linux/kernel/rcutiny_plugin.h
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   1/*
   2 * Read-Copy Update mechanism for mutual exclusion, the Bloatwatch edition
   3 * Internal non-public definitions that provide either classic
   4 * or preemptible semantics.
   5 *
   6 * This program is free software; you can redistribute it and/or modify
   7 * it under the terms of the GNU General Public License as published by
   8 * the Free Software Foundation; either version 2 of the License, or
   9 * (at your option) any later version.
  10 *
  11 * This program is distributed in the hope that it will be useful,
  12 * but WITHOUT ANY WARRANTY; without even the implied warranty of
  13 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
  14 * GNU General Public License for more details.
  15 *
  16 * You should have received a copy of the GNU General Public License
  17 * along with this program; if not, write to the Free Software
  18 * Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA 02111-1307, USA.
  19 *
  20 * Copyright (c) 2010 Linaro
  21 *
  22 * Author: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
  23 */
  24
  25#include <linux/kthread.h>
  26#include <linux/module.h>
  27#include <linux/debugfs.h>
  28#include <linux/seq_file.h>
  29
  30/* Global control variables for rcupdate callback mechanism. */
  31struct rcu_ctrlblk {
  32        struct rcu_head *rcucblist;     /* List of pending callbacks (CBs). */
  33        struct rcu_head **donetail;     /* ->next pointer of last "done" CB. */
  34        struct rcu_head **curtail;      /* ->next pointer of last CB. */
  35        RCU_TRACE(long qlen);           /* Number of pending CBs. */
  36        RCU_TRACE(char *name);          /* Name of RCU type. */
  37};
  38
  39/* Definition for rcupdate control block. */
  40static struct rcu_ctrlblk rcu_sched_ctrlblk = {
  41        .donetail       = &rcu_sched_ctrlblk.rcucblist,
  42        .curtail        = &rcu_sched_ctrlblk.rcucblist,
  43        RCU_TRACE(.name = "rcu_sched")
  44};
  45
  46static struct rcu_ctrlblk rcu_bh_ctrlblk = {
  47        .donetail       = &rcu_bh_ctrlblk.rcucblist,
  48        .curtail        = &rcu_bh_ctrlblk.rcucblist,
  49        RCU_TRACE(.name = "rcu_bh")
  50};
  51
  52#ifdef CONFIG_DEBUG_LOCK_ALLOC
  53int rcu_scheduler_active __read_mostly;
  54EXPORT_SYMBOL_GPL(rcu_scheduler_active);
  55#endif /* #ifdef CONFIG_DEBUG_LOCK_ALLOC */
  56
  57#ifdef CONFIG_TINY_PREEMPT_RCU
  58
  59#include <linux/delay.h>
  60
  61/* Global control variables for preemptible RCU. */
  62struct rcu_preempt_ctrlblk {
  63        struct rcu_ctrlblk rcb; /* curtail: ->next ptr of last CB for GP. */
  64        struct rcu_head **nexttail;
  65                                /* Tasks blocked in a preemptible RCU */
  66                                /*  read-side critical section while an */
  67                                /*  preemptible-RCU grace period is in */
  68                                /*  progress must wait for a later grace */
  69                                /*  period.  This pointer points to the */
  70                                /*  ->next pointer of the last task that */
  71                                /*  must wait for a later grace period, or */
  72                                /*  to &->rcb.rcucblist if there is no */
  73                                /*  such task. */
  74        struct list_head blkd_tasks;
  75                                /* Tasks blocked in RCU read-side critical */
  76                                /*  section.  Tasks are placed at the head */
  77                                /*  of this list and age towards the tail. */
  78        struct list_head *gp_tasks;
  79                                /* Pointer to the first task blocking the */
  80                                /*  current grace period, or NULL if there */
  81                                /*  is no such task. */
  82        struct list_head *exp_tasks;
  83                                /* Pointer to first task blocking the */
  84                                /*  current expedited grace period, or NULL */
  85                                /*  if there is no such task.  If there */
  86                                /*  is no current expedited grace period, */
  87                                /*  then there cannot be any such task. */
  88#ifdef CONFIG_RCU_BOOST
  89        struct list_head *boost_tasks;
  90                                /* Pointer to first task that needs to be */
  91                                /*  priority-boosted, or NULL if no priority */
  92                                /*  boosting is needed.  If there is no */
  93                                /*  current or expedited grace period, there */
  94                                /*  can be no such task. */
  95#endif /* #ifdef CONFIG_RCU_BOOST */
  96        u8 gpnum;               /* Current grace period. */
  97        u8 gpcpu;               /* Last grace period blocked by the CPU. */
  98        u8 completed;           /* Last grace period completed. */
  99                                /*  If all three are equal, RCU is idle. */
 100#ifdef CONFIG_RCU_BOOST
 101        unsigned long boost_time; /* When to start boosting (jiffies) */
 102#endif /* #ifdef CONFIG_RCU_BOOST */
 103#ifdef CONFIG_RCU_TRACE
 104        unsigned long n_grace_periods;
 105#ifdef CONFIG_RCU_BOOST
 106        unsigned long n_tasks_boosted;
 107                                /* Total number of tasks boosted. */
 108        unsigned long n_exp_boosts;
 109                                /* Number of tasks boosted for expedited GP. */
 110        unsigned long n_normal_boosts;
 111                                /* Number of tasks boosted for normal GP. */
 112        unsigned long n_balk_blkd_tasks;
 113                                /* Refused to boost: no blocked tasks. */
 114        unsigned long n_balk_exp_gp_tasks;
 115                                /* Refused to boost: nothing blocking GP. */
 116        unsigned long n_balk_boost_tasks;
 117                                /* Refused to boost: already boosting. */
 118        unsigned long n_balk_notyet;
 119                                /* Refused to boost: not yet time. */
 120        unsigned long n_balk_nos;
 121                                /* Refused to boost: not sure why, though. */
 122                                /*  This can happen due to race conditions. */
 123#endif /* #ifdef CONFIG_RCU_BOOST */
 124#endif /* #ifdef CONFIG_RCU_TRACE */
 125};
 126
 127static struct rcu_preempt_ctrlblk rcu_preempt_ctrlblk = {
 128        .rcb.donetail = &rcu_preempt_ctrlblk.rcb.rcucblist,
 129        .rcb.curtail = &rcu_preempt_ctrlblk.rcb.rcucblist,
 130        .nexttail = &rcu_preempt_ctrlblk.rcb.rcucblist,
 131        .blkd_tasks = LIST_HEAD_INIT(rcu_preempt_ctrlblk.blkd_tasks),
 132        RCU_TRACE(.rcb.name = "rcu_preempt")
 133};
 134
 135static int rcu_preempted_readers_exp(void);
 136static void rcu_report_exp_done(void);
 137
 138/*
 139 * Return true if the CPU has not yet responded to the current grace period.
