Root/kernel/rcutiny.c

1/*
2 * Read-Copy Update mechanism for mutual exclusion, the Bloatwatch edition.
3 *
4 * This program is free software; you can redistribute it and/or modify
5 * it under the terms of the GNU General Public License as published by
6 * the Free Software Foundation; either version 2 of the License, or
7 * (at your option) any later version.
8 *
9 * This program is distributed in the hope that it will be useful,
10 * but WITHOUT ANY WARRANTY; without even the implied warranty of
11 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
12 * GNU General Public License for more details.
13 *
14 * You should have received a copy of the GNU General Public License
15 * along with this program; if not, write to the Free Software
16 * Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA 02111-1307, USA.
17 *
18 * Copyright IBM Corporation, 2008
19 *
20 * Author: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
21 *
22 * For detailed explanation of Read-Copy Update mechanism see -
23 * Documentation/RCU
24 */
25#include <linux/moduleparam.h>
26#include <linux/completion.h>
27#include <linux/interrupt.h>
28#include <linux/notifier.h>
29#include <linux/rcupdate.h>
30#include <linux/kernel.h>
31#include <linux/module.h>
32#include <linux/mutex.h>
33#include <linux/sched.h>
34#include <linux/types.h>
35#include <linux/init.h>
36#include <linux/time.h>
37#include <linux/cpu.h>
38
39/* Controls for rcu_kthread() kthread, replacing RCU_SOFTIRQ used previously. */
40static struct task_struct *rcu_kthread_task;
41static DECLARE_WAIT_QUEUE_HEAD(rcu_kthread_wq);
42static unsigned long have_rcu_kthread_work;
43static void invoke_rcu_kthread(void);
44
45/* Forward declarations for rcutiny_plugin.h. */
46struct rcu_ctrlblk;
47static void rcu_process_callbacks(struct rcu_ctrlblk *rcp);
48static int rcu_kthread(void *arg);
49static void __call_rcu(struct rcu_head *head,
50               void (*func)(struct rcu_head *rcu),
51               struct rcu_ctrlblk *rcp);
52
53#include "rcutiny_plugin.h"
54
55#ifdef CONFIG_NO_HZ
56
57static long rcu_dynticks_nesting = 1;
58
59/*
60 * Enter dynticks-idle mode, which is an extended quiescent state
61 * if we have fully entered that mode (i.e., if the new value of
62 * dynticks_nesting is zero).
63 */
64void rcu_enter_nohz(void)
65{
66    if (--rcu_dynticks_nesting == 0)
67        rcu_sched_qs(0); /* implies rcu_bh_qsctr_inc(0) */
68}
69
70/*
71 * Exit dynticks-idle mode, so that we are no longer in an extended
72 * quiescent state.
73 */
74void rcu_exit_nohz(void)
75{
76    rcu_dynticks_nesting++;
77}
78
79#endif /* #ifdef CONFIG_NO_HZ */
80
81/*
82 * Helper function for rcu_qsctr_inc() and rcu_bh_qsctr_inc().
83 * Also disable irqs to avoid confusion due to interrupt handlers
84 * invoking call_rcu().
85 */
86static int rcu_qsctr_help(struct rcu_ctrlblk *rcp)
87{
88    unsigned long flags;
89
90    local_irq_save(flags);
91    if (rcp->rcucblist != NULL &&
92        rcp->donetail != rcp->curtail) {
93        rcp->donetail = rcp->curtail;
94        local_irq_restore(flags);
95        return 1;
96    }
97    local_irq_restore(flags);
98
99    return 0;
100}
101
102/*
103 * Record an rcu quiescent state. And an rcu_bh quiescent state while we
104 * are at it, given that any rcu quiescent state is also an rcu_bh
105 * quiescent state. Use "+" instead of "||" to defeat short circuiting.
106 */
107void rcu_sched_qs(int cpu)
108{
109    if (rcu_qsctr_help(&rcu_sched_ctrlblk) +
110        rcu_qsctr_help(&rcu_bh_ctrlblk))
111        invoke_rcu_kthread();
112}
113
114/*
115 * Record an rcu_bh quiescent state.
116 */
117void rcu_bh_qs(int cpu)
118{
119    if (rcu_qsctr_help(&rcu_bh_ctrlblk))
120        invoke_rcu_kthread();
121}
122
123/*
124 * Check to see if the scheduling-clock interrupt came from an extended
125 * quiescent state, and, if so, tell RCU about it.
126 */
127void rcu_check_callbacks(int cpu, int user)
128{
129    if (user ||
130        (idle_cpu(cpu) &&
131         !in_softirq() &&
132         hardirq_count() <= (1 << HARDIRQ_SHIFT)))
133        rcu_sched_qs(cpu);
134    else if (!in_softirq())
135        rcu_bh_qs(cpu);
136    rcu_preempt_check_callbacks();
137}
138
139/*
140 * Invoke the RCU callbacks on the specified rcu_ctrlkblk structure
141 * whose grace period has elapsed.
142 */
143static void rcu_process_callbacks(struct rcu_ctrlblk *rcp)
144{
145    struct rcu_head *next, *list;
146    unsigned long flags;
147    RCU_TRACE(int cb_count = 0);
148
149    /* If no RCU callbacks ready to invoke, just return. */
150    if (&rcp->rcucblist == rcp->donetail)
151        return;
152
153    /* Move the ready-to-invoke callbacks to a local list. */
154    local_irq_save(flags);
155    list = rcp->rcucblist;
156    rcp->rcucblist = *rcp->donetail;
157    *rcp->donetail = NULL;
158    if (rcp->curtail == rcp->donetail)
159        rcp->curtail = &rcp->rcucblist;
160    rcu_preempt_remove_callbacks(rcp);
161    rcp->donetail = &rcp->rcucblist;
162    local_irq_restore(flags);
163
164    /* Invoke the callbacks on the local list. */
165    while (list) {
166        next = list->next;
167        prefetch(next);
168        debug_rcu_head_unqueue(list);
169        local_bh_disable();
170        list->func(list);
171        local_bh_enable();
172        list = next;
173        RCU_TRACE(cb_count++);
174    }
175    RCU_TRACE(rcu_trace_sub_qlen(rcp, cb_count));
176}
177
178/*
179 * This kthread invokes RCU callbacks whose grace periods have
180 * elapsed. It is awakened as needed, and takes the place of the
181 * RCU_SOFTIRQ that was used previously for this purpose.
