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/module.h>
27#include <linux/debugfs.h>
28#include <linux/seq_file.h>
29
30/* Global control variables for rcupdate callback mechanism. */
31struct rcu_ctrlblk {
32    struct rcu_head *rcucblist; /* List of pending callbacks (CBs). */
33    struct rcu_head **donetail; /* ->next pointer of last "done" CB. */
34    struct rcu_head **curtail; /* ->next pointer of last CB. */
35    RCU_TRACE(long qlen); /* Number of pending CBs. */
36    RCU_TRACE(char *name); /* Name of RCU type. */
37};
38
39/* Definition for rcupdate control block. */
40static struct rcu_ctrlblk rcu_sched_ctrlblk = {
41    .donetail = &rcu_sched_ctrlblk.rcucblist,
42    .curtail = &rcu_sched_ctrlblk.rcucblist,
43    RCU_TRACE(.name = "rcu_sched")
44};
45
46static struct rcu_ctrlblk rcu_bh_ctrlblk = {
47    .donetail = &rcu_bh_ctrlblk.rcucblist,
48    .curtail = &rcu_bh_ctrlblk.rcucblist,
49    RCU_TRACE(.name = "rcu_bh")
50};
51
52#ifdef CONFIG_DEBUG_LOCK_ALLOC
53int rcu_scheduler_active __read_mostly;
54EXPORT_SYMBOL_GPL(rcu_scheduler_active);
55#endif /* #ifdef CONFIG_DEBUG_LOCK_ALLOC */
56
57#ifdef CONFIG_TINY_PREEMPT_RCU
58
59#include <linux/delay.h>
60
61/* Global control variables for preemptible RCU. */
62struct rcu_preempt_ctrlblk {
63    struct rcu_ctrlblk rcb; /* curtail: ->next ptr of last CB for GP. */
64    struct rcu_head **nexttail;
65                /* Tasks blocked in a preemptible RCU */
66                /* read-side critical section while an */
67                /* preemptible-RCU grace period is in */
68                /* progress must wait for a later grace */
69                /* period. This pointer points to the */
70                /* ->next pointer of the last task that */
71                /* must wait for a later grace period, or */
72                /* to &->rcb.rcucblist if there is no */
73                /* such task. */
74    struct list_head blkd_tasks;
75                /* Tasks blocked in RCU read-side critical */
76                /* section. Tasks are placed at the head */
77                /* of this list and age towards the tail. */
78    struct list_head *gp_tasks;
79                /* Pointer to the first task blocking the */
80                /* current grace period, or NULL if there */
81                /* is no such task. */
82    struct list_head *exp_tasks;
83                /* Pointer to first task blocking the */
84                /* current expedited grace period, or NULL */
85                /* if there is no such task. If there */
86                /* is no current expedited grace period, */
87                /* then there cannot be any such task. */
88#ifdef CONFIG_RCU_BOOST
89    struct list_head *boost_tasks;
90                /* Pointer to first task that needs to be */
91                /* priority-boosted, or NULL if no priority */
92                /* boosting is needed. If there is no */
93                /* current or expedited grace period, there */
94                /* can be no such task. */
95#endif /* #ifdef CONFIG_RCU_BOOST */
96    u8 gpnum; /* Current grace period. */
97    u8 gpcpu; /* Last grace period blocked by the CPU. */
98    u8 completed; /* Last grace period completed. */
99                /* If all three are equal, RCU is idle. */
100#ifdef CONFIG_RCU_BOOST
101    unsigned long boost_time; /* When to start boosting (jiffies) */
102#endif /* #ifdef CONFIG_RCU_BOOST */
103#ifdef CONFIG_RCU_TRACE
104    unsigned long n_grace_periods;
105#ifdef CONFIG_RCU_BOOST
106    unsigned long n_tasks_boosted;
107                /* Total number of tasks boosted. */
108    unsigned long n_exp_boosts;
109                /* Number of tasks boosted for expedited GP. */
110    unsigned long n_normal_boosts;
111                /* Number of tasks boosted for normal GP. */
112    unsigned long n_balk_blkd_tasks;
113                /* Refused to boost: no blocked tasks. */
114    unsigned long n_balk_exp_gp_tasks;
115                /* Refused to boost: nothing blocking GP. */
116    unsigned long n_balk_boost_tasks;
117                /* Refused to boost: already boosting. */
118    unsigned long n_balk_notyet;
119                /* Refused to boost: not yet time. */
120    unsigned long n_balk_nos;
121                /* Refused to boost: not sure why, though. */
122                /* This can happen due to race conditions. */
123#endif /* #ifdef CONFIG_RCU_BOOST */
124#endif /* #ifdef CONFIG_RCU_TRACE */
125};
126
127static struct rcu_preempt_ctrlblk rcu_preempt_ctrlblk = {
128    .rcb.donetail = &rcu_preempt_ctrlblk.rcb.rcucblist,
129    .rcb.curtail = &rcu_preempt_ctrlblk.rcb.rcucblist,
130    .nexttail = &rcu_preempt_ctrlblk.rcb.rcucblist,
131    .blkd_tasks = LIST_HEAD_INIT(rcu_preempt_ctrlblk.blkd_tasks),
132    RCU_TRACE(.rcb.name = "rcu_preempt")
133};
134
135static void rcu_read_unlock_special(struct task_struct *t);
136static int rcu_preempted_readers_exp(void);
137static void rcu_report_exp_done(void);
138
139/*
140 * Return true if the CPU has not yet responded to the current grace period.
