jiffies_stall;
472 	unsigned long jiffies_start;
473 	unsigned long mask;
474 	int ndetected;
475 	struct rcu_node *rnp;
476 	struct rcu_node *rnp_root = rcu_get_root();
477 	int ret;
478 
479 	trace_rcu_exp_grace_period(rcu_state.name, rcu_exp_gp_seq_endval(), TPS("startwait"));
480 	jiffies_stall = rcu_jiffies_till_stall_check();
481 	jiffies_start = jiffies;
482 
483 	for (;;) {
484 		ret = swait_event_timeout_exclusive(
485 				rcu_state.expedited_wq,
486 				sync_rcu_preempt_exp_done_unlocked(rnp_root),
487 				jiffies_stall);
488 		if (ret > 0 || sync_rcu_preempt_exp_done_unlocked(rnp_root))
489 			return;
490 		WARN_ON(ret < 0);  /* workqueues should not be signaled. */
491 		if (rcu_cpu_stall_suppress)
492 			continue;
493 		panic_on_rcu_stall();
494 		pr_err("INFO: %s detected expedited stalls on CPUs/tasks: {",
495 		       rcu_state.name);
496 		ndetected = 0;
497 		rcu_for_each_leaf_node(rnp) {
498 			ndetected += rcu_print_task_exp_stall(rnp);
499 			for_each_leaf_node_possible_cpu(rnp, cpu) {
500 				struct rcu_data *rdp;
501 
502 				mask = leaf_node_cpu_bit(rnp, cpu);
503 				if (!(rnp->expmask & mask))
504 					continue;
505 				ndetected++;
506 				rdp = per_cpu_ptr(&rcu_data, cpu);
507 				pr_cont(" %d-%c%c%c", cpu,
508 					"O."[!!cpu_online(cpu)],
509 					"o."[!!(rdp->grpmask & rnp->expmaskinit)],
510 					"N."[!!(rdp->grpmask & rnp->expmaskinitnext)]);
511 			}
512 		}
513 		pr_cont(" } %lu jiffies s: %lu root: %#lx/%c\n",
514 			jiffies - jiffies_start, rcu_state.expedited_sequence,
515 			rnp_root->expmask, ".T"[!!rnp_root->exp_tasks]);
516 		if (ndetected) {
517 			pr_err("blocking rcu_node structures:");
518 			rcu_for_each_node_breadth_first(rnp) {
519 				if (rnp == rnp_root)
520 					continue; /* printed unconditionally */
521 				if (sync_rcu_preempt_exp_done_unlocked(rnp))
522 					continue;
523 				pr_cont(" l=%u:%d-%d:%#lx/%c",
524 					rnp->level, rnp->grplo, rnp->grphi,
525 					rnp->expmask,
526 					".T"[!!rnp->exp_tasks]);
527 			}
528 			pr_cont("\n");
529 		}
530 		rcu_for_each_leaf_node(rnp) {
531 			for_each_leaf_node_possible_cpu(rnp, cpu) {
532 				mask = leaf_node_cpu_bit(rnp, cpu);
533 				if (!(rnp->expmask & mask))
534 					continue;
535 				dump_cpu_task(cpu);
536 			}
537 		}
538 		jiffies_stall = 3 * rcu_jiffies_till_stall_check() + 3;
539 	}
540 }
541 
542 /*
543  * Wait for the current expedited grace period to complete, and then
544  * wake up everyone who piggybacked on the just-completed expedited
545  * grace period.  Also update all the ->exp_seq_rq counters as needed
546  * in order to avoid counter-wrap problems.
547  */
548 static void rcu_exp_wait_wake(unsigned long s)
549 {
550 	struct rcu_node *rnp;
551 
552 	synchronize_sched_expedited_wait();
553 	rcu_exp_gp_seq_end();
554 	trace_rcu_exp_grace_period(rcu_state.name, s, TPS("end"));
555 
556 	/*
557 	 * Switch over to wakeup mode, allowing the next GP, but -only- the
558 	 * next GP, to proceed.
