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
  26 #include <linux/types.h>
  27 #include <linux/kernel.h>
  28 #include <linux/init.h>
  29 #include <linux/rcupdate.h>
  30 #include <linux/interrupt.h>
  31 #include <linux/sched.h>
  32 #include <linux/module.h>
  33 #include <linux/completion.h>
  34 #include <linux/moduleparam.h>
  35 #include <linux/notifier.h>
  36 #include <linux/cpu.h>
  37 #include <linux/mutex.h>
  38 #include <linux/time.h>
  39
  40 /* Global control variables for rcupdate callback mechanism. */
  41 struct rcu_ctrlblk {
  42         struct rcu_head *rcucblist;     /* List of pending callbacks (CBs). */
  43         struct rcu_head **donetail;     /* ->next pointer of last "done" CB. */
  44         struct rcu_head **curtail;      /* ->next pointer of last CB. */
  45 };
  46
  47 /* Definition for rcupdate control block. */
  48 static struct rcu_ctrlblk rcu_ctrlblk = {
  49         .rcucblist = NULL,
  50         .donetail = &rcu_ctrlblk.rcucblist,
  51         .curtail = &rcu_ctrlblk.rcucblist,
  52 };
  53 static struct rcu_ctrlblk rcu_bh_ctrlblk = {
  54         .rcucblist = NULL,
  55         .donetail = &rcu_bh_ctrlblk.rcucblist,
  56         .curtail = &rcu_bh_ctrlblk.rcucblist,
  57 };
  58
  59 #ifdef CONFIG_NO_HZ
  60
  61 static long rcu_dynticks_nesting = 1;
  62
  63 /*
  64  * Enter dynticks-idle mode, which is an extended quiescent state
  65  * if we have fully entered that mode (i.e., if the new value of
  66  * dynticks_nesting is zero).
  67  */
  68 void rcu_enter_nohz(void)
  69 {
  70         if (--rcu_dynticks_nesting == 0)
  71                 rcu_sched_qs(0); /* implies rcu_bh_qsctr_inc(0) */
  72 }
  73
  74 /*
  75  * Exit dynticks-idle mode, so that we are no longer in an extended
  76  * quiescent state.
  77  */
  78 void rcu_exit_nohz(void)
  79 {
  80         rcu_dynticks_nesting++;
  81 }
  82
  83 #endif /* #ifdef CONFIG_NO_HZ */
  84
  85 /*
  86  * Helper function for rcu_qsctr_inc() and rcu_bh_qsctr_inc().
  87  * Also disable irqs to avoid confusion due to interrupt handlers invoking
  88  * call_rcu().
  89  */
  90 static int rcu_qsctr_help(struct rcu_ctrlblk *rcp)
  91 {
  92         unsigned long flags;
  93
  94         local_irq_save(flags);
  95         if (rcp->rcucblist != NULL &&
  96             rcp->donetail != rcp->curtail) {
  97                 rcp->donetail = rcp->curtail;
  98                 local_irq_restore(flags);
  99                 return 1;
 100         }
 101         local_irq_restore(flags);
 102         return 0;
 103 }
 104
 105 /*
 106  * Record an rcu quiescent state.  And an rcu_bh quiescent state while we
 107  * are at it, given that any rcu quiescent state is also an rcu_bh
 108  * quiescent state.  Use "+" instead of "||" to defeat short circuiting.
 109  */
 110 void rcu_sched_qs(int cpu)
 111 {
 112         if (rcu_qsctr_help(&rcu_ctrlblk) + rcu_qsctr_help(&rcu_bh_ctrlblk))
 113                 raise_softirq(RCU_SOFTIRQ);
 114 }
 115
 116 /*
 117  * Record an rcu_bh quiescent state.
 118  */
 119 void rcu_bh_qs(int cpu)
 120 {
 121         if (rcu_qsctr_help(&rcu_bh_ctrlblk))
 122                 raise_softirq(RCU_SOFTIRQ);
 123 }
 124
 125 /*
 126  * Check to see if the scheduling-clock interrupt came from an extended
 127  * quiescent state, and, if so, tell RCU about it.
 128  */
 129 void rcu_check_callbacks(int cpu, int user)
 130 {
 131         if (user ||
 132             (idle_cpu(cpu) &&
 133              !in_softirq() &&
 134              hardirq_count() <= (1 << HARDIRQ_SHIFT)))
 135                 rcu_sched_qs(cpu);
 136         else if (!in_softirq())
 137                 rcu_bh_qs(cpu);
 138 }
 139
 140 /*
 141  * Helper function for rcu_process_callbacks() that operates on the
 142  * specified rcu_ctrlkblk structure.
