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 /* Global control variables for rcupdate callback mechanism. */
  40 struct rcu_ctrlblk {
  41         struct rcu_head *rcucblist;     /* List of pending callbacks (CBs). */
  42         struct rcu_head **donetail;     /* ->next pointer of last "done" CB. */
  43         struct rcu_head **curtail;      /* ->next pointer of last CB. */
  44 };
  45
  46 /* Definition for rcupdate control block. */
  47 static struct rcu_ctrlblk rcu_sched_ctrlblk = {
  48         .donetail       = &rcu_sched_ctrlblk.rcucblist,
  49         .curtail        = &rcu_sched_ctrlblk.rcucblist,
  50 };
  51
  52 static struct rcu_ctrlblk rcu_bh_ctrlblk = {
  53         .donetail       = &rcu_bh_ctrlblk.rcucblist,
  54         .curtail        = &rcu_bh_ctrlblk.rcucblist,
  55 };
  56
  57 #ifdef CONFIG_DEBUG_LOCK_ALLOC
  58 int rcu_scheduler_active __read_mostly;
  59 EXPORT_SYMBOL_GPL(rcu_scheduler_active);
  60 #endif /* #ifdef CONFIG_DEBUG_LOCK_ALLOC */
  61
  62 /* Forward declarations for rcutiny_plugin.h. */
  63 static void __rcu_process_callbacks(struct rcu_ctrlblk *rcp);
  64 static void __call_rcu(struct rcu_head *head,
  65                        void (*func)(struct rcu_head *rcu),
  66                        struct rcu_ctrlblk *rcp);
  67
  68 #include "rcutiny_plugin.h"
  69
  70 #ifdef CONFIG_NO_HZ
  71
  72 static long rcu_dynticks_nesting = 1;
  73
  74 /*
  75  * Enter dynticks-idle mode, which is an extended quiescent state
  76  * if we have fully entered that mode (i.e., if the new value of
  77  * dynticks_nesting is zero).
  78  */
  79 void rcu_enter_nohz(void)
  80 {
  81         if (--rcu_dynticks_nesting == 0)
  82                 rcu_sched_qs(0); /* implies rcu_bh_qsctr_inc(0) */
  83 }
  84
  85 /*
  86  * Exit dynticks-idle mode, so that we are no longer in an extended
  87  * quiescent state.
  88  */
  89 void rcu_exit_nohz(void)
  90 {
  91         rcu_dynticks_nesting++;
  92 }
  93
  94 #endif /* #ifdef CONFIG_NO_HZ */
  95
  96 /*
  97  * Helper function for rcu_qsctr_inc() and rcu_bh_qsctr_inc().
  98  * Also disable irqs to avoid confusion due to interrupt handlers
  99  * invoking call_rcu().
 100  */
 101 static int rcu_qsctr_help(struct rcu_ctrlblk *rcp)
 102 {
 103         unsigned long flags;
 104
 105         local_irq_save(flags);
 106         if (rcp->rcucblist != NULL &&
 107             rcp->donetail != rcp->curtail) {
 108                 rcp->donetail = rcp->curtail;
 109                 local_irq_restore(flags);
 110                 return 1;
 111         }
 112         local_irq_restore(flags);
 113
 114         return 0;
 115 }
 116
 117 /*
 118  * Record an rcu quiescent state.  And an rcu_bh quiescent state while we
 119  * are at it, given that any rcu quiescent state is also an rcu_bh
 120  * quiescent state.  Use "+" instead of "||" to defeat short circuiting.
 121  */
 122 void rcu_sched_qs(int cpu)
 123 {
 124         if (rcu_qsctr_help(&rcu_sched_ctrlblk) +
 125             rcu_qsctr_help(&rcu_bh_ctrlblk))
 126                 raise_softirq(RCU_SOFTIRQ);
 127 }
 128
 129 /*
 130  * Record an rcu_bh quiescent state.
 131  */
 132 void rcu_bh_qs(int cpu)
 133 {
 134         if (rcu_qsctr_help(&rcu_bh_ctrlblk))
 135                 raise_softirq(RCU_SOFTIRQ);
 136 }
 137
 138 /*
 139  * Check to see if the scheduling-clock interrupt came from an extended
 140  * quiescent state, and, if so, tell RCU about it.
 141  */
 142 void rcu_check_callbacks(int cpu, int user)
 143 {
 144         if (user ||
 145             (idle_cpu(cpu) &&
 146              !in_softirq() &&
 147              hardirq_count() <= (1 << HARDIRQ_SHIFT)))
 148                 rcu_sched_qs(cpu);
 149         else if (!in_softirq())
 150                 rcu_bh_qs(cpu);
 151         rcu_preempt_check_callbacks();
 152 }
 153
 154 /*
 155  * Helper function for rcu_process_callbacks() that operates on the
 156  * specified rcu_ctrlkblk structure.