559 	 */
560 	mutex_lock(&rcu_state.exp_wake_mutex);
561 
562 	rcu_for_each_node_breadth_first(rnp) {
563 		if (ULONG_CMP_LT(READ_ONCE(rnp->exp_seq_rq), s)) {
564 			spin_lock(&rnp->exp_lock);
565 			/* Recheck, avoid hang in case someone just arrived. */
566 			if (ULONG_CMP_LT(rnp->exp_seq_rq, s))
567 				rnp->exp_seq_rq = s;
568 			spin_unlock(&rnp->exp_lock);
569 		}
570 		smp_mb(); /* All above changes before wakeup. */
571 		wake_up_all(&rnp->exp_wq[rcu_seq_ctr(rcu_state.expedited_sequence) & 0x3]);
572 	}
573 	trace_rcu_exp_grace_period(rcu_state.name, s, TPS("endwake"));
574 	mutex_unlock(&rcu_state.exp_wake_mutex);
575 }
576 
577 /*
578  * Common code to drive an expedited grace period forward, used by
579  * workqueues and mid-boot-time tasks.
580  */
581 static void rcu_exp_sel_wait_wake(smp_call_func_t func, unsigned long s)
582 {
583 	/* Initialize the rcu_node tree in preparation for the wait. */
584 	sync_rcu_exp_select_cpus(func);
585 
586 	/* Wait and clean up, including waking everyone. */
587 	rcu_exp_wait_wake(s);
588 }
589 
590 /*
591  * Work-queue handler to drive an expedited grace period forward.
592  */
593 static void wait_rcu_exp_gp(struct work_struct *wp)
594 {
595 	struct rcu_exp_work *rewp;
596 
597 	rewp = container_of(wp, struct rcu_exp_work, rew_work);
598 	rcu_exp_sel_wait_wake(rewp->rew_func, rewp->rew_s);
599 }
600 
601 /*
602  * Given a smp_call_function() handler, kick off the specified
603  * implementation of expedited grace period.
604  */
605 static void _synchronize_rcu_expedited(smp_call_func_t func)
606 {
607 	struct rcu_data *rdp;
608 	struct rcu_exp_work rew;
609 	struct rcu_node *rnp;
610 	unsigned long s;
611 
612 	/* If expedited grace periods are prohibited, fall back to normal. */
613 	if (rcu_gp_is_normal()) {
614 		wait_rcu_gp(call_rcu);
615 		return;
616 	}
617 
618 	/* Take a snapshot of the sequence number.  */
619 	s = rcu_exp_gp_seq_snap();
620 	if (exp_funnel_lock(s))
621 		return;  /* Someone else did our work for us. */
622 
623 	/* Ensure that load happens before action based on it. */
624 	if (unlikely(rcu_scheduler_active == RCU_SCHEDULER_INIT)) {
625 		/* Direct call during scheduler init and early_initcalls(). */
626 		rcu_exp_sel_wait_wake(func, s);
627 	} else {
628 		/* Marshall arguments & schedule the expedited grace period. */
629 		rew.rew_func = func;
630 		rew.rew_s = s;
631 		INIT_WORK_ONSTACK(&rew.rew_work, wait_rcu_exp_gp);
632 		queue_work(rcu_gp_wq, &rew.rew_work);
633 	}
634 
635 	/* Wait for expedited grace period to complete. */
636 	rdp = per_cpu_ptr(&rcu_data, raw_smp_processor_id());
637 	rnp = rcu_get_root();
638 	wait_event(rnp->exp_wq[rcu_seq_ctr(s) & 0x3],
639 		   sync_exp_work_done(s));
640 	smp_mb(); /* Workqueue actions happen before return. */
641 
642 	/* Let the next expedited grace period start. */
643 	mutex_unlock(&rcu_state.exp_mutex);
644 }
645 
646 #ifdef CONFIG_PREEMPT_RCU
647 
648 /*
649  * Remote handler for smp_call_function_single().  If there is an
650  * RCU read-side critical section in effect, request that the
651  * next rcu_read_unlock() record the quiescent state up the
652  * ->expmask fields in the rcu_node tree.  Otherwise, immediately
653  * report the quiescent state.
