Root/kernel/rcutiny_plugin.h

1/*
2 * Read-Copy Update mechanism for mutual exclusion, the Bloatwatch edition
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 (c) 2010 Linaro
21 *
22 * Author: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
23 */
24
25#include <linux/kthread.h>
26#include <linux/debugfs.h>
27#include <linux/seq_file.h>
28
29#ifdef CONFIG_RCU_TRACE
30#define RCU_TRACE(stmt) stmt
31#else /* #ifdef CONFIG_RCU_TRACE */
32#define RCU_TRACE(stmt)
33#endif /* #else #ifdef CONFIG_RCU_TRACE */
34
35/* Global control variables for rcupdate callback mechanism. */
36struct rcu_ctrlblk {
37    struct rcu_head *rcucblist; /* List of pending callbacks (CBs). */
38    struct rcu_head **donetail; /* ->next pointer of last "done" CB. */
39    struct rcu_head **curtail; /* ->next pointer of last CB. */
40    RCU_TRACE(long qlen); /* Number of pending CBs. */
41};
42
43/* Definition for rcupdate control block. */
44static struct rcu_ctrlblk rcu_sched_ctrlblk = {
45    .donetail = &rcu_sched_ctrlblk.rcucblist,
46    .curtail = &rcu_sched_ctrlblk.rcucblist,
47};
48
49static struct rcu_ctrlblk rcu_bh_ctrlblk = {
50    .donetail = &rcu_bh_ctrlblk.rcucblist,
51    .curtail = &rcu_bh_ctrlblk.rcucblist,
52};
53
54#ifdef CONFIG_DEBUG_LOCK_ALLOC
55int rcu_scheduler_active __read_mostly;
56EXPORT_SYMBOL_GPL(rcu_scheduler_active);
57#endif /* #ifdef CONFIG_DEBUG_LOCK_ALLOC */
58
59#ifdef CONFIG_TINY_PREEMPT_RCU
60
61#include <linux/delay.h>
62
63/* Global control variables for preemptible RCU. */
64struct rcu_preempt_ctrlblk {
65    struct rcu_ctrlblk rcb; /* curtail: ->next ptr of last CB for GP. */
66    struct rcu_head **nexttail;
67                /* Tasks blocked in a preemptible RCU */
68                /* read-side critical section while an */
69                /* preemptible-RCU grace period is in */
70                /* progress must wait for a later grace */
71                /* period. This pointer points to the */
72                /* ->next pointer of the last task that */
73                /* must wait for a later grace period, or */
74                /* to &->rcb.rcucblist if there is no */
75                /* such task. */
76    struct list_head blkd_tasks;
77                /* Tasks blocked in RCU read-side critical */
78                /* section. Tasks are placed at the head */
79                /* of this list and age towards the tail. */
80    struct list_head *gp_tasks;
81                /* Pointer to the first task blocking the */
82                /* current grace period, or NULL if there */
83                /* is no such task. */
84    struct list_head *exp_tasks;
85                /* Pointer to first task blocking the */
86                /* current expedited grace period, or NULL */
87                /* if there is no such task. If there */
88                /* is no current expedited grace period, */
89                /* then there cannot be any such task. */
90#ifdef CONFIG_RCU_BOOST
91    struct list_head *boost_tasks;
92                /* Pointer to first task that needs to be */
93                /* priority-boosted, or NULL if no priority */
94                /* boosting is needed. If there is no */
95                /* current or expedited grace period, there */
96                /* can be no such task. */
97#endif /* #ifdef CONFIG_RCU_BOOST */
98    u8 gpnum; /* Current grace period. */
99    u8 gpcpu; /* Last grace period blocked by the CPU. */
100    u8 completed; /* Last grace period completed. */
101                /* If all three are equal, RCU is idle. */
102#ifdef CONFIG_RCU_BOOST
103    s8 boosted_this_gp; /* Has boosting already happened? */
104    unsigned long boost_time; /* When to start boosting (jiffies) */
105#endif /* #ifdef CONFIG_RCU_BOOST */
106#ifdef CONFIG_RCU_TRACE
107    unsigned long n_grace_periods;
108#ifdef CONFIG_RCU_BOOST
109    unsigned long n_tasks_boosted;
110    unsigned long n_exp_boosts;
111    unsigned long n_normal_boosts;
112    unsigned long n_normal_balk_blkd_tasks;
113    unsigned long n_normal_balk_gp_tasks;
114    unsigned long n_normal_balk_boost_tasks;
115    unsigned long n_normal_balk_boosted;
116    unsigned long n_normal_balk_notyet;
117    unsigned long n_normal_balk_nos;
118    unsigned long n_exp_balk_blkd_tasks;
119    unsigned long n_exp_balk_nos;
120#endif /* #ifdef CONFIG_RCU_BOOST */
121#endif /* #ifdef CONFIG_RCU_TRACE */
122};
123
124static struct rcu_preempt_ctrlblk rcu_preempt_ctrlblk = {
125    .rcb.donetail = &rcu_preempt_ctrlblk.rcb.rcucblist,
126    .rcb.curtail = &rcu_preempt_ctrlblk.rcb.rcucblist,
127    .nexttail = &rcu_preempt_ctrlblk.rcb.rcucblist,
128    .blkd_tasks = LIST_HEAD_INIT(rcu_preempt_ctrlblk.blkd_tasks),
129};
130
131static int rcu_preempted_readers_exp(void);
132static void rcu_report_exp_done(void);
133
134/*
135 * Return true if the CPU has not yet responded to the current grace period.