141 */
142static int rcu_cpu_blocking_cur_gp(void)
143{
144    return rcu_preempt_ctrlblk.gpcpu != rcu_preempt_ctrlblk.gpnum;
145}
146
147/*
148 * Check for a running RCU reader. Because there is only one CPU,
149 * there can be but one running RCU reader at a time. ;-)
150 *
151 * Returns zero if there are no running readers. Returns a positive
152 * number if there is at least one reader within its RCU read-side
153 * critical section. Returns a negative number if an outermost reader
154 * is in the midst of exiting from its RCU read-side critical section
155 *
156 * Returns zero if there are no running readers. Returns a positive
157 * number if there is at least one reader within its RCU read-side
158 * critical section. Returns a negative number if an outermost reader
159 * is in the midst of exiting from its RCU read-side critical section.
160 */
161static int rcu_preempt_running_reader(void)
162{
163    return current->rcu_read_lock_nesting;
164}
165
166/*
167 * Check for preempted RCU readers blocking any grace period.
168 * If the caller needs a reliable answer, it must disable hard irqs.
169 */
170static int rcu_preempt_blocked_readers_any(void)
171{
172    return !list_empty(&rcu_preempt_ctrlblk.blkd_tasks);
173}
174
175/*
176 * Check for preempted RCU readers blocking the current grace period.
177 * If the caller needs a reliable answer, it must disable hard irqs.
178 */
179static int rcu_preempt_blocked_readers_cgp(void)
180{
181    return rcu_preempt_ctrlblk.gp_tasks != NULL;
182}
183
184/*
185 * Return true if another preemptible-RCU grace period is needed.
186 */
187static int rcu_preempt_needs_another_gp(void)
188{
189    return *rcu_preempt_ctrlblk.rcb.curtail != NULL;
190}
191
192/*
193 * Return true if a preemptible-RCU grace period is in progress.
194 * The caller must disable hardirqs.
195 */
196static int rcu_preempt_gp_in_progress(void)
197{
198    return rcu_preempt_ctrlblk.completed != rcu_preempt_ctrlblk.gpnum;
199}
200
201/*
202 * Advance a ->blkd_tasks-list pointer to the next entry, instead
203 * returning NULL if at the end of the list.
204 */
205static struct list_head *rcu_next_node_entry(struct task_struct *t)
206{
207    struct list_head *np;
208
209    np = t->rcu_node_entry.next;
210    if (np == &rcu_preempt_ctrlblk.blkd_tasks)
211        np = NULL;
212    return np;
213}
214
215#ifdef CONFIG_RCU_TRACE
216
217#ifdef CONFIG_RCU_BOOST
218static void rcu_initiate_boost_trace(void);
219#endif /* #ifdef CONFIG_RCU_BOOST */
220
221/*
222 * Dump additional statistice for TINY_PREEMPT_RCU.