654  */
655 static void sync_rcu_exp_handler(void *unused)
656 {
657 	unsigned long flags;
658 	struct rcu_data *rdp = this_cpu_ptr(&rcu_data);
659 	struct rcu_node *rnp = rdp->mynode;
660 	struct task_struct *t = current;
661 
662 	/*
663 	 * First, the common case of not being in an RCU read-side
664 	 * critical section.  If also enabled or idle, immediately
665 	 * report the quiescent state, otherwise defer.
666 	 */
667 	if (!t->rcu_read_lock_nesting) {
668 		if (!(preempt_count() & (PREEMPT_MASK | SOFTIRQ_MASK)) ||
669 		    rcu_dynticks_curr_cpu_in_eqs()) {
670 			rcu_report_exp_rdp(rdp);
671 		} else {
672 			rdp->deferred_qs = true;
673 			set_tsk_need_resched(t);
674 			set_preempt_need_resched();
675 		}
676 		return;
677 	}
678 
679 	/*
680 	 * Second, the less-common case of being in an RCU read-side
681 	 * critical section.  In this case we can count on a future
682 	 * rcu_read_unlock().  However, this rcu_read_unlock() might
683 	 * execute on some other CPU, but in that case there will be
684 	 * a future context switch.  Either way, if the expedited
685 	 * grace period is still waiting on this CPU, set ->deferred_qs
686 	 * so that the eventual quiescent state will be reported.
687 	 * Note that there is a large group of race conditions that
688 	 * can have caused this quiescent state to already have been
689 	 * reported, so we really do need to check ->expmask.
690 	 */
691 	if (t->rcu_read_lock_nesting > 0) {
692 		raw_spin_lock_irqsave_rcu_node(rnp, flags);
693 		if (rnp->expmask & rdp->grpmask)
694 			rdp->deferred_qs = true;
695 		raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
696 	}
697 
698 	/*
699 	 * The final and least likely case is where the interrupted
700 	 * code was just about to or just finished exiting the RCU-preempt
701 	 * read-side critical section, and no, we can't tell which.
702 	 * So either way, set ->deferred_qs to flag later code that
703 	 * a quiescent state is required.
704 	 *
705 	 * If the CPU is fully enabled (or if some buggy RCU-preempt
706 	 * read-side critical section is being used from idle), just
707 	 * invoke rcu_preempt_defer_qs() to immediately report the
708 	 * quiescent state.  We cannot use rcu_read_unlock_special()
709 	 * because we are in an interrupt handler, which will cause that
710 	 * function to take an early exit without doing anything.
711 	 *
712 	 * Otherwise, force a context switch after the CPU enables everything.
713 	 */
714 	rdp->deferred_qs = true;
715 	if (!(preempt_count() & (PREEMPT_MASK | SOFTIRQ_MASK)) ||
716 	    WARN_ON_ONCE(rcu_dynticks_curr_cpu_in_eqs())) {
717 		rcu_preempt_deferred_qs(t);
718 	} else {
719 		set_tsk_need_resched(t);
720 		set_preempt_need_resched();
721 	}
722 }
723 
724 /* PREEMPT=y, so no PREEMPT=n expedited grace period to clean up after. */
725 static void sync_sched_exp_online_cleanup(int cpu)
726 {
727 }
728 
729 /**
730  * synchronize_rcu_expedited - Brute-force RCU grace period
731  *
732  * Wait for an RCU-preempt grace period, but expedite it.  The basic
733  * idea is to IPI all non-idle non-nohz online CPUs.  The IPI handler
734  * checks whether the CPU is in an RCU-preempt critical section, and
735  * if so, it sets a flag that causes the outermost rcu_read_unlock()
736  * to report the quiescent state.  On the other hand, if the CPU is
737  * not in an RCU read-side critical section, the IPI handler reports
738  * the quiescent state immediately.
739  *
740  * Although this is a greate improvement over previous expedited
741  * implementations, it is still unfriendly to real-time workloads, so is
742  * thus not recommended for any sort of common-case code.  In fact, if
743  * you are using synchronize_rcu_expedited() in a loop, please restructure
744  * your code to batch your updates, and then Use a single synchronize_rcu()
745  * instead.
