linux/kernel/rcutiny.c
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   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#include <linux/prefetch.h>
  39
  40/* Controls for rcu_kthread() kthread, replacing RCU_SOFTIRQ used previously. */
  41static struct task_struct *rcu_kthread_task;
  42static DECLARE_WAIT_QUEUE_HEAD(rcu_kthread_wq);
  43static unsigned long have_rcu_kthread_work;
  44
  45/* Forward declarations for rcutiny_plugin.h. */
  46struct rcu_ctrlblk;
  47static void invoke_rcu_kthread(void);
  48static void rcu_process_callbacks(struct rcu_ctrlblk *rcp);
  49static int rcu_kthread(void *arg);
  50static void __call_rcu(struct rcu_head *head,
  51                       void (*func)(struct rcu_head *rcu),
  52                       struct rcu_ctrlblk *rcp);
  53
  54#include "rcutiny_plugin.h"
  55
  56#ifdef CONFIG_NO_HZ
  57
  58static long rcu_dynticks_nesting = 1;
  59
  60/*
  61 * Enter dynticks-idle mode, which is an extended quiescent state
  62 * if we have fully entered that mode (i.e., if the new value of
  63 * dynticks_nesting is zero).
  64 */
  65void rcu_enter_nohz(void)
  66{
  67        if (--rcu_dynticks_nesting == 0)
  68                rcu_sched_qs(0); /* implies rcu_bh_qsctr_inc(0) */
  69}
  70
  71/*
  72 * Exit dynticks-idle mode, so that we are no longer in an extended
  73 * quiescent state.
  74 */
  75void rcu_exit_nohz(void)
  76{
  77        rcu_dynticks_nesting++;
  78}
  79
  80#endif /* #ifdef CONFIG_NO_HZ */
  81
  82/*
  83 * Helper function for rcu_sched_qs() and rcu_bh_qs().
  84 * Also irqs are disabled to avoid confusion due to interrupt handlers
  85 * invoking call_rcu().
  86 */
  87static int rcu_qsctr_help(struct rcu_ctrlblk *rcp)
  88{
  89        if (rcp->rcucblist != NULL &&
  90            rcp->donetail != rcp->curtail) {
  91                rcp->donetail = rcp->curtail;
  92                return 1;
  93        }
  94
  95        return 0;
  96}
  97
  98/*
  99 * Wake up rcu_kthread() to process callbacks now eligible for invocation
 100 * or to boost readers.
 101 */
 102static void invoke_rcu_kthread(void)
 103{
 104        have_rcu_kthread_work = 1;
 105        wake_up(&rcu_kthread_wq);
 106}
 107
 108/*
 109 * Record an rcu quiescent state.  And an rcu_bh quiescent state while we
 110 * are at it, given that any rcu quiescent state is also an rcu_bh
 111 * quiescent state.  Use "+" instead of "||" to defeat short circuiting.
 112 */
 113void rcu_sched_qs(int cpu)
 114{
 115        unsigned long flags;
 116
 117        local_irq_save(flags);
 118        if (rcu_qsctr_help(&rcu_sched_ctrlblk) +
 119            rcu_qsctr_help(&rcu_bh_ctrlblk))
 120                invoke_rcu_kthread();
 121        local_irq_restore(flags);
 122}
 123
 124/*
 125 * Record an rcu_bh quiescent state.
 126 */
 127void rcu_bh_qs(int cpu)
 128{
 129        unsigned long flags;
 130
 131        local_irq_save(flags);
 132        if (rcu_qsctr_help(&rcu_bh_ctrlblk))
 133                invoke_rcu_kthread();
 134        local_irq_restore(flags);
 135}
 136
 137/*
 138 * Check to see if the scheduling-clock interrupt came from an extended
 139 * quiescent state, and, if so, tell RCU about it.
 140 */
 141void rcu_check_callbacks(int cpu, int user)
 142{
 143        if (user ||
 144            (idle_cpu(cpu) &&
 145             !in_softirq() &&
 146             hardirq_count() <= (1 << HARDIRQ_SHIFT)))
 147                rcu_sched_qs(cpu);
 148        else if (!in_softirq())
 149                rcu_bh_qs(cpu);
 150        rcu_preempt_check_callbacks();
 151}
 152
 153/*
 154 * Invoke the RCU callbacks on the specified rcu_ctrlkblk structure
 155 * whose grace period has elapsed.
 156 */
 157static void rcu_process_callbacks(struct rcu_ctrlblk *rcp)
 158{
 159        struct rcu_head *next, *list;
 160        unsigned long flags;
 161        RCU_TRACE(int cb_count = 0);
 162
 163        /* If no RCU callbacks ready to invoke, just return. */
 164        if (&rcp->rcucblist == rcp->donetail)
 165                return;
 166
 167        /* Move the ready-to-invoke callbacks to a local list. */
 168        local_irq_save(flags);
 169        list = rcp->rcucblist;
 170        rcp->rcucblist = *rcp->donetail;
 171        *rcp->donetail = NULL;
 172        if (rcp->curtail == rcp->donetail)
 173                rcp->curtail = &rcp->rcucblist;
 174        rcu_preempt_remove_callbacks(rcp);
 175        rcp->donetail = &rcp->rcucblist;
 176        local_irq_restore(flags);
 177
 178        /* Invoke the callbacks on the local list. */
 179        while (list) {
 180                next = list->next;
 181                prefetch(next);
 182                debug_rcu_head_unqueue(list);
 183                local_bh_disable();
 184                __rcu_reclaim(list);
 185                local_bh_enable();
 186                list = next;
 187                RCU_TRACE(cb_count++);
 188        }
 189        RCU_TRACE(rcu_trace_sub_qlen(rcp, cb_count));
 190}
 191
 192/*
 193 * This kthread invokes RCU callbacks whose grace periods have
 194 * elapsed.  It is awakened as needed, and takes the place of the
 195 * RCU_SOFTIRQ that was used previously for this purpose.
 196 * This is a kthread, but it is never stopped, at least not until
 197 * the system goes down.
 198 */
 199static int rcu_kthread(void *arg)
 200{
 201        unsigned long work;
 202        unsigned long morework;
 203        unsigned long flags;
 204
 205        for (;;) {
 206                wait_event_interruptible(rcu_kthread_wq,
 207                                         have_rcu_kthread_work != 0);
 208                morework = rcu_boost();
 209                local_irq_save(flags);
 210                work = have_rcu_kthread_work;
 211                have_rcu_kthread_work = morework;
 212                local_irq_restore(flags);
 213                if (work) {
 214                        rcu_process_callbacks(&rcu_sched_ctrlblk);
 215                        rcu_process_callbacks(&rcu_bh_ctrlblk);
 216                        rcu_preempt_process_callbacks();
 217                }
 218                schedule_timeout_interruptible(1); /* Leave CPU for others. */
 219        }
 220
 221        return 0;  /* Not reached, but needed to shut gcc up. */
 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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