 140 */
 141static int rcu_cpu_blocking_cur_gp(void)
 142{
 143        return rcu_preempt_ctrlblk.gpcpu != rcu_preempt_ctrlblk.gpnum;
 144}
 145
 146/*
 147 * Check for a running RCU reader.  Because there is only one CPU,
 148 * there can be but one running RCU reader at a time.  ;-)
 149 *
 150 * Returns zero if there are no running readers.  Returns a positive
 151 * number if there is at least one reader within its RCU read-side
 152 * critical section.  Returns a negative number if an outermost reader
 153 * is in the midst of exiting from its RCU read-side critical section
 154 *
 155 * Returns zero if there are no running readers.  Returns a positive
 156 * number if there is at least one reader within its RCU read-side
 157 * critical section.  Returns a negative number if an outermost reader
 158 * is in the midst of exiting from its RCU read-side critical section.
 159 */
 160static int rcu_preempt_running_reader(void)
 161{
 162        return current->rcu_read_lock_nesting;
 163}
 164
 165/*
 166 * Check for preempted RCU readers blocking any grace period.
 167 * If the caller needs a reliable answer, it must disable hard irqs.
 168 */
 169static int rcu_preempt_blocked_readers_any(void)
 170{
 171        return !list_empty(&rcu_preempt_ctrlblk.blkd_tasks);
 172}
 173
 174/*
 175 * Check for preempted RCU readers blocking the current grace period.
 176 * If the caller needs a reliable answer, it must disable hard irqs.
 177 */
 178static int rcu_preempt_blocked_readers_cgp(void)
 179{
 180        return rcu_preempt_ctrlblk.gp_tasks != NULL;
 181}
 182
 183/*
 184 * Return true if another preemptible-RCU grace period is needed.
 185 */
 186static int rcu_preempt_needs_another_gp(void)
 187{
 188        return *rcu_preempt_ctrlblk.rcb.curtail != NULL;
 189}
 190
 191/*
 192 * Return true if a preemptible-RCU grace period is in progress.
 193 * The caller must disable hardirqs.
 194 */
 195static int rcu_preempt_gp_in_progress(void)
 196{
 197        return rcu_preempt_ctrlblk.completed != rcu_preempt_ctrlblk.gpnum;
 198}
 199
 200/*
 201 * Advance a ->blkd_tasks-list pointer to the next entry, instead
 202 * returning NULL if at the end of the list.
 203 */
 204static struct list_head *rcu_next_node_entry(struct task_struct *t)
 205{
 206        struct list_head *np;
 207
 208        np = t->rcu_node_entry.next;
 209        if (np == &rcu_preempt_ctrlblk.blkd_tasks)
 210                np = NULL;
 211        return np;
 212}
 213
 214#ifdef CONFIG_RCU_TRACE
 215
 216#ifdef CONFIG_RCU_BOOST
 217static void rcu_initiate_boost_trace(void);
 218#endif /* #ifdef CONFIG_RCU_BOOST */
 219
 220/*
 221 * Dump additional statistice for TINY_PREEMPT_RCU.
 222 */
 223static void show_tiny_preempt_stats(struct seq_file *m)
 224{
 225        seq_printf(m, "rcu_preempt: qlen=%ld gp=%lu g%u/p%u/c%u tasks=%c%c%c\n",
 226                   rcu_preempt_ctrlblk.rcb.qlen,
 227                   rcu_preempt_ctrlblk.n_grace_periods,
 228                   rcu_preempt_ctrlblk.gpnum,
 229                   rcu_preempt_ctrlblk.gpcpu,
 230                   rcu_preempt_ctrlblk.completed,
 231                   "T."[list_empty(&rcu_preempt_ctrlblk.blkd_tasks)],
 232                   "N."[!rcu_preempt_ctrlblk.gp_tasks],
 233                   "E."[!rcu_preempt_ctrlblk.exp_tasks]);
 234#ifdef CONFIG_RCU_BOOST
 235        seq_printf(m, "%sttb=%c ntb=%lu neb=%lu nnb=%lu j=%04x bt=%04x\n",
 236                   "             ",
 237                   "B."[!rcu_preempt_ctrlblk.boost_tasks],
 238                   rcu_preempt_ctrlblk.n_tasks_boosted,
 239                   rcu_preempt_ctrlblk.n_exp_boosts,
 240                   rcu_preempt_ctrlblk.n_normal_boosts,
 241                   (int)(jiffies & 0xffff),
 242                   (int)(rcu_preempt_ctrlblk.boost_time & 0xffff));
 243        seq_printf(m, "%s: nt=%lu egt=%lu bt=%lu ny=%lu nos=%lu\n",
 244                   "             balk",
 245                   rcu_preempt_ctrlblk.n_balk_blkd_tasks,
 246                   rcu_preempt_ctrlblk.n_balk_exp_gp_tasks,
 247                   rcu_preempt_ctrlblk.n_balk_boost_tasks,
 248                   rcu_preempt_ctrlblk.n_balk_notyet,
 249                   rcu_preempt_ctrlblk.n_balk_nos);
 250#endif /* #ifdef CONFIG_RCU_BOOST */
 251}
 252
 253#endif /* #ifdef CONFIG_RCU_TRACE */
 254
 255#ifdef CONFIG_RCU_BOOST
 256
 257#include "rtmutex_common.h"
 258
 259#define RCU_BOOST_PRIO CONFIG_RCU_BOOST_PRIO
 260
 261/* Controls for rcu_kthread() kthread. */
 262static struct task_struct *rcu_kthread_task;
 263static DECLARE_WAIT_QUEUE_HEAD(rcu_kthread_wq);
 264static unsigned long have_rcu_kthread_work;
 265
 266/*
 267 * Carry out RCU priority boosting on the task indicated by ->boost_tasks,
 268 * and advance ->boost_tasks to the next task in the ->blkd_tasks list.
