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