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

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