 269 */
 270static int rcu_boost(void)
 271{
 272        unsigned long flags;
 273        struct rt_mutex mtx;
 274        struct task_struct *t;
 275        struct list_head *tb;
 276
 277        if (rcu_preempt_ctrlblk.boost_tasks == NULL &&
 278            rcu_preempt_ctrlblk.exp_tasks == NULL)
 279                return 0;  /* Nothing to boost. */
 280
 281        local_irq_save(flags);
 282
 283        /*
 284         * Recheck with irqs disabled: all tasks in need of boosting
 285         * might exit their RCU read-side critical sections on their own
 286         * if we are preempted just before disabling irqs.
 287         */
 288        if (rcu_preempt_ctrlblk.boost_tasks == NULL &&
 289            rcu_preempt_ctrlblk.exp_tasks == NULL) {
 290                local_irq_restore(flags);
 291                return 0;
 292        }
 293
 294        /*
 295         * Preferentially boost tasks blocking expedited grace periods.
 296         * This cannot starve the normal grace periods because a second
 297         * expedited grace period must boost all blocked tasks, including
 298         * those blocking the pre-existing normal grace period.
 299         */
 300        if (rcu_preempt_ctrlblk.exp_tasks != NULL) {
 301                tb = rcu_preempt_ctrlblk.exp_tasks;
 302                RCU_TRACE(rcu_preempt_ctrlblk.n_exp_boosts++);
 303        } else {
 304                tb = rcu_preempt_ctrlblk.boost_tasks;
 305                RCU_TRACE(rcu_preempt_ctrlblk.n_normal_boosts++);
 306        }
 307        RCU_TRACE(rcu_preempt_ctrlblk.n_tasks_boosted++);
 308
 309        /*
 310         * We boost task t by manufacturing an rt_mutex that appears to
 311         * be held by task t.  We leave a pointer to that rt_mutex where
 312         * task t can find it, and task t will release the mutex when it
 313         * exits its outermost RCU read-side critical section.  Then
 314         * simply acquiring this artificial rt_mutex will boost task
 315         * t's priority.  (Thanks to tglx for suggesting this approach!)
 316         */
 317        t = container_of(tb, struct task_struct, rcu_node_entry);
 318        rt_mutex_init_proxy_locked(&mtx, t);
 319        t->rcu_boost_mutex = &mtx;
 320        local_irq_restore(flags);
 321        rt_mutex_lock(&mtx);
 322        rt_mutex_unlock(&mtx);  /* Keep lockdep happy. */
 323
 324        return ACCESS_ONCE(rcu_preempt_ctrlblk.boost_tasks) != NULL ||
 325               ACCESS_ONCE(rcu_preempt_ctrlblk.exp_tasks) != NULL;
 326}
 327
 328/*
 329 * Check to see if it is now time to start boosting RCU readers blocking
 330 * the current grace period, and, if so, tell the rcu_kthread_task to
 331 * start boosting them.  If there is an expedited boost in progress,
 332 * we wait for it to complete.
 333 *
 334 * If there are no blocked readers blocking the current grace period,
 335 * return 0 to let the caller know, otherwise return 1.  Note that this
 336 * return value is independent of whether or not boosting was done.
 337 */
 338static int rcu_initiate_boost(void)
 339{
 340        if (!rcu_preempt_blocked_readers_cgp() &&
 341            rcu_preempt_ctrlblk.exp_tasks == NULL) {
 342                RCU_TRACE(rcu_preempt_ctrlblk.n_balk_exp_gp_tasks++);
 343                return 0;
 344        }
 345        if (rcu_preempt_ctrlblk.exp_tasks != NULL ||
 346            (rcu_preempt_ctrlblk.gp_tasks != NULL &&
 347             rcu_preempt_ctrlblk.boost_tasks == NULL &&
 348             ULONG_CMP_GE(jiffies, rcu_preempt_ctrlblk.boost_time))) {
 349                if (rcu_preempt_ctrlblk.exp_tasks == NULL)
 350                        rcu_preempt_ctrlblk.boost_tasks =
 351                                rcu_preempt_ctrlblk.gp_tasks;
 352                invoke_rcu_callbacks();
 353        } else {
 354                RCU_TRACE(rcu_initiate_boost_trace());
 355        }
 356        return 1;
 357}
 358
 359#define RCU_BOOST_DELAY_JIFFIES DIV_ROUND_UP(CONFIG_RCU_BOOST_DELAY * HZ, 1000)
 360
 361/*
 362 * Do priority-boost accounting for the start of a new grace period.