746  *
747  * This has the same semantics as (but is more brutal than) synchronize_rcu().
748  */
749 void synchronize_rcu_expedited(void)
750 {
751 	RCU_LOCKDEP_WARN(lock_is_held(&rcu_bh_lock_map) ||
752 			 lock_is_held(&rcu_lock_map) ||
753 			 lock_is_held(&rcu_sched_lock_map),
754 			 "Illegal synchronize_rcu_expedited() in RCU read-side critical section");
755 
756 	if (rcu_scheduler_active == RCU_SCHEDULER_INACTIVE)
757 		return;
758 	_synchronize_rcu_expedited(sync_rcu_exp_handler);
759 }
760 EXPORT_SYMBOL_GPL(synchronize_rcu_expedited);
761 
762 #else /* #ifdef CONFIG_PREEMPT_RCU */
763 
764 /* Invoked on each online non-idle CPU for expedited quiescent state. */
765 static void sync_sched_exp_handler(void *unused)
766 {
767 	struct rcu_data *rdp;
768 	struct rcu_node *rnp;
769 
770 	rdp = this_cpu_ptr(&rcu_data);
771 	rnp = rdp->mynode;
772 	if (!(READ_ONCE(rnp->expmask) & rdp->grpmask) ||
773 	    __this_cpu_read(rcu_data.cpu_no_qs.b.exp))
774 		return;
775 	if (rcu_is_cpu_rrupt_from_idle()) {
776 		rcu_report_exp_rdp(this_cpu_ptr(&rcu_data));
777 		return;
778 	}
779 	__this_cpu_write(rcu_data.cpu_no_qs.b.exp, true);
780 	/* Store .exp before .rcu_urgent_qs. */
781 	smp_store_release(this_cpu_ptr(&rcu_data.rcu_urgent_qs), true);
782 	set_tsk_need_resched(current);
783 	set_preempt_need_resched();
784 }
785 
786 /* Send IPI for expedited cleanup if needed at end of CPU-hotplug operation. */
787 static void sync_sched_exp_online_cleanup(int cpu)
788 {
789 	struct rcu_data *rdp;
790 	int ret;
791 	struct rcu_node *rnp;
792 
793 	rdp = per_cpu_ptr(&rcu_data, cpu);
794 	rnp = rdp->mynode;
795 	if (!(READ_ONCE(rnp->expmask) & rdp->grpmask))
796 		return;
797 	ret = smp_call_function_single(cpu, sync_sched_exp_handler, NULL, 0);
798 	WARN_ON_ONCE(ret);
799 }
800 
801 /*
802  * Because a context switch is a grace period for !PREEMPT, any
803  * blocking grace-period wait automatically implies a grace period if
804  * there is only one CPU online at any point time during execution of
805  * either synchronize_rcu() or synchronize_rcu_expedited().  It is OK to
806  * occasionally incorrectly indicate that there are multiple CPUs online
807  * when there was in fact only one the whole time, as this just adds some
808  * overhead: RCU still operates correctly.
809  */
810 static int rcu_blocking_is_gp(void)
811 {
812 	int ret;
813 
814 	might_sleep();  /* Check for RCU read-side critical section. */
815 	preempt_disable();
816 	ret = num_online_cpus() <= 1;
817 	preempt_enable();
818 	return ret;
819 }
820 
821 /* PREEMPT=n implementation of synchronize_rcu_expedited(). */
822 void synchronize_rcu_expedited(void)
823 {
824 	RCU_LOCKDEP_WARN(lock_is_held(&rcu_bh_lock_map) ||
825 			 lock_is_held(&rcu_lock_map) ||
826 			 lock_is_held(&rcu_sched_lock_map),
827 			 "Illegal synchronize_rcu_expedited() in RCU read-side critical section");
828 
829 	/* If only one CPU, this is automatically a grace period. */
830 	if (rcu_blocking_is_gp())
831 		return;
832 
833 	_synchronize_rcu_expedited(sync_sched_exp_handler);
834 }
835 EXPORT_SYMBOL_GPL(synchronize_rcu_expedited);
836 
837 #endif /* #else #ifdef CONFIG_PREEMPT_RCU */
838