223 */
224static void show_tiny_preempt_stats(struct seq_file *m)
225{
226    seq_printf(m, "rcu_preempt: qlen=%ld gp=%lu g%u/p%u/c%u tasks=%c%c%c\n",
227           rcu_preempt_ctrlblk.rcb.qlen,
228           rcu_preempt_ctrlblk.n_grace_periods,
229           rcu_preempt_ctrlblk.gpnum,
230           rcu_preempt_ctrlblk.gpcpu,
231           rcu_preempt_ctrlblk.completed,
232           "T."[list_empty(&rcu_preempt_ctrlblk.blkd_tasks)],
233           "N."[!rcu_preempt_ctrlblk.gp_tasks],
234           "E."[!rcu_preempt_ctrlblk.exp_tasks]);
235#ifdef CONFIG_RCU_BOOST
236    seq_printf(m, "%sttb=%c ntb=%lu neb=%lu nnb=%lu j=%04x bt=%04x\n",
237           " ",
238           "B."[!rcu_preempt_ctrlblk.boost_tasks],
239           rcu_preempt_ctrlblk.n_tasks_boosted,
240           rcu_preempt_ctrlblk.n_exp_boosts,
241           rcu_preempt_ctrlblk.n_normal_boosts,
242           (int)(jiffies & 0xffff),
243           (int)(rcu_preempt_ctrlblk.boost_time & 0xffff));
244    seq_printf(m, "%s: nt=%lu egt=%lu bt=%lu ny=%lu nos=%lu\n",
245           " balk",
246           rcu_preempt_ctrlblk.n_balk_blkd_tasks,
247           rcu_preempt_ctrlblk.n_balk_exp_gp_tasks,
248           rcu_preempt_ctrlblk.n_balk_boost_tasks,
249           rcu_preempt_ctrlblk.n_balk_notyet,
250           rcu_preempt_ctrlblk.n_balk_nos);
251#endif /* #ifdef CONFIG_RCU_BOOST */
252}
253
254#endif /* #ifdef CONFIG_RCU_TRACE */
255
256#ifdef CONFIG_RCU_BOOST
257
258#include "rtmutex_common.h"
259
260#define RCU_BOOST_PRIO CONFIG_RCU_BOOST_PRIO
261
262/* Controls for rcu_kthread() kthread. */
263static struct task_struct *rcu_kthread_task;
264static DECLARE_WAIT_QUEUE_HEAD(rcu_kthread_wq);
265static unsigned long have_rcu_kthread_work;
266
267/*
268 * Carry out RCU priority boosting on the task indicated by ->boost_tasks,
269 * and advance ->boost_tasks to the next task in the ->blkd_tasks list.
270 */
271static int rcu_boost(void)
272{
273    unsigned long flags;
274    struct rt_mutex mtx;
275    struct task_struct *t;
276    struct list_head *tb;
277
278    if (rcu_preempt_ctrlblk.boost_tasks == NULL &&
279        rcu_preempt_ctrlblk.exp_tasks == NULL)
280        return 0; /* Nothing to boost. */
281
282    raw_local_irq_save(flags);
283
284    /*
285     * Recheck with irqs disabled: all tasks in need of boosting
286     * might exit their RCU read-side critical sections on their own
287     * if we are preempted just before disabling irqs.
288     */
289    if (rcu_preempt_ctrlblk.boost_tasks == NULL &&
290        rcu_preempt_ctrlblk.exp_tasks == NULL) {
291        raw_local_irq_restore(flags);
292        return 0;
293    }
294
295    /*
296     * Preferentially boost tasks blocking expedited grace periods.
297     * This cannot starve the normal grace periods because a second
298     * expedited grace period must boost all blocked tasks, including
299     * those blocking the pre-existing normal grace period.
300     */
301    if (rcu_preempt_ctrlblk.exp_tasks != NULL) {
302        tb = rcu_preempt_ctrlblk.exp_tasks;
303        RCU_TRACE(rcu_preempt_ctrlblk.n_exp_boosts++);
304    } else {
305        tb = rcu_preempt_ctrlblk.boost_tasks;
306        RCU_TRACE(rcu_preempt_ctrlblk.n_normal_boosts++);
307    }
308    RCU_TRACE(rcu_preempt_ctrlblk.n_tasks_boosted++);
309
310    /*
311     * We boost task t by manufacturing an rt_mutex that appears to
312     * be held by task t. We leave a pointer to that rt_mutex where
313     * task t can find it, and task t will release the mutex when it
314     * exits its outermost RCU read-side critical section. Then
315     * simply acquiring this artificial rt_mutex will boost task
316     * t's priority. (Thanks to tglx for suggesting this approach!)
317     */
318    t = container_of(tb, struct task_struct, rcu_node_entry);
319    rt_mutex_init_proxy_locked(&mtx, t);
320    t->rcu_boost_mutex = &mtx;
321    raw_local_irq_restore(flags);
322    rt_mutex_lock(&mtx);
323    rt_mutex_unlock(&mtx); /* Keep lockdep happy. */
324
325    return ACCESS_ONCE(rcu_preempt_ctrlblk.boost_tasks) != NULL ||
326           ACCESS_ONCE(rcu_preempt_ctrlblk.exp_tasks) != NULL;
327}
328
329/*
330 * Check to see if it is now time to start boosting RCU readers blocking
331 * the current grace period, and, if so, tell the rcu_kthread_task to
332 * start boosting them. If there is an expedited boost in progress,
333 * we wait for it to complete.
334 *
335 * If there are no blocked readers blocking the current grace period,
336 * return 0 to let the caller know, otherwise return 1. Note that this
337 * return value is independent of whether or not boosting was done.