 363 */
 364static void rcu_preempt_boost_start_gp(void)
 365{
 366        rcu_preempt_ctrlblk.boost_time = jiffies + RCU_BOOST_DELAY_JIFFIES;
 367}
 368
 369#else /* #ifdef CONFIG_RCU_BOOST */
 370
 371/*
 372 * If there is no RCU priority boosting, we don't initiate boosting,
 373 * but we do indicate whether there are blocked readers blocking the
 374 * current grace period.
 375 */
 376static int rcu_initiate_boost(void)
 377{
 378        return rcu_preempt_blocked_readers_cgp();
 379}
 380
 381/*
 382 * If there is no RCU priority boosting, nothing to do at grace-period start.
 383 */
 384static void rcu_preempt_boost_start_gp(void)
 385{
 386}
 387
 388#endif /* else #ifdef CONFIG_RCU_BOOST */
 389
 390/*
 391 * Record a preemptible-RCU quiescent state for the specified CPU.  Note
 392 * that this just means that the task currently running on the CPU is
 393 * in a quiescent state.  There might be any number of tasks blocked
 394 * while in an RCU read-side critical section.
 395 *
 396 * Unlike the other rcu_*_qs() functions, callers to this function
 397 * must disable irqs in order to protect the assignment to
 398 * ->rcu_read_unlock_special.
 399 *
 400 * Because this is a single-CPU implementation, the only way a grace
 401 * period can end is if the CPU is in a quiescent state.  The reason is
 402 * that a blocked preemptible-RCU reader can exit its critical section
 403 * only if the CPU is running it at the time.  Therefore, when the
 404 * last task blocking the current grace period exits its RCU read-side
 405 * critical section, neither the CPU nor blocked tasks will be stopping
 406 * the current grace period.  (In contrast, SMP implementations
 407 * might have CPUs running in RCU read-side critical sections that
 408 * block later grace periods -- but this is not possible given only
 409 * one CPU.)
 410 */
 411static void rcu_preempt_cpu_qs(void)
 412{
 413        /* Record both CPU and task as having responded to current GP. */
 414        rcu_preempt_ctrlblk.gpcpu = rcu_preempt_ctrlblk.gpnum;
 415        current->rcu_read_unlock_special &= ~RCU_READ_UNLOCK_NEED_QS;
 416
 417        /* If there is no GP then there is nothing more to do.  */
 418        if (!rcu_preempt_gp_in_progress())
 419                return;
 420        /*
 421         * Check up on boosting.  If there are readers blocking the
 422         * current grace period, leave.
 423         */
 424        if (rcu_initiate_boost())
 425                return;
 426
 427        /* Advance callbacks. */
 428        rcu_preempt_ctrlblk.completed = rcu_preempt_ctrlblk.gpnum;
 429        rcu_preempt_ctrlblk.rcb.donetail = rcu_preempt_ctrlblk.rcb.curtail;
 430        rcu_preempt_ctrlblk.rcb.curtail = rcu_preempt_ctrlblk.nexttail;
 431
 432        /* If there are no blocked readers, next GP is done instantly. */
 433        if (!rcu_preempt_blocked_readers_any())
 434                rcu_preempt_ctrlblk.rcb.donetail = rcu_preempt_ctrlblk.nexttail;
 435
 436        /* If there are done callbacks, cause them to be invoked. */
 437        if (*rcu_preempt_ctrlblk.rcb.donetail != NULL)
 438                invoke_rcu_callbacks();
 439}
 440
 441/*
 442 * Start a new RCU grace period if warranted.  Hard irqs must be disabled.
 443 */
 444static void rcu_preempt_start_gp(void)
 445{
 446        if (!rcu_preempt_gp_in_progress() && rcu_preempt_needs_another_gp()) {
 447
 448                /* Official start of GP. */
 449                rcu_preempt_ctrlblk.gpnum++;
 450                RCU_TRACE(rcu_preempt_ctrlblk.n_grace_periods++);
 451
 452                /* Any blocked RCU readers block new GP. */
 453                if (rcu_preempt_blocked_readers_any())
 454                        rcu_preempt_ctrlblk.gp_tasks =
 455                                rcu_preempt_ctrlblk.blkd_tasks.next;
 456
 457                /* Set up for RCU priority boosting. */
 458                rcu_preempt_boost_start_gp();
 459
 460                /* If there is no running reader, CPU is done with GP. */
 461                if (!rcu_preempt_running_reader())
 462                        rcu_preempt_cpu_qs();
 463        }
 464}
 465
 466/*
 467 * We have entered the scheduler, and the current task might soon be
 468 * context-switched away from.  If this task is in an RCU read-side
 469 * critical section, we will no longer be able to rely on the CPU to
 470 * record that fact, so we enqueue the task on the blkd_tasks list.
 471 * If the task started after the current grace period began, as recorded
 472 * by ->gpcpu, we enqueue at the beginning of the list.  Otherwise
 473 * before the element referenced by ->gp_tasks (or at the tail if
 474 * ->gp_tasks is NULL) and point ->gp_tasks at the newly added element.
 475 * The task will dequeue itself when it exits the outermost enclosing
 476 * RCU read-side critical section.  Therefore, the current grace period
 477 * cannot be permitted to complete until the ->gp_tasks pointer becomes
 478 * NULL.
 479 *
 480 * Caller must disable preemption.