182 * This is a kthread, but it is never stopped, at least not until
183 * the system goes down.
184 */
185static int rcu_kthread(void *arg)
186{
187    unsigned long work;
188    unsigned long morework;
189    unsigned long flags;
190
191    for (;;) {
192        wait_event_interruptible(rcu_kthread_wq,
193                     have_rcu_kthread_work != 0);
194        morework = rcu_boost();
195        local_irq_save(flags);
196        work = have_rcu_kthread_work;
197        have_rcu_kthread_work = morework;
198        local_irq_restore(flags);
199        if (work) {
200            rcu_process_callbacks(&rcu_sched_ctrlblk);
201            rcu_process_callbacks(&rcu_bh_ctrlblk);
202            rcu_preempt_process_callbacks();
203        }
204        schedule_timeout_interruptible(1); /* Leave CPU for others. */
205    }
206
207    return 0; /* Not reached, but needed to shut gcc up. */
208}
209
210/*
211 * Wake up rcu_kthread() to process callbacks now eligible for invocation
212 * or to boost readers.
213 */
214static void invoke_rcu_kthread(void)
215{
216    unsigned long flags;
217
218    local_irq_save(flags);
219    have_rcu_kthread_work = 1;
220    wake_up(&rcu_kthread_wq);
221    local_irq_restore(flags);
222}
223
224/*
225 * Wait for a grace period to elapse. But it is illegal to invoke
226 * synchronize_sched() from within an RCU read-side critical section.
227 * Therefore, any legal call to synchronize_sched() is a quiescent
228 * state, and so on a UP system, synchronize_sched() need do nothing.
229 * Ditto for synchronize_rcu_bh(). (But Lai Jiangshan points out the
230 * benefits of doing might_sleep() to reduce latency.)
231 *
232 * Cool, huh? (Due to Josh Triplett.)
233 *
234 * But we want to make this a static inline later. The cond_resched()
235 * currently makes this problematic.
236 */
237void synchronize_sched(void)
238{
239    cond_resched();
240}
241EXPORT_SYMBOL_GPL(synchronize_sched);
242
243/*
244 * Helper function for call_rcu() and call_rcu_bh().
245 */
246static void __call_rcu(struct rcu_head *head,
247               void (*func)(struct rcu_head *rcu),
248               struct rcu_ctrlblk *rcp)
249{
250    unsigned long flags;
251
252    debug_rcu_head_queue(head);
253    head->func = func;
254    head->next = NULL;
255
256    local_irq_save(flags);
257    *rcp->curtail = head;
258    rcp->curtail = &head->next;
259    RCU_TRACE(rcp->qlen++);
260    local_irq_restore(flags);
261}
262
263/*
264 * Post an RCU callback to be invoked after the end of an RCU-sched grace
265 * period. But since we have but one CPU, that would be after any
266 * quiescent state.
267 */
268void call_rcu_sched(struct rcu_head *head, void (*func)(struct rcu_head *rcu))
269{
270    __call_rcu(head, func, &rcu_sched_ctrlblk);
271}
272EXPORT_SYMBOL_GPL(call_rcu_sched);
273
274/*
275 * Post an RCU bottom-half callback to be invoked after any subsequent
276 * quiescent state.
277 */
278void call_rcu_bh(struct rcu_head *head, void (*func)(struct rcu_head *rcu))
279{
280    __call_rcu(head, func, &rcu_bh_ctrlblk);
281}
282EXPORT_SYMBOL_GPL(call_rcu_bh);
283
284void rcu_barrier_bh(void)
285{
286    struct rcu_synchronize rcu;
287
288    init_rcu_head_on_stack(&rcu.head);
289    init_completion(&rcu.completion);
290    /* Will wake me after RCU finished. */
291    call_rcu_bh(&rcu.head, wakeme_after_rcu);
292    /* Wait for it. */
293    wait_for_completion(&rcu.completion);
294    destroy_rcu_head_on_stack(&rcu.head);
295}
296EXPORT_SYMBOL_GPL(rcu_barrier_bh);
297
298void rcu_barrier_sched(void)
299{
300    struct rcu_synchronize rcu;
301
302    init_rcu_head_on_stack(&rcu.head);
303    init_completion(&rcu.completion);
304    /* Will wake me after RCU finished. */
305    call_rcu_sched(&rcu.head, wakeme_after_rcu);
306    /* Wait for it. */
307    wait_for_completion(&rcu.completion);
308    destroy_rcu_head_on_stack(&rcu.head);
309}
310EXPORT_SYMBOL_GPL(rcu_barrier_sched);
311
312/*
313 * Spawn the kthread that invokes RCU callbacks.
314 */
315static int __init rcu_spawn_kthreads(void)
316{
317    struct sched_param sp;
318
319    rcu_kthread_task = kthread_run(rcu_kthread, NULL, "rcu_kthread");
320    sp.sched_priority = RCU_BOOST_PRIO;
321    sched_setscheduler_nocheck(rcu_kthread_task, SCHED_FIFO, &sp);
322    return 0;
323}
324early_initcall(rcu_spawn_kthreads);
325

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