136 */
137static int rcu_cpu_blocking_cur_gp(void)
138{
139    return rcu_preempt_ctrlblk.gpcpu != rcu_preempt_ctrlblk.gpnum;
140}
141
142/*
143 * Check for a running RCU reader. Because there is only one CPU,
144 * there can be but one running RCU reader at a time. ;-)
145 */
146static int rcu_preempt_running_reader(void)
147{
148    return current->rcu_read_lock_nesting;
149}
150
151/*
152 * Check for preempted RCU readers blocking any grace period.
153 * If the caller needs a reliable answer, it must disable hard irqs.
154 */
155static int rcu_preempt_blocked_readers_any(void)
156{
157    return !list_empty(&rcu_preempt_ctrlblk.blkd_tasks);
158}
159
160/*
161 * Check for preempted RCU readers blocking the current grace period.
162 * If the caller needs a reliable answer, it must disable hard irqs.
163 */
164static int rcu_preempt_blocked_readers_cgp(void)
165{
166    return rcu_preempt_ctrlblk.gp_tasks != NULL;
167}
168
169/*
170 * Return true if another preemptible-RCU grace period is needed.
171 */
172static int rcu_preempt_needs_another_gp(void)
173{
174    return *rcu_preempt_ctrlblk.rcb.curtail != NULL;
175}
176
177/*
178 * Return true if a preemptible-RCU grace period is in progress.
179 * The caller must disable hardirqs.
180 */
181static int rcu_preempt_gp_in_progress(void)
182{
183    return rcu_preempt_ctrlblk.completed != rcu_preempt_ctrlblk.gpnum;
184}
185
186/*
187 * Advance a ->blkd_tasks-list pointer to the next entry, instead
188 * returning NULL if at the end of the list.
189 */
190static struct list_head *rcu_next_node_entry(struct task_struct *t)
191{
192    struct list_head *np;
193
194    np = t->rcu_node_entry.next;
195    if (np == &rcu_preempt_ctrlblk.blkd_tasks)
196        np = NULL;
197    return np;
198}
199
200#ifdef CONFIG_RCU_TRACE
201
202#ifdef CONFIG_RCU_BOOST
203static void rcu_initiate_boost_trace(void);
204static void rcu_initiate_exp_boost_trace(void);
205#endif /* #ifdef CONFIG_RCU_BOOST */
206
207/*
208 * Dump additional statistice for TINY_PREEMPT_RCU.
209 */
210static void show_tiny_preempt_stats(struct seq_file *m)
211{
212    seq_printf(m, "rcu_preempt: qlen=%ld gp=%lu g%u/p%u/c%u tasks=%c%c%c\n",
213           rcu_preempt_ctrlblk.rcb.qlen,
214           rcu_preempt_ctrlblk.n_grace_periods,
215           rcu_preempt_ctrlblk.gpnum,
216           rcu_preempt_ctrlblk.gpcpu,
217           rcu_preempt_ctrlblk.completed,
218           "T."[list_empty(&rcu_preempt_ctrlblk.blkd_tasks)],
219           "N."[!rcu_preempt_ctrlblk.gp_tasks],
220           "E."[!rcu_preempt_ctrlblk.exp_tasks]);
221#ifdef CONFIG_RCU_BOOST
222    seq_printf(m, " ttb=%c btg=",
223           "B."[!rcu_preempt_ctrlblk.boost_tasks]);
224    switch (rcu_preempt_ctrlblk.boosted_this_gp) {
225    case -1:
226        seq_puts(m, "exp");
227        break;
228    case 0:
229        seq_puts(m, "no");
230        break;
231    case 1:
232        seq_puts(m, "begun");
233        break;
234    case 2:
235        seq_puts(m, "done");
236        break;
237    default:
238        seq_printf(m, "?%d?", rcu_preempt_ctrlblk.boosted_this_gp);
239    }
240    seq_printf(m, " ntb=%lu neb=%lu nnb=%lu j=%04x bt=%04x\n",
241           rcu_preempt_ctrlblk.n_tasks_boosted,
242           rcu_preempt_ctrlblk.n_exp_boosts,
243           rcu_preempt_ctrlblk.n_normal_boosts,
244           (int)(jiffies & 0xffff),
245           (int)(rcu_preempt_ctrlblk.boost_time & 0xffff));
246    seq_printf(m, " %s: nt=%lu gt=%lu bt=%lu b=%lu ny=%lu nos=%lu\n",
247           "normal balk",
248           rcu_preempt_ctrlblk.n_normal_balk_blkd_tasks,
249           rcu_preempt_ctrlblk.n_normal_balk_gp_tasks,
250           rcu_preempt_ctrlblk.n_normal_balk_boost_tasks,
251           rcu_preempt_ctrlblk.n_normal_balk_boosted,
252           rcu_preempt_ctrlblk.n_normal_balk_notyet,
253           rcu_preempt_ctrlblk.n_normal_balk_nos);
254    seq_printf(m, " exp balk: bt=%lu nos=%lu\n",
255           rcu_preempt_ctrlblk.n_exp_balk_blkd_tasks,
256           rcu_preempt_ctrlblk.n_exp_balk_nos);
257#endif /* #ifdef CONFIG_RCU_BOOST */
258}
259
260#endif /* #ifdef CONFIG_RCU_TRACE */
261
262#ifdef CONFIG_RCU_BOOST
263
264#include "rtmutex_common.h"
265
266/*
267 * Carry out RCU priority boosting on the task indicated by ->boost_tasks,
268 * and advance ->boost_tasks to the next task in the ->blkd_tasks list.