338 */
339static int rcu_initiate_boost(void)
340{
341    if (!rcu_preempt_blocked_readers_cgp() &&
342        rcu_preempt_ctrlblk.exp_tasks == NULL) {
343        RCU_TRACE(rcu_preempt_ctrlblk.n_balk_exp_gp_tasks++);
344        return 0;
345    }
346    if (rcu_preempt_ctrlblk.exp_tasks != NULL ||
347        (rcu_preempt_ctrlblk.gp_tasks != NULL &&
348         rcu_preempt_ctrlblk.boost_tasks == NULL &&
349         ULONG_CMP_GE(jiffies, rcu_preempt_ctrlblk.boost_time))) {
350        if (rcu_preempt_ctrlblk.exp_tasks == NULL)
351            rcu_preempt_ctrlblk.boost_tasks =
352                rcu_preempt_ctrlblk.gp_tasks;
353        invoke_rcu_callbacks();
354    } else
355        RCU_TRACE(rcu_initiate_boost_trace());
356    return 1;
357}
358
359#define RCU_BOOST_DELAY_JIFFIES DIV_ROUND_UP(CONFIG_RCU_BOOST_DELAY * HZ, 1000)
360
361/*
362 * Do priority-boost accounting for the start of a new grace period.
363 */
364static void rcu_preempt_boost_start_gp(void)
365{
366    rcu_preempt_ctrlblk.boost_time = jiffies + RCU_BOOST_DELAY_JIFFIES;
367}
368
369#else /* #ifdef CONFIG_RCU_BOOST */
370
371/*
372 * If there is no RCU priority boosting, we don't initiate boosting,
373 * but we do indicate whether there are blocked readers blocking the
374 * current grace period.
375 */
376static int rcu_initiate_boost(void)
377{
378    return rcu_preempt_blocked_readers_cgp();
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 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_callbacks();
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() > 0 &&
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    } else if (rcu_preempt_running_reader() < 0 &&
508           t->rcu_read_unlock_special) {
509        /*
510         * Complete exit from RCU read-side critical section on
511         * behalf of preempted instance of __rcu_read_unlock().
512         */
513        rcu_read_unlock_special(t);
514    }
515
516    /*
517     * Either we were not in an RCU read-side critical section to
518     * begin with, or we have now recorded that critical section
519     * globally. Either way, we can now note a quiescent state
520     * for this CPU. Again, if we were in an RCU read-side critical
521     * section, and if that critical section was blocking the current
522     * grace period, then the fact that the task has been enqueued
523     * means that current grace period continues to be blocked.
524     */
525    rcu_preempt_cpu_qs();
526    local_irq_restore(flags);
527}
528
529/*
530 * Tiny-preemptible RCU implementation for rcu_read_lock().
531 * Just increment ->rcu_read_lock_nesting, shared state will be updated
532 * if we block.
533 */
534void __rcu_read_lock(void)
535{
536    current->rcu_read_lock_nesting++;
537    barrier(); /* needed if we ever invoke rcu_read_lock in rcutiny.c */
538}
539EXPORT_SYMBOL_GPL(__rcu_read_lock);
540
541/*
542 * Handle special cases during rcu_read_unlock(), such as needing to
543 * notify RCU core processing or task having blocked during the RCU
544 * read-side critical section.
545 */
546static noinline void rcu_read_unlock_special(struct task_struct *t)
547{
548    int empty;
549    int empty_exp;
550    unsigned long flags;
551    struct list_head *np;
552#ifdef CONFIG_RCU_BOOST
553    struct rt_mutex *rbmp = NULL;
554#endif /* #ifdef CONFIG_RCU_BOOST */
555    int special;
556
557    /*
558     * NMI handlers cannot block and cannot safely manipulate state.
559     * They therefore cannot possibly be special, so just leave.
560     */
561    if (in_nmi())
562        return;
563
564    local_irq_save(flags);
565
566    /*
567     * If RCU core is waiting for this CPU to exit critical section,
568     * let it know that we have done so.
569     */
570    special = t->rcu_read_unlock_special;
571    if (special & RCU_READ_UNLOCK_NEED_QS)
572        rcu_preempt_cpu_qs();
573
574    /* Hardware IRQ handlers cannot block. */
575    if (in_irq() || in_serving_softirq()) {
576        local_irq_restore(flags);
577        return;
578    }
579
580    /* Clean up if blocked during RCU read-side critical section. */
581    if (special & RCU_READ_UNLOCK_BLOCKED) {
582        t->rcu_read_unlock_special &= ~RCU_READ_UNLOCK_BLOCKED;
583
584        /*
585         * Remove this task from the ->blkd_tasks list and adjust
586         * any pointers that might have been referencing it.
587         */
588        empty = !rcu_preempt_blocked_readers_cgp();
589        empty_exp = rcu_preempt_ctrlblk.exp_tasks == NULL;
590        np = rcu_next_node_entry(t);
591        list_del_init(&t->rcu_node_entry);
592        if (&t->rcu_node_entry == rcu_preempt_ctrlblk.gp_tasks)
593            rcu_preempt_ctrlblk.gp_tasks = np;
594        if (&t->rcu_node_entry == rcu_preempt_ctrlblk.exp_tasks)
595            rcu_preempt_ctrlblk.exp_tasks = np;
596#ifdef CONFIG_RCU_BOOST
597        if (&t->rcu_node_entry == rcu_preempt_ctrlblk.boost_tasks)
598            rcu_preempt_ctrlblk.boost_tasks = np;
599#endif /* #ifdef CONFIG_RCU_BOOST */
600
601        /*
602         * If this was the last task on the current list, and if
603         * we aren't waiting on the CPU, report the quiescent state
604         * and start a new grace period if needed.
