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_ctrlblk = {
  48         .donetail       = &rcu_ctrlblk.rcucblist,
  49         .curtail        = &rcu_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_NO_HZ
  58
  59 static long rcu_dynticks_nesting = 1;
  60
  61 /*
  62  * Enter dynticks-idle mode, which is an extended quiescent state
  63  * if we have fully entered that mode (i.e., if the new value of
  64  * dynticks_nesting is zero).
  65  */
  66 void rcu_enter_nohz(void)
  67 {
  68         if (--rcu_dynticks_nesting == 0)
  69                 rcu_sched_qs(0); /* implies rcu_bh_qsctr_inc(0) */
  70 }
  71
  72 /*
  73  * Exit dynticks-idle mode, so that we are no longer in an extended
  74  * quiescent state.
  75  */
  76 void rcu_exit_nohz(void)
  77 {
  78         rcu_dynticks_nesting++;
  79 }
  80
  81 #endif /* #ifdef CONFIG_NO_HZ */
  82
  83 /*
  84  * Helper function for rcu_qsctr_inc() and rcu_bh_qsctr_inc().
  85  * Also disable irqs to avoid confusion due to interrupt handlers
  86  * invoking call_rcu().
  87  */
  88 static int rcu_qsctr_help(struct rcu_ctrlblk *rcp)
  89 {
  90         unsigned long flags;
  91
  92         local_irq_save(flags);
  93         if (rcp->rcucblist != NULL &&
  94             rcp->donetail != rcp->curtail) {
  95                 rcp->donetail = rcp->curtail;
  96                 local_irq_restore(flags);
  97                 return 1;
  98         }
  99         local_irq_restore(flags);
 100
 101         return 0;
 102 }
 103
 104 /*
 105  * Record an rcu quiescent state.  And an rcu_bh quiescent state while we
 106  * are at it, given that any rcu quiescent state is also an rcu_bh
 107  * quiescent state.  Use "+" instead of "||" to defeat short circuiting.
 108  */
 109 void rcu_sched_qs(int cpu)
 110 {
 111         if (rcu_qsctr_help(&rcu_ctrlblk) + rcu_qsctr_help(&rcu_bh_ctrlblk))
 112                 raise_softirq(RCU_SOFTIRQ);
 113 }
 114
 115 /*
 116  * Record an rcu_bh quiescent state.
 117  */
 118 void rcu_bh_qs(int cpu)
 119 {
 120         if (rcu_qsctr_help(&rcu_bh_ctrlblk))
 121                 raise_softirq(RCU_SOFTIRQ);
 122 }
 123
 124 /*
 125  * Check to see if the scheduling-clock interrupt came from an extended
 126  * quiescent state, and, if so, tell RCU about it.
 127  */
 128 void rcu_check_callbacks(int cpu, int user)
 129 {
 130         if (user ||
 131             (idle_cpu(cpu) &&
 132              !in_softirq() &&
 133              hardirq_count() <= (1 << HARDIRQ_SHIFT)))
 134                 rcu_sched_qs(cpu);
 135         else if (!in_softirq())
 136                 rcu_bh_qs(cpu);
 137 }
 138
 139 /*
 140  * Helper function for rcu_process_callbacks() that operates on the
 141  * specified rcu_ctrlkblk structure.
 142  */
 143 static void __rcu_process_callbacks(struct rcu_ctrlblk *rcp)
 144 {
 145         struct rcu_head *next, *list;
 146         unsigned long flags;
 147
 148         /* If no RCU callbacks ready to invoke, just return. */
 149         if (&rcp->rcucblist == rcp->donetail)
 150                 return;
 151
 152         /* Move the ready-to-invoke callbacks to a local list. */
 153         local_irq_save(flags);
 154         list = rcp->rcucblist;
 155         rcp->rcucblist = *rcp->donetail;
 156         *rcp->donetail = NULL;
 157         if (rcp->curtail == rcp->donetail)
 158                 rcp->curtail = &rcp->rcucblist;
 159         rcp->donetail = &rcp->rcucblist;
 160         local_irq_restore(flags);
 161
 162         /* Invoke the callbacks on the local list. */
 163         while (list) {
 164                 next = list->next;
 165                 prefetch(next);
 166                 list->func(list);
 167                 list = next;
 168         }
 169 }
 170
 171 /*
 172  * Invoke any callbacks whose grace period has completed.