269 */
270static int rcu_boost(void)
271{
272    unsigned long flags;
273    struct rt_mutex mtx;
274    struct list_head *np;
275    struct task_struct *t;
276
277    if (rcu_preempt_ctrlblk.boost_tasks == NULL)
278        return 0; /* Nothing to boost. */
279    raw_local_irq_save(flags);
280    rcu_preempt_ctrlblk.boosted_this_gp++;
281    t = container_of(rcu_preempt_ctrlblk.boost_tasks, struct task_struct,
282             rcu_node_entry);
283    np = rcu_next_node_entry(t);
284    rt_mutex_init_proxy_locked(&mtx, t);
285    t->rcu_boost_mutex = &mtx;
286    t->rcu_read_unlock_special |= RCU_READ_UNLOCK_BOOSTED;
287    raw_local_irq_restore(flags);
288    rt_mutex_lock(&mtx);
289    RCU_TRACE(rcu_preempt_ctrlblk.n_tasks_boosted++);
290    rcu_preempt_ctrlblk.boosted_this_gp++;
291    rt_mutex_unlock(&mtx);
292    return rcu_preempt_ctrlblk.boost_tasks != NULL;
293}
294
295/*
296 * Check to see if it is now time to start boosting RCU readers blocking
297 * the current grace period, and, if so, tell the rcu_kthread_task to
298 * start boosting them. If there is an expedited boost in progress,
299 * we wait for it to complete.
300 *
301 * If there are no blocked readers blocking the current grace period,
302 * return 0 to let the caller know, otherwise return 1. Note that this
303 * return value is independent of whether or not boosting was done.
304 */
305static int rcu_initiate_boost(void)
306{
307    if (!rcu_preempt_blocked_readers_cgp()) {
308        RCU_TRACE(rcu_preempt_ctrlblk.n_normal_balk_blkd_tasks++);
309        return 0;
310    }
311    if (rcu_preempt_ctrlblk.gp_tasks != NULL &&
312        rcu_preempt_ctrlblk.boost_tasks == NULL &&
313        rcu_preempt_ctrlblk.boosted_this_gp == 0 &&
314        ULONG_CMP_GE(jiffies, rcu_preempt_ctrlblk.boost_time)) {
315        rcu_preempt_ctrlblk.boost_tasks = rcu_preempt_ctrlblk.gp_tasks;
316        invoke_rcu_kthread();
317        RCU_TRACE(rcu_preempt_ctrlblk.n_normal_boosts++);
318    } else
319        RCU_TRACE(rcu_initiate_boost_trace());
320    return 1;
321}
322
323/*
324 * Initiate boosting for an expedited grace period.
325 */
326static void rcu_initiate_expedited_boost(void)
327{
328    unsigned long flags;
329
330    raw_local_irq_save(flags);
331    if (!list_empty(&rcu_preempt_ctrlblk.blkd_tasks)) {
332        rcu_preempt_ctrlblk.boost_tasks =
333            rcu_preempt_ctrlblk.blkd_tasks.next;
334        rcu_preempt_ctrlblk.boosted_this_gp = -1;
335        invoke_rcu_kthread();
336        RCU_TRACE(rcu_preempt_ctrlblk.n_exp_boosts++);
337    } else
338        RCU_TRACE(rcu_initiate_exp_boost_trace());
339    raw_local_irq_restore(flags);
340}
341
342#define RCU_BOOST_DELAY_JIFFIES DIV_ROUND_UP(CONFIG_RCU_BOOST_DELAY * HZ, 1000);
343
344/*
345 * Do priority-boost accounting for the start of a new grace period.
346 */
347static void rcu_preempt_boost_start_gp(void)
348{
349    rcu_preempt_ctrlblk.boost_time = jiffies + RCU_BOOST_DELAY_JIFFIES;
350    if (rcu_preempt_ctrlblk.boosted_this_gp > 0)
351        rcu_preempt_ctrlblk.boosted_this_gp = 0;
352}
353
354#else /* #ifdef CONFIG_RCU_BOOST */
355
356/*
357 * If there is no RCU priority boosting, we don't boost.
358 */
359static int rcu_boost(void)
360{
361    return 0;
362}
363
364/*
365 * If there is no RCU priority boosting, we don't initiate boosting,
366 * but we do indicate whether there are blocked readers blocking the
367 * current grace period.