 157  */
 158 static void __rcu_process_callbacks(struct rcu_ctrlblk *rcp)
 159 {
 160         struct rcu_head *next, *list;
 161         unsigned long flags;
 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                 list->func(list);
 184                 list = next;
 185         }
 186 }
 187
 188 /*
 189  * Invoke any callbacks whose grace period has completed.
 190  */
 191 static void rcu_process_callbacks(struct softirq_action *unused)
 192 {
 193         __rcu_process_callbacks(&rcu_sched_ctrlblk);
 194         __rcu_process_callbacks(&rcu_bh_ctrlblk);
 195         rcu_preempt_process_callbacks();
 196 }
 197
 198 /*
 199  * Wait for a grace period to elapse.  But it is illegal to invoke
 200  * synchronize_sched() from within an RCU read-side critical section.
 201  * Therefore, any legal call to synchronize_sched() is a quiescent
 202  * state, and so on a UP system, synchronize_sched() need do nothing.
 203  * Ditto for synchronize_rcu_bh().  (But Lai Jiangshan points out the
 204  * benefits of doing might_sleep() to reduce latency.)
 205  *
 206  * Cool, huh?  (Due to Josh Triplett.)
 207  *
 208  * But we want to make this a static inline later.  The cond_resched()
 209  * currently makes this problematic.
 210  */
 211 void synchronize_sched(void)
 212 {
 213         cond_resched();
 214 }
 215 EXPORT_SYMBOL_GPL(synchronize_sched);
 216
 217 /*
 218  * Helper function for call_rcu() and call_rcu_bh().
 219  */
 220 static void __call_rcu(struct rcu_head *head,
 221                        void (*func)(struct rcu_head *rcu),
 222                        struct rcu_ctrlblk *rcp)
 223 {
 224         unsigned long flags;
 225
 226         debug_rcu_head_queue(head);
 227         head->func = func;
 228         head->next = NULL;
 229
 230         local_irq_save(flags);
 231         *rcp->curtail = head;
 232         rcp->curtail = &head->next;
 233         local_irq_restore(flags);
 234 }
 235
 236 /*
 237  * Post an RCU callback to be invoked after the end of an RCU-sched grace
 238  * period.  But since we have but one CPU, that would be after any
 239  * quiescent state.
 240  */
 241 void call_rcu_sched(struct rcu_head *head, void (*func)(struct rcu_head *rcu))
 242 {
 243         __call_rcu(head, func, &rcu_sched_ctrlblk);
 244 }
 245 EXPORT_SYMBOL_GPL(call_rcu_sched);
 246
 247 /*
 248  * Post an RCU bottom-half callback to be invoked after any subsequent
 249  * quiescent state.
 250  */
 251 void call_rcu_bh(struct rcu_head *head, void (*func)(struct rcu_head *rcu))
 252 {
 253         __call_rcu(head, func, &rcu_bh_ctrlblk);
 254 }
 255 EXPORT_SYMBOL_GPL(call_rcu_bh);
 256
 257 void rcu_barrier_bh(void)
 258 {
 259         struct rcu_synchronize rcu;
 260
 261         init_rcu_head_on_stack(&rcu.head);
 262         init_completion(&rcu.completion);
 263         /* Will wake me after RCU finished. */
 264         call_rcu_bh(&rcu.head, wakeme_after_rcu);
 265         /* Wait for it. */
 266         wait_for_completion(&rcu.completion);
 267         destroy_rcu_head_on_stack(&rcu.head);
 268 }
 269 EXPORT_SYMBOL_GPL(rcu_barrier_bh);
 270
 271 void rcu_barrier_sched(void)
 272 {
 273         struct rcu_synchronize rcu;
 274
 275         init_rcu_head_on_stack(&rcu.head);
 276         init_completion(&rcu.completion);
 277         /* Will wake me after RCU finished. */
 278         call_rcu_sched(&rcu.head, wakeme_after_rcu);
 279         /* Wait for it. */
 280         wait_for_completion(&rcu.completion);
 281         destroy_rcu_head_on_stack(&rcu.head);
 282 }
 283 EXPORT_SYMBOL_GPL(rcu_barrier_sched);
 284
 285 void __init rcu_init(void)
 286 {
 287         open_softirq(RCU_SOFTIRQ, rcu_process_callbacks);
 288 }