 143  */
 144 static void __rcu_process_callbacks(struct rcu_ctrlblk *rcp)
 145 {
 146         unsigned long flags;
 147         struct rcu_head *next, *list;
 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         rcp->donetail = &rcp->rcucblist;
 161         local_irq_restore(flags);
 162
 163         /* Invoke the callbacks on the local list. */
 164         while (list) {
 165                 next = list->next;
 166                 prefetch(next);
 167                 list->func(list);
 168                 list = next;
 169         }
 170 }
 171
 172 /*
 173  * Invoke any callbacks whose grace period has completed.
 174  */
 175 static void rcu_process_callbacks(struct softirq_action *unused)
 176 {
 177         __rcu_process_callbacks(&rcu_ctrlblk);
 178         __rcu_process_callbacks(&rcu_bh_ctrlblk);
 179 }
 180
 181 /*
 182  * Null function to handle CPU being onlined.  Longer term, we want to
 183  * make TINY_RCU avoid using rcupdate.c, but later...
 184  */
 185 int rcu_cpu_notify(struct notifier_block *self,
 186                    unsigned long action, void *hcpu)
 187 {
 188         return NOTIFY_OK;
 189 }
 190
 191 /*
 192  * Wait for a grace period to elapse.  But it is illegal to invoke
 193  * synchronize_sched() from within an RCU read-side critical section.
 194  * Therefore, any legal call to synchronize_sched() is a quiescent
 195  * state, and so on a UP system, synchronize_sched() need do nothing.
 196  * Ditto for synchronize_rcu_bh().  (But Lai Jiangshan points out the
 197  * benefits of doing might_sleep() to reduce latency.)
 198  *
 199  * Cool, huh?  (Due to Josh Triplett.)
 200  *
 201  * But we want to make this a static inline later.
 202  */
 203 void synchronize_sched(void)
 204 {
 205         cond_resched();
 206 }
 207 EXPORT_SYMBOL_GPL(synchronize_sched);
 208
 209 void synchronize_rcu_bh(void)
 210 {
 211         synchronize_sched();
 212 }
 213 EXPORT_SYMBOL_GPL(synchronize_rcu_bh);
 214
 215 /*
 216  * Helper function for call_rcu() and call_rcu_bh().
 217  */
 218 static void __call_rcu(struct rcu_head *head,
 219                        void (*func)(struct rcu_head *rcu),
 220                        struct rcu_ctrlblk *rcp)
 221 {
 222         unsigned long flags;
 223
 224         head->func = func;
 225         head->next = NULL;
 226         local_irq_save(flags);
 227         *rcp->curtail = head;
 228         rcp->curtail = &head->next;
 229         local_irq_restore(flags);
 230 }
 231
 232 /*
 233  * Post an RCU callback to be invoked after the end of an RCU grace
 234  * period.  But since we have but one CPU, that would be after any
 235  * quiescent state.
 236  */
 237 void call_rcu(struct rcu_head *head,
 238               void (*func)(struct rcu_head *rcu))
 239 {
 240         __call_rcu(head, func, &rcu_ctrlblk);
 241 }
 242 EXPORT_SYMBOL_GPL(call_rcu);
 243
 244 /*
 245  * Post an RCU bottom-half callback to be invoked after any subsequent
 246  * quiescent state.
 247  */
 248 void call_rcu_bh(struct rcu_head *head,
 249                  void (*func)(struct rcu_head *rcu))
 250 {
 251         __call_rcu(head, func, &rcu_bh_ctrlblk);
 252 }
 253 EXPORT_SYMBOL_GPL(call_rcu_bh);
 254
 255 void rcu_barrier(void)
 256 {
 257         struct rcu_synchronize rcu;
 258
 259         init_completion(&rcu.completion);
 260         /* Will wake me after RCU finished. */
 261         call_rcu(&rcu.head, wakeme_after_rcu);
 262         /* Wait for it. */
 263         wait_for_completion(&rcu.completion);
 264 }
 265 EXPORT_SYMBOL_GPL(rcu_barrier);
 266
 267 void rcu_barrier_bh(void)
 268 {
 269         struct rcu_synchronize rcu;
 270
 271         init_completion(&rcu.completion);
 272         /* Will wake me after RCU finished. */
 273         call_rcu_bh(&rcu.head, wakeme_after_rcu);
 274         /* Wait for it. */
 275         wait_for_completion(&rcu.completion);
 276 }
 277 EXPORT_SYMBOL_GPL(rcu_barrier_bh);
 278
 279 void rcu_barrier_sched(void)
 280 {
 281         struct rcu_synchronize rcu;
 282
 283         init_completion(&rcu.completion);
 284         /* Will wake me after RCU finished. */
 285         call_rcu_sched(&rcu.head, wakeme_after_rcu);
 286         /* Wait for it. */
 287         wait_for_completion(&rcu.completion);
 288 }
 289 EXPORT_SYMBOL_GPL(rcu_barrier_sched);
 290
 291 void __rcu_init(void)
 292 {
 293         open_softirq(RCU_SOFTIRQ, rcu_process_callbacks);
 294 }