368 */
369static int rcu_initiate_boost(void)
370{
371    return rcu_preempt_blocked_readers_cgp();
372}
373
374/*
375 * If there is no RCU priority boosting, we don't initiate expedited boosting.
376 */
377static void rcu_initiate_expedited_boost(void)
378{
379}
380
381/*
382 * If there is no RCU priority boosting, nothing to do at grace-period start.
383 */
384static void rcu_preempt_boost_start_gp(void)
385{
386}
387
388#endif /* else #ifdef CONFIG_RCU_BOOST */
389
390/*
391 * Record a preemptible-RCU quiescent state for the specified CPU. Note
392 * that this just means that the task currently running on the CPU is
393 * in a quiescent state. There might be any number of tasks blocked
394 * while in an RCU read-side critical section.
395 *
396 * Unlike the other rcu_*_qs() functions, callers to this function
397 * must disable irqs in order to protect the assignment to
398 * ->rcu_read_unlock_special.
399 *
400 * Because this is a single-CPU implementation, the only way a grace
401 * period can end is if the CPU is in a quiescent state. The reason is
402 * that a blocked preemptible-RCU reader can exit its critical section
403 * only if the CPU is running it at the time. Therefore, when the
404 * last task blocking the current grace period exits its RCU read-side
405 * critical section, neither the CPU nor blocked tasks will be stopping
406 * the current grace period. (In contrast, SMP implementations
407 * might have CPUs running in RCU read-side critical sections that
408 * block later grace periods -- but this is not possible given only
409 * one CPU.)
410 */
411static void rcu_preempt_cpu_qs(void)
412{
413    /* Record both CPU and task as having responded to current GP. */
414    rcu_preempt_ctrlblk.gpcpu = rcu_preempt_ctrlblk.gpnum;
415    current->rcu_read_unlock_special &= ~RCU_READ_UNLOCK_NEED_QS;
416
417    /* If there is no GP then there is nothing more to do. */
418    if (!rcu_preempt_gp_in_progress())
419        return;
420    /*
421     * Check up on boosting. If there are no readers blocking the
422     * current grace period, leave.
423     */
424    if (rcu_initiate_boost())
425        return;
426
427    /* Advance callbacks. */
428    rcu_preempt_ctrlblk.completed = rcu_preempt_ctrlblk.gpnum;
429    rcu_preempt_ctrlblk.rcb.donetail = rcu_preempt_ctrlblk.rcb.curtail;
430    rcu_preempt_ctrlblk.rcb.curtail = rcu_preempt_ctrlblk.nexttail;
431
432    /* If there are no blocked readers, next GP is done instantly. */
433    if (!rcu_preempt_blocked_readers_any())
434        rcu_preempt_ctrlblk.rcb.donetail = rcu_preempt_ctrlblk.nexttail;
435
436    /* If there are done callbacks, cause them to be invoked. */
437    if (*rcu_preempt_ctrlblk.rcb.donetail != NULL)
438        invoke_rcu_kthread();
439}
440
441/*
442 * Start a new RCU grace period if warranted. Hard irqs must be disabled.
443 */
444static void rcu_preempt_start_gp(void)
445{
446    if (!rcu_preempt_gp_in_progress() && rcu_preempt_needs_another_gp()) {
447
448        /* Official start of GP. */
449        rcu_preempt_ctrlblk.gpnum++;
450        RCU_TRACE(rcu_preempt_ctrlblk.n_grace_periods++);
451
452        /* Any blocked RCU readers block new GP. */
453        if (rcu_preempt_blocked_readers_any())
454            rcu_preempt_ctrlblk.gp_tasks =
455                rcu_preempt_ctrlblk.blkd_tasks.next;
456
457        /* Set up for RCU priority boosting. */
458        rcu_preempt_boost_start_gp();
459
460        /* If there is no running reader, CPU is done with GP. */
461        if (!rcu_preempt_running_reader())
462            rcu_preempt_cpu_qs();
463    }
464}
465
466/*
467 * We have entered the scheduler, and the current task might soon be
468 * context-switched away from. If this task is in an RCU read-side
469 * critical section, we will no longer be able to rely on the CPU to
470 * record that fact, so we enqueue the task on the blkd_tasks list.
471 * If the task started after the current grace period began, as recorded
472 * by ->gpcpu, we enqueue at the beginning of the list. Otherwise
473 * before the element referenced by ->gp_tasks (or at the tail if
474 * ->gp_tasks is NULL) and point ->gp_tasks at the newly added element.
475 * The task will dequeue itself when it exits the outermost enclosing
476 * RCU read-side critical section. Therefore, the current grace period
477 * cannot be permitted to complete until the ->gp_tasks pointer becomes
478 * NULL.
479 *
480 * Caller must disable preemption.
481 */
482void rcu_preempt_note_context_switch(void)
483{
484    struct task_struct *t = current;
485    unsigned long flags;
486
487    local_irq_save(flags); /* must exclude scheduler_tick(). */
488    if (rcu_preempt_running_reader() &&
489        (t->rcu_read_unlock_special & RCU_READ_UNLOCK_BLOCKED) == 0) {
490
491        /* Possibly blocking in an RCU read-side critical section. */
492        t->rcu_read_unlock_special |= RCU_READ_UNLOCK_BLOCKED;
493
494        /*
495         * If this CPU has already checked in, then this task
496         * will hold up the next grace period rather than the
497         * current grace period. Queue the task accordingly.
