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 #include <linux/prefetch.h>
  39
  40 /* Controls for rcu_kthread() kthread, replacing RCU_SOFTIRQ used previously. */
  41 static struct task_struct *rcu_kthread_task;
  42 static DECLARE_WAIT_QUEUE_HEAD(rcu_kthread_wq);
  43 static unsigned long have_rcu_kthread_work;
  44
  45 /* Forward declarations for rcutiny_plugin.h. */
  46 struct rcu_ctrlblk;
  47 static void invoke_rcu_kthread(void);
  48 static void rcu_process_callbacks(struct rcu_ctrlblk *rcp);
  49 static int rcu_kthread(void *arg);
  50 static 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
  58 static 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  */
  65 void 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  */
  75 void 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  */
  87 static 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  */
 102 static 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  */
 113 void 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  */
 127 void 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  */
 141 void 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  */
 157 static 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  */
 199 static 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  */
 237 void synchronize_sched(void)
 238 {
 239         cond_resched();
 240 }
 241 EXPORT_SYMBOL_GPL(synchronize_sched);
 242
 243 /*
 244  * Helper function for call_rcu() and call_rcu_bh().
 245  */
 246 static 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  */
 268 void call_rcu_sched(struct rcu_head *head, void (*func)(struct rcu_head *rcu))
 269 {
 270         __call_rcu(head, func, &rcu_sched_ctrlblk);
 271 }
 272 EXPORT_SYMBOL_GPL(call_rcu_sched);
 273
 274 /*
 275  * Post an RCU bottom-half callback to be invoked after any subsequent
 276  * quiescent state.
 277  */
 278 void call_rcu_bh(struct rcu_head *head, void (*func)(struct rcu_head *rcu))
 279 {
 280         __call_rcu(head, func, &rcu_bh_ctrlblk);
 281 }
 282 EXPORT_SYMBOL_GPL(call_rcu_bh);
 283
 284 void 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 }
 296 EXPORT_SYMBOL_GPL(rcu_barrier_bh);
 297
 298 void 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 }
 310 EXPORT_SYMBOL_GPL(rcu_barrier_sched);
 311
 312 /*
 313  * Spawn the kthread that invokes RCU callbacks.
 314  */
 315 static 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 }
 324 early_initcall(rcu_spawn_kthreads);