 481 */
 482void rcu_preempt_note_context_switch(void)
 483{
 484        struct task_struct *t = current;
 485        unsigned long flags;
 486
 487        local_irq_save(flags); /* must exclude scheduler_tick(). */
 488        if (rcu_preempt_running_reader() > 0 &&
 489            (t->rcu_read_unlock_special & RCU_READ_UNLOCK_BLOCKED) == 0) {
 490
 491                /* Possibly blocking in an RCU read-side critical section. */
 492                t->rcu_read_unlock_special |= RCU_READ_UNLOCK_BLOCKED;
 493
 494                /*
 495                 * If this CPU has already checked in, then this task
 496                 * will hold up the next grace period rather than the
 497                 * current grace period.  Queue the task accordingly.
 498                 * If the task is queued for the current grace period
 499                 * (i.e., this CPU has not yet passed through a quiescent
 500                 * state for the current grace period), then as long
 501                 * as that task remains queued, the current grace period
 502                 * cannot end.
 503                 */
 504                list_add(&t->rcu_node_entry, &rcu_preempt_ctrlblk.blkd_tasks);
 505                if (rcu_cpu_blocking_cur_gp())
 506                        rcu_preempt_ctrlblk.gp_tasks = &t->rcu_node_entry;
 507        } else if (rcu_preempt_running_reader() < 0 &&
 508                   t->rcu_read_unlock_special) {
 509                /*
 510                 * Complete exit from RCU read-side critical section on
 511                 * behalf of preempted instance of __rcu_read_unlock().
 512                 */
 513                rcu_read_unlock_special(t);
 514        }
 515
 516        /*
 517         * Either we were not in an RCU read-side critical section to
 518         * begin with, or we have now recorded that critical section
 519         * globally.  Either way, we can now note a quiescent state
 520         * for this CPU.  Again, if we were in an RCU read-side critical
 521         * section, and if that critical section was blocking the current
 522         * grace period, then the fact that the task has been enqueued
 523         * means that current grace period continues to be blocked.
 524         */
 525        rcu_preempt_cpu_qs();
 526        local_irq_restore(flags);
 527}
 528
 529/*
 530 * Handle special cases during rcu_read_unlock(), such as needing to
 531 * notify RCU core processing or task having blocked during the RCU
 532 * read-side critical section.
 533 */
 534void rcu_read_unlock_special(struct task_struct *t)
 535{
 536        int empty;
 537        int empty_exp;
 538        unsigned long flags;
 539        struct list_head *np;
 540#ifdef CONFIG_RCU_BOOST
 541        struct rt_mutex *rbmp = NULL;
 542#endif /* #ifdef CONFIG_RCU_BOOST */
 543        int special;
 544
 545        /*
 546         * NMI handlers cannot block and cannot safely manipulate state.
 547         * They therefore cannot possibly be special, so just leave.
 548         */
 549        if (in_nmi())
 550                return;
 551
 552        local_irq_save(flags);
 553
 554        /*
 555         * If RCU core is waiting for this CPU to exit critical section,
 556         * let it know that we have done so.
 557         */
 558        special = t->rcu_read_unlock_special;
 559        if (special & RCU_READ_UNLOCK_NEED_QS)
 560                rcu_preempt_cpu_qs();
 561
 562        /* Hardware IRQ handlers cannot block. */
 563        if (in_irq() || in_serving_softirq()) {
 564                local_irq_restore(flags);
 565                return;
 566        }
 567
 568        /* Clean up if blocked during RCU read-side critical section. */
 569        if (special & RCU_READ_UNLOCK_BLOCKED) {
 570                t->rcu_read_unlock_special &= ~RCU_READ_UNLOCK_BLOCKED;
 571
 572                /*
 573                 * Remove this task from the ->blkd_tasks list and adjust
 574                 * any pointers that might have been referencing it.
 575                 */
 576                empty = !rcu_preempt_blocked_readers_cgp();
 577                empty_exp = rcu_preempt_ctrlblk.exp_tasks == NULL;
 578                np = rcu_next_node_entry(t);
 579                list_del_init(&t->rcu_node_entry);
 580                if (&t->rcu_node_entry == rcu_preempt_ctrlblk.gp_tasks)
 581                        rcu_preempt_ctrlblk.gp_tasks = np;
 582                if (&t->rcu_node_entry == rcu_preempt_ctrlblk.exp_tasks)
 583                        rcu_preempt_ctrlblk.exp_tasks = np;
 584#ifdef CONFIG_RCU_BOOST
 585                if (&t->rcu_node_entry == rcu_preempt_ctrlblk.boost_tasks)
 586                        rcu_preempt_ctrlblk.boost_tasks = np;
 587#endif /* #ifdef CONFIG_RCU_BOOST */
 588
 589                /*
 590                 * If this was the last task on the current list, and if
 591                 * we aren't waiting on the CPU, report the quiescent state
 592                 * and start a new grace period if needed.
 593                 */
 594                if (!empty && !rcu_preempt_blocked_readers_cgp()) {
 595                        rcu_preempt_cpu_qs();
 596                        rcu_preempt_start_gp();
 597                }
 598
 599                /*
 600                 * If this was the last task on the expedited lists,
 601                 * then we need wake up the waiting task.
 602                 */
 603                if (!empty_exp && rcu_preempt_ctrlblk.exp_tasks == NULL)
 604                        rcu_report_exp_done();
 605        }
 606#ifdef CONFIG_RCU_BOOST
 607        /* Unboost self if was boosted. */
 608        if (t->rcu_boost_mutex != NULL) {
 609                rbmp = t->rcu_boost_mutex;
 610                t->rcu_boost_mutex = NULL;
 611                rt_mutex_unlock(rbmp);
 612        }
 613#endif /* #ifdef CONFIG_RCU_BOOST */
 614        local_irq_restore(flags);
 615}
 616
 617/*
 618 * Check for a quiescent state from the current CPU.  When a task blocks,
 619 * the task is recorded in the rcu_preempt_ctrlblk structure, which is
 620 * checked elsewhere.  This is called from the scheduling-clock interrupt.