498         * If the task is queued for the current grace period
499         * (i.e., this CPU has not yet passed through a quiescent
500         * state for the current grace period), then as long
501         * as that task remains queued, the current grace period
502         * cannot end.
503         */
504        list_add(&t->rcu_node_entry, &rcu_preempt_ctrlblk.blkd_tasks);
505        if (rcu_cpu_blocking_cur_gp())
506            rcu_preempt_ctrlblk.gp_tasks = &t->rcu_node_entry;
507    }
508
509    /*
510     * Either we were not in an RCU read-side critical section to
511     * begin with, or we have now recorded that critical section
512     * globally. Either way, we can now note a quiescent state
513     * for this CPU. Again, if we were in an RCU read-side critical
514     * section, and if that critical section was blocking the current
515     * grace period, then the fact that the task has been enqueued
516     * means that current grace period continues to be blocked.
517     */
518    rcu_preempt_cpu_qs();
519    local_irq_restore(flags);
520}
521
522/*
523 * Tiny-preemptible RCU implementation for rcu_read_lock().
524 * Just increment ->rcu_read_lock_nesting, shared state will be updated
525 * if we block.
526 */
527void __rcu_read_lock(void)
528{
529    current->rcu_read_lock_nesting++;
530    barrier(); /* needed if we ever invoke rcu_read_lock in rcutiny.c */
531}
532EXPORT_SYMBOL_GPL(__rcu_read_lock);
533
534/*
535 * Handle special cases during rcu_read_unlock(), such as needing to
536 * notify RCU core processing or task having blocked during the RCU
537 * read-side critical section.
538 */
539static void rcu_read_unlock_special(struct task_struct *t)
540{
541    int empty;
542    int empty_exp;
543    unsigned long flags;
544    struct list_head *np;
545    int special;
546
547    /*
548     * NMI handlers cannot block and cannot safely manipulate state.
549     * They therefore cannot possibly be special, so just leave.
550     */
551    if (in_nmi())
552        return;
553
554    local_irq_save(flags);
555
556    /*
557     * If RCU core is waiting for this CPU to exit critical section,
558     * let it know that we have done so.
559     */
560    special = t->rcu_read_unlock_special;
561    if (special & RCU_READ_UNLOCK_NEED_QS)
562        rcu_preempt_cpu_qs();
563
564    /* Hardware IRQ handlers cannot block. */
565    if (in_irq()) {
566        local_irq_restore(flags);
567        return;
568    }
569
570    /* Clean up if blocked during RCU read-side critical section. */
571    if (special & RCU_READ_UNLOCK_BLOCKED) {
572        t->rcu_read_unlock_special &= ~RCU_READ_UNLOCK_BLOCKED;
573
574        /*
575         * Remove this task from the ->blkd_tasks list and adjust
576         * any pointers that might have been referencing it.
577         */
578        empty = !rcu_preempt_blocked_readers_cgp();
579        empty_exp = rcu_preempt_ctrlblk.exp_tasks == NULL;
580        np = rcu_next_node_entry(t);
581        list_del(&t->rcu_node_entry);
582        if (&t->rcu_node_entry == rcu_preempt_ctrlblk.gp_tasks)
583            rcu_preempt_ctrlblk.gp_tasks = np;
584        if (&t->rcu_node_entry == rcu_preempt_ctrlblk.exp_tasks)
585            rcu_preempt_ctrlblk.exp_tasks = np;
586#ifdef CONFIG_RCU_BOOST
587        if (&t->rcu_node_entry == rcu_preempt_ctrlblk.boost_tasks)
588            rcu_preempt_ctrlblk.boost_tasks = np;
589#endif /* #ifdef CONFIG_RCU_BOOST */
590        INIT_LIST_HEAD(&t->rcu_node_entry);
591
592        /*
593         * If this was the last task on the current list, and if
594         * we aren't waiting on the CPU, report the quiescent state
595         * and start a new grace period if needed.
596         */
597        if (!empty && !rcu_preempt_blocked_readers_cgp()) {
598            rcu_preempt_cpu_qs();
599            rcu_preempt_start_gp();
600        }
601
602        /*
603         * If this was the last task on the expedited lists,
604         * then we need wake up the waiting task.
605         */
606        if (!empty_exp && rcu_preempt_ctrlblk.exp_tasks == NULL)
607            rcu_report_exp_done();
608    }
609#ifdef CONFIG_RCU_BOOST
610    /* Unboost self if was boosted. */
611    if (special & RCU_READ_UNLOCK_BOOSTED) {
612        t->rcu_read_unlock_special &= ~RCU_READ_UNLOCK_BOOSTED;
613        rt_mutex_unlock(t->rcu_boost_mutex);
614        t->rcu_boost_mutex = NULL;
615    }
616#endif /* #ifdef CONFIG_RCU_BOOST */
617    local_irq_restore(flags);
618}
619
620/*
621 * Tiny-preemptible RCU implementation for rcu_read_unlock().