605         */
606        if (!empty && !rcu_preempt_blocked_readers_cgp()) {
607            rcu_preempt_cpu_qs();
608            rcu_preempt_start_gp();
609        }
610
611        /*
612         * If this was the last task on the expedited lists,
613         * then we need wake up the waiting task.
614         */
615        if (!empty_exp && rcu_preempt_ctrlblk.exp_tasks == NULL)
616            rcu_report_exp_done();
617    }
618#ifdef CONFIG_RCU_BOOST
619    /* Unboost self if was boosted. */
620    if (t->rcu_boost_mutex != NULL) {
621        rbmp = t->rcu_boost_mutex;
622        t->rcu_boost_mutex = NULL;
623        rt_mutex_unlock(rbmp);
624    }
625#endif /* #ifdef CONFIG_RCU_BOOST */
626    local_irq_restore(flags);
627}
628
629/*
630 * Tiny-preemptible RCU implementation for rcu_read_unlock().
631 * Decrement ->rcu_read_lock_nesting. If the result is zero (outermost
632 * rcu_read_unlock()) and ->rcu_read_unlock_special is non-zero, then
633 * invoke rcu_read_unlock_special() to clean up after a context switch
634 * in an RCU read-side critical section and other special cases.
635 */
636void __rcu_read_unlock(void)
637{
638    struct task_struct *t = current;
639
640    barrier(); /* needed if we ever invoke rcu_read_unlock in rcutiny.c */
641    if (t->rcu_read_lock_nesting != 1)
642        --t->rcu_read_lock_nesting;
643    else {
644        t->rcu_read_lock_nesting = INT_MIN;
645        barrier(); /* assign before ->rcu_read_unlock_special load */
646        if (unlikely(ACCESS_ONCE(t->rcu_read_unlock_special)))
647            rcu_read_unlock_special(t);
648        barrier(); /* ->rcu_read_unlock_special load before assign */
649        t->rcu_read_lock_nesting = 0;
650    }
651#ifdef CONFIG_PROVE_LOCKING
652    {
653        int rrln = ACCESS_ONCE(t->rcu_read_lock_nesting);
654
655        WARN_ON_ONCE(rrln < 0 && rrln > INT_MIN / 2);
656    }
657#endif /* #ifdef CONFIG_PROVE_LOCKING */
658}
659EXPORT_SYMBOL_GPL(__rcu_read_unlock);
660
661/*
662 * Check for a quiescent state from the current CPU. When a task blocks,
663 * the task is recorded in the rcu_preempt_ctrlblk structure, which is
664 * checked elsewhere. This is called from the scheduling-clock interrupt.
665 *
666 * Caller must disable hard irqs.
667 */
668static void rcu_preempt_check_callbacks(void)
669{
670    struct task_struct *t = current;
671
672    if (rcu_preempt_gp_in_progress() &&
673        (!rcu_preempt_running_reader() ||
674         !rcu_cpu_blocking_cur_gp()))
675        rcu_preempt_cpu_qs();
676    if (&rcu_preempt_ctrlblk.rcb.rcucblist !=
677        rcu_preempt_ctrlblk.rcb.donetail)
678        invoke_rcu_callbacks();
679    if (rcu_preempt_gp_in_progress() &&
680        rcu_cpu_blocking_cur_gp() &&
681        rcu_preempt_running_reader() > 0)
682        t->rcu_read_unlock_special |= RCU_READ_UNLOCK_NEED_QS;
683}
684
685/*
686 * TINY_PREEMPT_RCU has an extra callback-list tail pointer to
687 * update, so this is invoked from rcu_process_callbacks() to
688 * handle that case. Of course, it is invoked for all flavors of
689 * RCU, but RCU callbacks can appear only on one of the lists, and
690 * neither ->nexttail nor ->donetail can possibly be NULL, so there
691 * is no need for an explicit check.
692 */
693static void rcu_preempt_remove_callbacks(struct rcu_ctrlblk *rcp)
694{
695    if (rcu_preempt_ctrlblk.nexttail == rcp->donetail)
696        rcu_preempt_ctrlblk.nexttail = &rcp->rcucblist;
697}
698
699/*
700 * Process callbacks for preemptible RCU.
701 */
702static void rcu_preempt_process_callbacks(void)
703{
704    __rcu_process_callbacks(&rcu_preempt_ctrlblk.rcb);
705}
706
707/*
708 * Queue a preemptible -RCU callback for invocation after a grace period.