 173  */
 174 static void rcu_process_callbacks(struct softirq_action *unused)
 175 {
 176         __rcu_process_callbacks(&rcu_ctrlblk);
 177         __rcu_process_callbacks(&rcu_bh_ctrlblk);
 178 }
 179
 180 /*
 181  * Wait for a grace period to elapse.  But it is illegal to invoke
 182  * synchronize_sched() from within an RCU read-side critical section.
 183  * Therefore, any legal call to synchronize_sched() is a quiescent
 184  * state, and so on a UP system, synchronize_sched() need do nothing.
 185  * Ditto for synchronize_rcu_bh().  (But Lai Jiangshan points out the
 186  * benefits of doing might_sleep() to reduce latency.)
 187  *
 188  * Cool, huh?  (Due to Josh Triplett.)
 189  *
 190  * But we want to make this a static inline later.
 191  */
 192 void synchronize_sched(void)
 193 {
 194         cond_resched();
 195 }
 196 EXPORT_SYMBOL_GPL(synchronize_sched);
 197
 198 void synchronize_rcu_bh(void)
 199 {
 200         synchronize_sched();
 201 }
 202 EXPORT_SYMBOL_GPL(synchronize_rcu_bh);
 203
 204 /*
 205  * Helper function for call_rcu() and call_rcu_bh().
 206  */
 207 static void __call_rcu(struct rcu_head *head,
 208                        void (*func)(struct rcu_head *rcu),
 209                        struct rcu_ctrlblk *rcp)
 210 {
 211         unsigned long flags;
 212
 213         head->func = func;
 214         head->next = NULL;
 215
 216         local_irq_save(flags);
 217         *rcp->curtail = head;
 218         rcp->curtail = &head->next;
 219         local_irq_restore(flags);
 220 }
 221
 222 /*
 223  * Post an RCU callback to be invoked after the end of an RCU grace
 224  * period.  But since we have but one CPU, that would be after any
 225  * quiescent state.
 226  */
 227 void call_rcu(struct rcu_head *head, void (*func)(struct rcu_head *rcu))
 228 {
 229         __call_rcu(head, func, &rcu_ctrlblk);
 230 }
 231 EXPORT_SYMBOL_GPL(call_rcu);
 232
 233 /*
 234  * Post an RCU bottom-half callback to be invoked after any subsequent
 235  * quiescent state.
 236  */
 237 void call_rcu_bh(struct rcu_head *head, void (*func)(struct rcu_head *rcu))
 238 {
 239         __call_rcu(head, func, &rcu_bh_ctrlblk);
 240 }
 241 EXPORT_SYMBOL_GPL(call_rcu_bh);
 242
 243 void rcu_barrier(void)
 244 {
 245         struct rcu_synchronize rcu;
 246
 247         init_completion(&rcu.completion);
 248         /* Will wake me after RCU finished. */
 249         call_rcu(&rcu.head, wakeme_after_rcu);
 250         /* Wait for it. */
 251         wait_for_completion(&rcu.completion);
 252 }
 253 EXPORT_SYMBOL_GPL(rcu_barrier);
 254
 255 void rcu_barrier_bh(void)
 256 {
 257         struct rcu_synchronize rcu;
 258
 259         init_completion(&rcu.completion);
 260         /* Will wake me after RCU finished. */
 261         call_rcu_bh(&rcu.head, wakeme_after_rcu);
 262         /* Wait for it. */
 263         wait_for_completion(&rcu.completion);
 264 }
 265 EXPORT_SYMBOL_GPL(rcu_barrier_bh);
 266
 267 void rcu_barrier_sched(void)
 268 {
 269         struct rcu_synchronize rcu;
 270
 271         init_completion(&rcu.completion);
 272         /* Will wake me after RCU finished. */
 273         call_rcu_sched(&rcu.head, wakeme_after_rcu);
 274         /* Wait for it. */
 275         wait_for_completion(&rcu.completion);
 276 }
 277 EXPORT_SYMBOL_GPL(rcu_barrier_sched);
 278
 279 void __init rcu_init(void)
 280 {
 281         open_softirq(RCU_SOFTIRQ, rcu_process_callbacks);
 282 }