 621 *
 622 * Caller must disable hard irqs.
 623 */
 624static void rcu_preempt_check_callbacks(void)
 625{
 626        struct task_struct *t = current;
 627
 628        if (rcu_preempt_gp_in_progress() &&
 629            (!rcu_preempt_running_reader() ||
 630             !rcu_cpu_blocking_cur_gp()))
 631                rcu_preempt_cpu_qs();
 632        if (&rcu_preempt_ctrlblk.rcb.rcucblist !=
 633            rcu_preempt_ctrlblk.rcb.donetail)
 634                invoke_rcu_callbacks();
 635        if (rcu_preempt_gp_in_progress() &&
 636            rcu_cpu_blocking_cur_gp() &&
 637            rcu_preempt_running_reader() > 0)
 638                t->rcu_read_unlock_special |= RCU_READ_UNLOCK_NEED_QS;
 639}
 640
 641/*
 642 * TINY_PREEMPT_RCU has an extra callback-list tail pointer to
 643 * update, so this is invoked from rcu_process_callbacks() to
 644 * handle that case.  Of course, it is invoked for all flavors of
 645 * RCU, but RCU callbacks can appear only on one of the lists, and
 646 * neither ->nexttail nor ->donetail can possibly be NULL, so there
 647 * is no need for an explicit check.
 648 */
 649static void rcu_preempt_remove_callbacks(struct rcu_ctrlblk *rcp)
 650{
 651        if (rcu_preempt_ctrlblk.nexttail == rcp->donetail)
 652                rcu_preempt_ctrlblk.nexttail = &rcp->rcucblist;
 653}
 654
 655/*
 656 * Process callbacks for preemptible RCU.
 657 */
 658static void rcu_preempt_process_callbacks(void)
 659{
 660        __rcu_process_callbacks(&rcu_preempt_ctrlblk.rcb);
 661}
 662
 663/*
 664 * Queue a preemptible -RCU callback for invocation after a grace period.
 665 */
 666void call_rcu(struct rcu_head *head, void (*func)(struct rcu_head *rcu))
 667{
 668        unsigned long flags;
 669
 670        debug_rcu_head_queue(head);
 671        head->func = func;
 672        head->next = NULL;
 673
 674        local_irq_save(flags);
 675        *rcu_preempt_ctrlblk.nexttail = head;
 676        rcu_preempt_ctrlblk.nexttail = &head->next;
 677        RCU_TRACE(rcu_preempt_ctrlblk.rcb.qlen++);
 678        rcu_preempt_start_gp();  /* checks to see if GP needed. */
 679        local_irq_restore(flags);
 680}
 681EXPORT_SYMBOL_GPL(call_rcu);
 682
 683/*
 684 * synchronize_rcu - wait until a grace period has elapsed.
 685 *
 686 * Control will return to the caller some time after a full grace
 687 * period has elapsed, in other words after all currently executing RCU
 688 * read-side critical sections have completed.  RCU read-side critical
 689 * sections are delimited by rcu_read_lock() and rcu_read_unlock(),
 690 * and may be nested.
 691 */
 692void synchronize_rcu(void)
 693{
 694        rcu_lockdep_assert(!lock_is_held(&rcu_bh_lock_map) &&
 695                           !lock_is_held(&rcu_lock_map) &&
 696                           !lock_is_held(&rcu_sched_lock_map),
 697                           "Illegal synchronize_rcu() in RCU read-side critical section");
 698
 699#ifdef CONFIG_DEBUG_LOCK_ALLOC
 700        if (!rcu_scheduler_active)
 701                return;
 702#endif /* #ifdef CONFIG_DEBUG_LOCK_ALLOC */
 703
 704        WARN_ON_ONCE(rcu_preempt_running_reader());
 705        if (!rcu_preempt_blocked_readers_any())
 706                return;
 707
 708        /* Once we get past the fastpath checks, same code as rcu_barrier(). */
 709        rcu_barrier();
 710}
 711EXPORT_SYMBOL_GPL(synchronize_rcu);
 712
 713static DECLARE_WAIT_QUEUE_HEAD(sync_rcu_preempt_exp_wq);
 714static unsigned long sync_rcu_preempt_exp_count;
 715static DEFINE_MUTEX(sync_rcu_preempt_exp_mutex);
 716
 717/*
 718 * Return non-zero if there are any tasks in RCU read-side critical
 719 * sections blocking the current preemptible-RCU expedited grace period.
 720 * If there is no preemptible-RCU expedited grace period currently in
 721 * progress, returns zero unconditionally.
 722 */
 723static int rcu_preempted_readers_exp(void)
 724{
 725        return rcu_preempt_ctrlblk.exp_tasks != NULL;
 726}
 727
 728/*
 729 * Report the exit from RCU read-side critical section for the last task
 730 * that queued itself during or before the current expedited preemptible-RCU
 731 * grace period.
 732 */
 733static void rcu_report_exp_done(void)
 734{
 735        wake_up(&sync_rcu_preempt_exp_wq);
 736}
 737
 738/*
 739 * Wait for an rcu-preempt grace period, but expedite it.  The basic idea
 740 * is to rely in the fact that there is but one CPU, and that it is
 741 * illegal for a task to invoke synchronize_rcu_expedited() while in a
 742 * preemptible-RCU read-side critical section.  Therefore, any such
 743 * critical sections must correspond to blocked tasks, which must therefore
 744 * be on the ->blkd_tasks list.  So just record the current head of the
 745 * list in the ->exp_tasks pointer, and wait for all tasks including and
 746 * after the task pointed to by ->exp_tasks to drain.