709 */
710void call_rcu(struct rcu_head *head, void (*func)(struct rcu_head *rcu))
711{
712    unsigned long flags;
713
714    debug_rcu_head_queue(head);
715    head->func = func;
716    head->next = NULL;
717
718    local_irq_save(flags);
719    *rcu_preempt_ctrlblk.nexttail = head;
720    rcu_preempt_ctrlblk.nexttail = &head->next;
721    RCU_TRACE(rcu_preempt_ctrlblk.rcb.qlen++);
722    rcu_preempt_start_gp(); /* checks to see if GP needed. */
723    local_irq_restore(flags);
724}
725EXPORT_SYMBOL_GPL(call_rcu);
726
727/*
728 * synchronize_rcu - wait until a grace period has elapsed.
729 *
730 * Control will return to the caller some time after a full grace
731 * period has elapsed, in other words after all currently executing RCU
732 * read-side critical sections have completed. RCU read-side critical
733 * sections are delimited by rcu_read_lock() and rcu_read_unlock(),
734 * and may be nested.
735 */
736void synchronize_rcu(void)
737{
738    rcu_lockdep_assert(!lock_is_held(&rcu_bh_lock_map) &&
739               !lock_is_held(&rcu_lock_map) &&
740               !lock_is_held(&rcu_sched_lock_map),
741               "Illegal synchronize_rcu() in RCU read-side critical section");
742
743#ifdef CONFIG_DEBUG_LOCK_ALLOC
744    if (!rcu_scheduler_active)
745        return;
746#endif /* #ifdef CONFIG_DEBUG_LOCK_ALLOC */
747
748    WARN_ON_ONCE(rcu_preempt_running_reader());
749    if (!rcu_preempt_blocked_readers_any())
750        return;
751
752    /* Once we get past the fastpath checks, same code as rcu_barrier(). */
753    rcu_barrier();
754}
755EXPORT_SYMBOL_GPL(synchronize_rcu);
756
757static DECLARE_WAIT_QUEUE_HEAD(sync_rcu_preempt_exp_wq);
758static unsigned long sync_rcu_preempt_exp_count;
759static DEFINE_MUTEX(sync_rcu_preempt_exp_mutex);
760
761/*
762 * Return non-zero if there are any tasks in RCU read-side critical
763 * sections blocking the current preemptible-RCU expedited grace period.
764 * If there is no preemptible-RCU expedited grace period currently in
765 * progress, returns zero unconditionally.
766 */
767static int rcu_preempted_readers_exp(void)
768{
769    return rcu_preempt_ctrlblk.exp_tasks != NULL;
770}
771
772/*
773 * Report the exit from RCU read-side critical section for the last task
774 * that queued itself during or before the current expedited preemptible-RCU
775 * grace period.
776 */
777static void rcu_report_exp_done(void)
778{
779    wake_up(&sync_rcu_preempt_exp_wq);
780}
781
782/*
783 * Wait for an rcu-preempt grace period, but expedite it. The basic idea
784 * is to rely in the fact that there is but one CPU, and that it is
785 * illegal for a task to invoke synchronize_rcu_expedited() while in a
786 * preemptible-RCU read-side critical section. Therefore, any such
787 * critical sections must correspond to blocked tasks, which must therefore
788 * be on the ->blkd_tasks list. So just record the current head of the
789 * list in the ->exp_tasks pointer, and wait for all tasks including and
790 * after the task pointed to by ->exp_tasks to drain.
791 */
792void synchronize_rcu_expedited(void)
793{
794    unsigned long flags;
795    struct rcu_preempt_ctrlblk *rpcp = &rcu_preempt_ctrlblk;
796    unsigned long snap;
797
798    barrier(); /* ensure prior action seen before grace period. */
799
800    WARN_ON_ONCE(rcu_preempt_running_reader());
801
802    /*
803     * Acquire lock so that there is only one preemptible RCU grace
804     * period in flight. Of course, if someone does the expedited
805     * grace period for us while we are acquiring the lock, just leave.
806     */
807    snap = sync_rcu_preempt_exp_count + 1;
808    mutex_lock(&sync_rcu_preempt_exp_mutex);
809    if (ULONG_CMP_LT(snap, sync_rcu_preempt_exp_count))
810        goto unlock_mb_ret; /* Others did our work for us. */
811
812    local_irq_save(flags);
813
814    /*
815     * All RCU readers have to already be on blkd_tasks because
816     * we cannot legally be executing in an RCU read-side critical
817     * section.