622 * Decrement ->rcu_read_lock_nesting. If the result is zero (outermost
623 * rcu_read_unlock()) and ->rcu_read_unlock_special is non-zero, then
624 * invoke rcu_read_unlock_special() to clean up after a context switch
625 * in an RCU read-side critical section and other special cases.
626 */
627void __rcu_read_unlock(void)
628{
629    struct task_struct *t = current;
630
631    barrier(); /* needed if we ever invoke rcu_read_unlock in rcutiny.c */
632    --t->rcu_read_lock_nesting;
633    barrier(); /* decrement before load of ->rcu_read_unlock_special */
634    if (t->rcu_read_lock_nesting == 0 &&
635        unlikely(ACCESS_ONCE(t->rcu_read_unlock_special)))
636        rcu_read_unlock_special(t);
637#ifdef CONFIG_PROVE_LOCKING
638    WARN_ON_ONCE(t->rcu_read_lock_nesting < 0);
639#endif /* #ifdef CONFIG_PROVE_LOCKING */
640}
641EXPORT_SYMBOL_GPL(__rcu_read_unlock);
642
643/*
644 * Check for a quiescent state from the current CPU. When a task blocks,
645 * the task is recorded in the rcu_preempt_ctrlblk structure, which is
646 * checked elsewhere. This is called from the scheduling-clock interrupt.
647 *
648 * Caller must disable hard irqs.
649 */
650static void rcu_preempt_check_callbacks(void)
651{
652    struct task_struct *t = current;
653
654    if (rcu_preempt_gp_in_progress() &&
655        (!rcu_preempt_running_reader() ||
656         !rcu_cpu_blocking_cur_gp()))
657        rcu_preempt_cpu_qs();
658    if (&rcu_preempt_ctrlblk.rcb.rcucblist !=
659        rcu_preempt_ctrlblk.rcb.donetail)
660        invoke_rcu_kthread();
661    if (rcu_preempt_gp_in_progress() &&
662        rcu_cpu_blocking_cur_gp() &&
663        rcu_preempt_running_reader())
664        t->rcu_read_unlock_special |= RCU_READ_UNLOCK_NEED_QS;
665}
666
667/*
668 * TINY_PREEMPT_RCU has an extra callback-list tail pointer to
669 * update, so this is invoked from rcu_process_callbacks() to
670 * handle that case. Of course, it is invoked for all flavors of
671 * RCU, but RCU callbacks can appear only on one of the lists, and
672 * neither ->nexttail nor ->donetail can possibly be NULL, so there
673 * is no need for an explicit check.
674 */
675static void rcu_preempt_remove_callbacks(struct rcu_ctrlblk *rcp)
676{
677    if (rcu_preempt_ctrlblk.nexttail == rcp->donetail)
678        rcu_preempt_ctrlblk.nexttail = &rcp->rcucblist;
679}
680
681/*
682 * Process callbacks for preemptible RCU.
683 */
684static void rcu_preempt_process_callbacks(void)
685{
686    rcu_process_callbacks(&rcu_preempt_ctrlblk.rcb);
687}
688
689/*
690 * Queue a preemptible -RCU callback for invocation after a grace period.
691 */
692void call_rcu(struct rcu_head *head, void (*func)(struct rcu_head *rcu))
693{
694    unsigned long flags;
695
696    debug_rcu_head_queue(head);
697    head->func = func;
698    head->next = NULL;
699
700    local_irq_save(flags);
701    *rcu_preempt_ctrlblk.nexttail = head;
702    rcu_preempt_ctrlblk.nexttail = &head->next;
703    RCU_TRACE(rcu_preempt_ctrlblk.rcb.qlen++);
704    rcu_preempt_start_gp(); /* checks to see if GP needed. */
705    local_irq_restore(flags);
706}
707EXPORT_SYMBOL_GPL(call_rcu);
708
709void rcu_barrier(void)
710{
711    struct rcu_synchronize rcu;
712
713    init_rcu_head_on_stack(&rcu.head);
714    init_completion(&rcu.completion);
715    /* Will wake me after RCU finished. */
716    call_rcu(&rcu.head, wakeme_after_rcu);
717    /* Wait for it. */
718    wait_for_completion(&rcu.completion);
719    destroy_rcu_head_on_stack(&rcu.head);
720}
721EXPORT_SYMBOL_GPL(rcu_barrier);
722
723/*
724 * synchronize_rcu - wait until a grace period has elapsed.
725 *
726 * Control will return to the caller some time after a full grace
727 * period has elapsed, in other words after all currently executing RCU
728 * read-side critical sections have completed. RCU read-side critical
729 * sections are delimited by rcu_read_lock() and rcu_read_unlock(),
730 * and may be nested.