 747 */
 748void synchronize_rcu_expedited(void)
 749{
 750        unsigned long flags;
 751        struct rcu_preempt_ctrlblk *rpcp = &rcu_preempt_ctrlblk;
 752        unsigned long snap;
 753
 754        barrier(); /* ensure prior action seen before grace period. */
 755
 756        WARN_ON_ONCE(rcu_preempt_running_reader());
 757
 758        /*
 759         * Acquire lock so that there is only one preemptible RCU grace
 760         * period in flight.  Of course, if someone does the expedited
 761         * grace period for us while we are acquiring the lock, just leave.
 762         */
 763        snap = sync_rcu_preempt_exp_count + 1;
 764        mutex_lock(&sync_rcu_preempt_exp_mutex);
 765        if (ULONG_CMP_LT(snap, sync_rcu_preempt_exp_count))
 766                goto unlock_mb_ret; /* Others did our work for us. */
 767
 768        local_irq_save(flags);
 769
 770        /*
 771         * All RCU readers have to already be on blkd_tasks because
 772         * we cannot legally be executing in an RCU read-side critical
 773         * section.
 774         */
 775
 776        /* Snapshot current head of ->blkd_tasks list. */
 777        rpcp->exp_tasks = rpcp->blkd_tasks.next;
 778        if (rpcp->exp_tasks == &rpcp->blkd_tasks)
 779                rpcp->exp_tasks = NULL;
 780
 781        /* Wait for tail of ->blkd_tasks list to drain. */
 782        if (!rcu_preempted_readers_exp()) {
 783                local_irq_restore(flags);
 784        } else {
 785                rcu_initiate_boost();
 786                local_irq_restore(flags);
 787                wait_event(sync_rcu_preempt_exp_wq,
 788                           !rcu_preempted_readers_exp());
 789        }
 790
 791        /* Clean up and exit. */
 792        barrier(); /* ensure expedited GP seen before counter increment. */
 793        sync_rcu_preempt_exp_count++;
 794unlock_mb_ret:
 795        mutex_unlock(&sync_rcu_preempt_exp_mutex);
 796        barrier(); /* ensure subsequent action seen after grace period. */
 797}
 798EXPORT_SYMBOL_GPL(synchronize_rcu_expedited);
 799
 800/*
 801 * Does preemptible RCU need the CPU to stay out of dynticks mode?
 802 */
 803int rcu_preempt_needs_cpu(void)
 804{
 805        return rcu_preempt_ctrlblk.rcb.rcucblist != NULL;
 806}
 807
 808#else /* #ifdef CONFIG_TINY_PREEMPT_RCU */
 809
 810#ifdef CONFIG_RCU_TRACE
 811
 812/*
 813 * Because preemptible RCU does not exist, it is not necessary to
 814 * dump out its statistics.
 815 */
 816static void show_tiny_preempt_stats(struct seq_file *m)
 817{
 818}
 819
 820#endif /* #ifdef CONFIG_RCU_TRACE */
 821
 822/*
 823 * Because preemptible RCU does not exist, it never has any callbacks
 824 * to check.
 825 */
 826static void rcu_preempt_check_callbacks(void)
 827{
 828}
 829
 830/*
 831 * Because preemptible RCU does not exist, it never has any callbacks
 832 * to remove.
 833 */
 834static void rcu_preempt_remove_callbacks(struct rcu_ctrlblk *rcp)
 835{
 836}
 837
 838/*
 839 * Because preemptible RCU does not exist, it never has any callbacks
 840 * to process.
 841 */
 842static void rcu_preempt_process_callbacks(void)
 843{
 844}
 845
 846#endif /* #else #ifdef CONFIG_TINY_PREEMPT_RCU */
 847
 848#ifdef CONFIG_RCU_BOOST
 849
 850/*
 851 * Wake up rcu_kthread() to process callbacks now eligible for invocation
 852 * or to boost readers.
 853 */
 854static void invoke_rcu_callbacks(void)
 855{
 856        have_rcu_kthread_work = 1;
 857        if (rcu_kthread_task != NULL)
 858                wake_up(&rcu_kthread_wq);
 859}
 860
 861#ifdef CONFIG_RCU_TRACE
 862
 863/*
 864 * Is the current CPU running the RCU-callbacks kthread?
 865 * Caller must have preemption disabled.
 866 */
 867static bool rcu_is_callbacks_kthread(void)
 868{
 869        return rcu_kthread_task == current;
 870}
 871
 872#endif /* #ifdef CONFIG_RCU_TRACE */
 873
 874/*
 875 * This kthread invokes RCU callbacks whose grace periods have
 876 * elapsed.  It is awakened as needed, and takes the place of the
 877 * RCU_SOFTIRQ that is used for this purpose when boosting is disabled.
 878 * This is a kthread, but it is never stopped, at least not until
 879 * the system goes down.
 880 */
 881static int rcu_kthread(void *arg)
 882{
 883        unsigned long work;
 884        unsigned long morework;
 885        unsigned long flags;
 886
 887        for (;;) {
 888                wait_event_interruptible(rcu_kthread_wq,
 889                                         have_rcu_kthread_work != 0);
 890                morework = rcu_boost();
 891                local_irq_save(flags);
 892                work = have_rcu_kthread_work;
 893                have_rcu_kthread_work = morework;
 894                local_irq_restore(flags);
 895                if (work)
 896                        rcu_process_callbacks(NULL);
 897                schedule_timeout_interruptible(1); /* Leave CPU for others. */
 898        }
 899
 900        return 0;  /* Not reached, but needed to shut gcc up. */
 901}
 902
 903/*
 904 * Spawn the kthread that invokes RCU callbacks.