818     */
819
820    /* Snapshot current head of ->blkd_tasks list. */
821    rpcp->exp_tasks = rpcp->blkd_tasks.next;
822    if (rpcp->exp_tasks == &rpcp->blkd_tasks)
823        rpcp->exp_tasks = NULL;
824
825    /* Wait for tail of ->blkd_tasks list to drain. */
826    if (!rcu_preempted_readers_exp())
827        local_irq_restore(flags);
828    else {
829        rcu_initiate_boost();
830        local_irq_restore(flags);
831        wait_event(sync_rcu_preempt_exp_wq,
832               !rcu_preempted_readers_exp());
833    }
834
835    /* Clean up and exit. */
836    barrier(); /* ensure expedited GP seen before counter increment. */
837    sync_rcu_preempt_exp_count++;
838unlock_mb_ret:
839    mutex_unlock(&sync_rcu_preempt_exp_mutex);
840    barrier(); /* ensure subsequent action seen after grace period. */
841}
842EXPORT_SYMBOL_GPL(synchronize_rcu_expedited);
843
844/*
845 * Does preemptible RCU need the CPU to stay out of dynticks mode?
846 */
847int rcu_preempt_needs_cpu(void)
848{
849    if (!rcu_preempt_running_reader())
850        rcu_preempt_cpu_qs();
851    return rcu_preempt_ctrlblk.rcb.rcucblist != NULL;
852}
853
854#else /* #ifdef CONFIG_TINY_PREEMPT_RCU */
855
856#ifdef CONFIG_RCU_TRACE
857
858/*
859 * Because preemptible RCU does not exist, it is not necessary to
860 * dump out its statistics.
861 */
862static void show_tiny_preempt_stats(struct seq_file *m)
863{
864}
865
866#endif /* #ifdef CONFIG_RCU_TRACE */
867
868/*
869 * Because preemptible RCU does not exist, it never has any callbacks
870 * to check.
871 */
872static void rcu_preempt_check_callbacks(void)
873{
874}
875
876/*
877 * Because preemptible RCU does not exist, it never has any callbacks
878 * to remove.
879 */
880static void rcu_preempt_remove_callbacks(struct rcu_ctrlblk *rcp)
881{
882}
883
884/*
885 * Because preemptible RCU does not exist, it never has any callbacks
886 * to process.
887 */
888static void rcu_preempt_process_callbacks(void)
889{
890}
891
892#endif /* #else #ifdef CONFIG_TINY_PREEMPT_RCU */
893
894#ifdef CONFIG_RCU_BOOST
895
896/*
897 * Wake up rcu_kthread() to process callbacks now eligible for invocation
898 * or to boost readers.
899 */
900static void invoke_rcu_callbacks(void)
901{
902    have_rcu_kthread_work = 1;
903    if (rcu_kthread_task != NULL)
904        wake_up(&rcu_kthread_wq);
905}
906
907#ifdef CONFIG_RCU_TRACE
908
909/*
910 * Is the current CPU running the RCU-callbacks kthread?
911 * Caller must have preemption disabled.
912 */
913static bool rcu_is_callbacks_kthread(void)
914{
915    return rcu_kthread_task == current;
916}
917
918#endif /* #ifdef CONFIG_RCU_TRACE */
919
920/*
921 * This kthread invokes RCU callbacks whose grace periods have
922 * elapsed. It is awakened as needed, and takes the place of the
923 * RCU_SOFTIRQ that is used for this purpose when boosting is disabled.
924 * This is a kthread, but it is never stopped, at least not until
925 * the system goes down.
926 */
927static int rcu_kthread(void *arg)
928{
929    unsigned long work;
930    unsigned long morework;
931    unsigned long flags;
932
933    for (;;) {
934        wait_event_interruptible(rcu_kthread_wq,
935                     have_rcu_kthread_work != 0);
936        morework = rcu_boost();
937        local_irq_save(flags);
938        work = have_rcu_kthread_work;
939        have_rcu_kthread_work = morework;
940        local_irq_restore(flags);
941        if (work)
942            rcu_process_callbacks(NULL);
943        schedule_timeout_interruptible(1); /* Leave CPU for others. */
944    }
945
946    return 0; /* Not reached, but needed to shut gcc up. */
947}
948
949/*
950 * Spawn the kthread that invokes RCU callbacks.
951 */
952static int __init rcu_spawn_kthreads(void)
953{
954    struct sched_param sp;
955
956    rcu_kthread_task = kthread_run(rcu_kthread, NULL, "rcu_kthread");
957    sp.sched_priority = RCU_BOOST_PRIO;
958    sched_setscheduler_nocheck(rcu_kthread_task, SCHED_FIFO, &sp);
959    return 0;
960}
961early_initcall(rcu_spawn_kthreads);
962
963#else /* #ifdef CONFIG_RCU_BOOST */
964
965/* Hold off callback invocation until early_initcall() time. */
966static int rcu_scheduler_fully_active __read_mostly;
967
968/*
969 * Start up softirq processing of callbacks.