731 */
732void synchronize_rcu(void)
733{
734#ifdef CONFIG_DEBUG_LOCK_ALLOC
735    if (!rcu_scheduler_active)
736        return;
737#endif /* #ifdef CONFIG_DEBUG_LOCK_ALLOC */
738
739    WARN_ON_ONCE(rcu_preempt_running_reader());
740    if (!rcu_preempt_blocked_readers_any())
741        return;
742
743    /* Once we get past the fastpath checks, same code as rcu_barrier(). */
744    rcu_barrier();
745}
746EXPORT_SYMBOL_GPL(synchronize_rcu);
747
748static DECLARE_WAIT_QUEUE_HEAD(sync_rcu_preempt_exp_wq);
749static unsigned long sync_rcu_preempt_exp_count;
750static DEFINE_MUTEX(sync_rcu_preempt_exp_mutex);
751
752/*
753 * Return non-zero if there are any tasks in RCU read-side critical
754 * sections blocking the current preemptible-RCU expedited grace period.
755 * If there is no preemptible-RCU expedited grace period currently in
756 * progress, returns zero unconditionally.
757 */
758static int rcu_preempted_readers_exp(void)
759{
760    return rcu_preempt_ctrlblk.exp_tasks != NULL;
761}
762
763/*
764 * Report the exit from RCU read-side critical section for the last task
765 * that queued itself during or before the current expedited preemptible-RCU
766 * grace period.
767 */
768static void rcu_report_exp_done(void)
769{
770    wake_up(&sync_rcu_preempt_exp_wq);
771}
772
773/*
774 * Wait for an rcu-preempt grace period, but expedite it. The basic idea
775 * is to rely in the fact that there is but one CPU, and that it is
776 * illegal for a task to invoke synchronize_rcu_expedited() while in a
777 * preemptible-RCU read-side critical section. Therefore, any such
778 * critical sections must correspond to blocked tasks, which must therefore
779 * be on the ->blkd_tasks list. So just record the current head of the
780 * list in the ->exp_tasks pointer, and wait for all tasks including and
781 * after the task pointed to by ->exp_tasks to drain.
782 */
783void synchronize_rcu_expedited(void)
784{
785    unsigned long flags;
786    struct rcu_preempt_ctrlblk *rpcp = &rcu_preempt_ctrlblk;
787    unsigned long snap;
788
789    barrier(); /* ensure prior action seen before grace period. */
790
791    WARN_ON_ONCE(rcu_preempt_running_reader());
792
793    /*
794     * Acquire lock so that there is only one preemptible RCU grace
795     * period in flight. Of course, if someone does the expedited
796     * grace period for us while we are acquiring the lock, just leave.
797     */
798    snap = sync_rcu_preempt_exp_count + 1;
799    mutex_lock(&sync_rcu_preempt_exp_mutex);
800    if (ULONG_CMP_LT(snap, sync_rcu_preempt_exp_count))
801        goto unlock_mb_ret; /* Others did our work for us. */
802
803    local_irq_save(flags);
804
805    /*
806     * All RCU readers have to already be on blkd_tasks because
807     * we cannot legally be executing in an RCU read-side critical
808     * section.
809     */
810
811    /* Snapshot current head of ->blkd_tasks list. */
812    rpcp->exp_tasks = rpcp->blkd_tasks.next;
813    if (rpcp->exp_tasks == &rpcp->blkd_tasks)
814        rpcp->exp_tasks = NULL;
815    local_irq_restore(flags);
816
817    /* Wait for tail of ->blkd_tasks list to drain. */
818    if (rcu_preempted_readers_exp())
819        rcu_initiate_expedited_boost();
820        wait_event(sync_rcu_preempt_exp_wq,
821               !rcu_preempted_readers_exp());
822
823    /* Clean up and exit. */
824    barrier(); /* ensure expedited GP seen before counter increment. */
825    sync_rcu_preempt_exp_count++;
826unlock_mb_ret:
827    mutex_unlock(&sync_rcu_preempt_exp_mutex);
828    barrier(); /* ensure subsequent action seen after grace period. */
829}
830EXPORT_SYMBOL_GPL(synchronize_rcu_expedited);
831
832/*
833 * Does preemptible RCU need the CPU to stay out of dynticks mode?
834 */
835int rcu_preempt_needs_cpu(void)
836{
837    if (!rcu_preempt_running_reader())
838        rcu_preempt_cpu_qs();
839    return rcu_preempt_ctrlblk.rcb.rcucblist != NULL;
840}
841
842/*
843 * Check for a task exiting while in a preemptible -RCU read-side
844 * critical section, clean up if so. No need to issue warnings,
845 * as debug_check_no_locks_held() already does this if lockdep
846 * is enabled.
847 */
848void exit_rcu(void)
849{
850    struct task_struct *t = current;
851
852    if (t->rcu_read_lock_nesting == 0)
853        return;
854    t->rcu_read_lock_nesting = 1;
855    rcu_read_unlock();
856}
857
858#else /* #ifdef CONFIG_TINY_PREEMPT_RCU */
859
860#ifdef CONFIG_RCU_TRACE
861
862/*
863 * Because preemptible RCU does not exist, it is not necessary to
864 * dump out its statistics.
865 */
866static void show_tiny_preempt_stats(struct seq_file *m)
867{
868}
869
870#endif /* #ifdef CONFIG_RCU_TRACE */
871
872/*
873 * Because preemptible RCU does not exist, it is never necessary to
874 * boost preempted RCU readers.