 905 */
 906static int __init rcu_spawn_kthreads(void)
 907{
 908        struct sched_param sp;
 909
 910        rcu_kthread_task = kthread_run(rcu_kthread, NULL, "rcu_kthread");
 911        sp.sched_priority = RCU_BOOST_PRIO;
 912        sched_setscheduler_nocheck(rcu_kthread_task, SCHED_FIFO, &sp);
 913        return 0;
 914}
 915early_initcall(rcu_spawn_kthreads);
 916
 917#else /* #ifdef CONFIG_RCU_BOOST */
 918
 919/* Hold off callback invocation until early_initcall() time. */
 920static int rcu_scheduler_fully_active __read_mostly;
 921
 922/*
 923 * Start up softirq processing of callbacks.
 924 */
 925void invoke_rcu_callbacks(void)
 926{
 927        if (rcu_scheduler_fully_active)
 928                raise_softirq(RCU_SOFTIRQ);
 929}
 930
 931#ifdef CONFIG_RCU_TRACE
 932
 933/*
 934 * There is no callback kthread, so this thread is never it.
 935 */
 936static bool rcu_is_callbacks_kthread(void)
 937{
 938        return false;
 939}
 940
 941#endif /* #ifdef CONFIG_RCU_TRACE */
 942
 943static int __init rcu_scheduler_really_started(void)
 944{
 945        rcu_scheduler_fully_active = 1;
 946        open_softirq(RCU_SOFTIRQ, rcu_process_callbacks);
 947        raise_softirq(RCU_SOFTIRQ);  /* Invoke any callbacks from early boot. */
 948        return 0;
 949}
 950early_initcall(rcu_scheduler_really_started);
 951
 952#endif /* #else #ifdef CONFIG_RCU_BOOST */
 953
 954#ifdef CONFIG_DEBUG_LOCK_ALLOC
 955#include <linux/kernel_stat.h>
 956
 957/*
 958 * During boot, we forgive RCU lockdep issues.  After this function is
 959 * invoked, we start taking RCU lockdep issues seriously.
 960 */
 961void __init rcu_scheduler_starting(void)
 962{
 963        WARN_ON(nr_context_switches() > 0);
 964        rcu_scheduler_active = 1;
 965}
 966
 967#endif /* #ifdef CONFIG_DEBUG_LOCK_ALLOC */
 968
 969#ifdef CONFIG_RCU_TRACE
 970
 971#ifdef CONFIG_RCU_BOOST
 972
 973static void rcu_initiate_boost_trace(void)
 974{
 975        if (list_empty(&rcu_preempt_ctrlblk.blkd_tasks))
 976                rcu_preempt_ctrlblk.n_balk_blkd_tasks++;
 977        else if (rcu_preempt_ctrlblk.gp_tasks == NULL &&
 978                 rcu_preempt_ctrlblk.exp_tasks == NULL)
 979                rcu_preempt_ctrlblk.n_balk_exp_gp_tasks++;
 980        else if (rcu_preempt_ctrlblk.boost_tasks != NULL)
 981                rcu_preempt_ctrlblk.n_balk_boost_tasks++;
 982        else if (!ULONG_CMP_GE(jiffies, rcu_preempt_ctrlblk.boost_time))
 983                rcu_preempt_ctrlblk.n_balk_notyet++;
 984        else
 985                rcu_preempt_ctrlblk.n_balk_nos++;
 986}
 987
 988#endif /* #ifdef CONFIG_RCU_BOOST */
 989
 990static void rcu_trace_sub_qlen(struct rcu_ctrlblk *rcp, int n)
 991{
 992        unsigned long flags;
 993
 994        local_irq_save(flags);
 995        rcp->qlen -= n;
 996        local_irq_restore(flags);
 997}
 998
 999/*
1000 * Dump statistics for TINY_RCU, such as they are.
1001 */
1002static int show_tiny_stats(struct seq_file *m, void *unused)
1003{
1004        show_tiny_preempt_stats(m);
1005        seq_printf(m, "rcu_sched: qlen: %ld\n", rcu_sched_ctrlblk.qlen);
1006        seq_printf(m, "rcu_bh: qlen: %ld\n", rcu_bh_ctrlblk.qlen);
1007        return 0;
1008}
1009
1010static int show_tiny_stats_open(struct inode *inode, struct file *file)
1011{
1012        return single_open(file, show_tiny_stats, NULL);
1013}
1014
1015static const struct file_operations show_tiny_stats_fops = {
1016        .owner = THIS_MODULE,
1017        .open = show_tiny_stats_open,
1018        .read = seq_read,
1019        .llseek = seq_lseek,
1020        .release = single_release,
1021};
1022
1023static struct dentry *rcudir;
1024
1025static int __init rcutiny_trace_init(void)
1026{
1027        struct dentry *retval;
1028
1029        rcudir = debugfs_create_dir("rcu", NULL);
1030        if (!rcudir)
1031                goto free_out;
1032        retval = debugfs_create_file("rcudata", 0444, rcudir,
1033                                     NULL, &show_tiny_stats_fops);
1034        if (!retval)
1035                goto free_out;
1036        return 0;
1037free_out:
1038        debugfs_remove_recursive(rcudir);
1039        return 1;
1040}
1041
1042static void __exit rcutiny_trace_cleanup(void)
1043{
1044        debugfs_remove_recursive(rcudir);
1045}
1046
1047module_init(rcutiny_trace_init);
1048module_exit(rcutiny_trace_cleanup);
1049
1050MODULE_AUTHOR("Paul E. McKenney");
1051MODULE_DESCRIPTION("Read-Copy Update tracing for tiny implementation");
1052MODULE_LICENSE("GPL");
1053
1054#endif /* #ifdef CONFIG_RCU_TRACE */
1055
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