970 */
971void invoke_rcu_callbacks(void)
972{
973    if (rcu_scheduler_fully_active)
974        raise_softirq(RCU_SOFTIRQ);
975}
976
977#ifdef CONFIG_RCU_TRACE
978
979/*
980 * There is no callback kthread, so this thread is never it.
981 */
982static bool rcu_is_callbacks_kthread(void)
983{
984    return false;
985}
986
987#endif /* #ifdef CONFIG_RCU_TRACE */
988
989static int __init rcu_scheduler_really_started(void)
990{
991    rcu_scheduler_fully_active = 1;
992    open_softirq(RCU_SOFTIRQ, rcu_process_callbacks);
993    raise_softirq(RCU_SOFTIRQ); /* Invoke any callbacks from early boot. */
994    return 0;
995}
996early_initcall(rcu_scheduler_really_started);
997
998#endif /* #else #ifdef CONFIG_RCU_BOOST */
999
1000#ifdef CONFIG_DEBUG_LOCK_ALLOC
1001#include <linux/kernel_stat.h>
1002
1003/*
1004 * During boot, we forgive RCU lockdep issues. After this function is
1005 * invoked, we start taking RCU lockdep issues seriously.
1006 */
1007void __init rcu_scheduler_starting(void)
1008{
1009    WARN_ON(nr_context_switches() > 0);
1010    rcu_scheduler_active = 1;
1011}
1012
1013#endif /* #ifdef CONFIG_DEBUG_LOCK_ALLOC */
1014
1015#ifdef CONFIG_RCU_TRACE
1016
1017#ifdef CONFIG_RCU_BOOST
1018
1019static void rcu_initiate_boost_trace(void)
1020{
1021    if (list_empty(&rcu_preempt_ctrlblk.blkd_tasks))
1022        rcu_preempt_ctrlblk.n_balk_blkd_tasks++;
1023    else if (rcu_preempt_ctrlblk.gp_tasks == NULL &&
1024         rcu_preempt_ctrlblk.exp_tasks == NULL)
1025        rcu_preempt_ctrlblk.n_balk_exp_gp_tasks++;
1026    else if (rcu_preempt_ctrlblk.boost_tasks != NULL)
1027        rcu_preempt_ctrlblk.n_balk_boost_tasks++;
1028    else if (!ULONG_CMP_GE(jiffies, rcu_preempt_ctrlblk.boost_time))
1029        rcu_preempt_ctrlblk.n_balk_notyet++;
1030    else
1031        rcu_preempt_ctrlblk.n_balk_nos++;
1032}
1033
1034#endif /* #ifdef CONFIG_RCU_BOOST */
1035
1036static void rcu_trace_sub_qlen(struct rcu_ctrlblk *rcp, int n)
1037{
1038    unsigned long flags;
1039
1040    raw_local_irq_save(flags);
1041    rcp->qlen -= n;
1042    raw_local_irq_restore(flags);
1043}
1044
1045/*
1046 * Dump statistics for TINY_RCU, such as they are.
1047 */
1048static int show_tiny_stats(struct seq_file *m, void *unused)
1049{
1050    show_tiny_preempt_stats(m);
1051    seq_printf(m, "rcu_sched: qlen: %ld\n", rcu_sched_ctrlblk.qlen);
1052    seq_printf(m, "rcu_bh: qlen: %ld\n", rcu_bh_ctrlblk.qlen);
1053    return 0;
1054}
1055
1056static int show_tiny_stats_open(struct inode *inode, struct file *file)
1057{
1058    return single_open(file, show_tiny_stats, NULL);
1059}
1060
1061static const struct file_operations show_tiny_stats_fops = {
1062    .owner = THIS_MODULE,
1063    .open = show_tiny_stats_open,
1064    .read = seq_read,
1065    .llseek = seq_lseek,
1066    .release = single_release,
1067};
1068
1069static struct dentry *rcudir;
1070
1071static int __init rcutiny_trace_init(void)
1072{
1073    struct dentry *retval;
1074
1075    rcudir = debugfs_create_dir("rcu", NULL);
1076    if (!rcudir)
1077        goto free_out;
1078    retval = debugfs_create_file("rcudata", 0444, rcudir,
1079                     NULL, &show_tiny_stats_fops);
1080    if (!retval)
1081        goto free_out;
1082    return 0;
1083free_out:
1084    debugfs_remove_recursive(rcudir);
1085    return 1;
1086}
1087
1088static void __exit rcutiny_trace_cleanup(void)
1089{
1090    debugfs_remove_recursive(rcudir);
1091}
1092
1093module_init(rcutiny_trace_init);
1094module_exit(rcutiny_trace_cleanup);
1095
1096MODULE_AUTHOR("Paul E. McKenney");
1097MODULE_DESCRIPTION("Read-Copy Update tracing for tiny implementation");
1098MODULE_LICENSE("GPL");
1099
1100#endif /* #ifdef CONFIG_RCU_TRACE */
1101

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