875 */
876static int rcu_boost(void)
877{
878    return 0;
879}
880
881/*
882 * Because preemptible RCU does not exist, it never has any callbacks
883 * to check.
884 */
885static void rcu_preempt_check_callbacks(void)
886{
887}
888
889/*
890 * Because preemptible RCU does not exist, it never has any callbacks
891 * to remove.
892 */
893static void rcu_preempt_remove_callbacks(struct rcu_ctrlblk *rcp)
894{
895}
896
897/*
898 * Because preemptible RCU does not exist, it never has any callbacks
899 * to process.
900 */
901static void rcu_preempt_process_callbacks(void)
902{
903}
904
905#endif /* #else #ifdef CONFIG_TINY_PREEMPT_RCU */
906
907#ifdef CONFIG_DEBUG_LOCK_ALLOC
908#include <linux/kernel_stat.h>
909
910/*
911 * During boot, we forgive RCU lockdep issues. After this function is
912 * invoked, we start taking RCU lockdep issues seriously.
913 */
914void __init rcu_scheduler_starting(void)
915{
916    WARN_ON(nr_context_switches() > 0);
917    rcu_scheduler_active = 1;
918}
919
920#endif /* #ifdef CONFIG_DEBUG_LOCK_ALLOC */
921
922#ifdef CONFIG_RCU_BOOST
923#define RCU_BOOST_PRIO CONFIG_RCU_BOOST_PRIO
924#else /* #ifdef CONFIG_RCU_BOOST */
925#define RCU_BOOST_PRIO 1
926#endif /* #else #ifdef CONFIG_RCU_BOOST */
927
928#ifdef CONFIG_RCU_TRACE
929
930#ifdef CONFIG_RCU_BOOST
931
932static void rcu_initiate_boost_trace(void)
933{
934    if (rcu_preempt_ctrlblk.gp_tasks == NULL)
935        rcu_preempt_ctrlblk.n_normal_balk_gp_tasks++;
936    else if (rcu_preempt_ctrlblk.boost_tasks != NULL)
937        rcu_preempt_ctrlblk.n_normal_balk_boost_tasks++;
938    else if (rcu_preempt_ctrlblk.boosted_this_gp != 0)
939        rcu_preempt_ctrlblk.n_normal_balk_boosted++;
940    else if (!ULONG_CMP_GE(jiffies, rcu_preempt_ctrlblk.boost_time))
941        rcu_preempt_ctrlblk.n_normal_balk_notyet++;
942    else
943        rcu_preempt_ctrlblk.n_normal_balk_nos++;
944}
945
946static void rcu_initiate_exp_boost_trace(void)
947{
948    if (list_empty(&rcu_preempt_ctrlblk.blkd_tasks))
949        rcu_preempt_ctrlblk.n_exp_balk_blkd_tasks++;
950    else
951        rcu_preempt_ctrlblk.n_exp_balk_nos++;
952}
953
954#endif /* #ifdef CONFIG_RCU_BOOST */
955
956static void rcu_trace_sub_qlen(struct rcu_ctrlblk *rcp, int n)
957{
958    unsigned long flags;
959
960    raw_local_irq_save(flags);
961    rcp->qlen -= n;
962    raw_local_irq_restore(flags);
963}
964
965/*
966 * Dump statistics for TINY_RCU, such as they are.
967 */
968static int show_tiny_stats(struct seq_file *m, void *unused)
969{
970    show_tiny_preempt_stats(m);
971    seq_printf(m, "rcu_sched: qlen: %ld\n", rcu_sched_ctrlblk.qlen);
972    seq_printf(m, "rcu_bh: qlen: %ld\n", rcu_bh_ctrlblk.qlen);
973    return 0;
974}
975
976static int show_tiny_stats_open(struct inode *inode, struct file *file)
977{
978    return single_open(file, show_tiny_stats, NULL);
979}
980
981static const struct file_operations show_tiny_stats_fops = {
982    .owner = THIS_MODULE,
983    .open = show_tiny_stats_open,
984    .read = seq_read,
985    .llseek = seq_lseek,
986    .release = single_release,
987};
988
989static struct dentry *rcudir;
990
991static int __init rcutiny_trace_init(void)
992{
993    struct dentry *retval;
994
995    rcudir = debugfs_create_dir("rcu", NULL);
996    if (!rcudir)
997        goto free_out;
998    retval = debugfs_create_file("rcudata", 0444, rcudir,
999                     NULL, &show_tiny_stats_fops);
1000    if (!retval)
1001        goto free_out;
1002    return 0;
1003free_out:
1004    debugfs_remove_recursive(rcudir);
1005    return 1;
1006}
1007
1008static void __exit rcutiny_trace_cleanup(void)
1009{
1010    debugfs_remove_recursive(rcudir);
1011}
1012
1013module_init(rcutiny_trace_init);
1014module_exit(rcutiny_trace_cleanup);
1015
1016MODULE_AUTHOR("Paul E. McKenney");
1017MODULE_DESCRIPTION("Read-Copy Update tracing for tiny implementation");
1018MODULE_LICENSE("GPL");
1019
1020#endif /* #ifdef CONFIG_RCU_TRACE */
1021

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