module/zfs/zthr.c

   1 /*
   2  * CDDL HEADER START
   3  *
   4  * This file and its contents are supplied under the terms of the
   5  * Common Development and Distribution License ("CDDL"), version 1.0.
   6  * You may only use this file in accordance with the terms of version
   7  * 1.0 of the CDDL.
   8  *
   9  * A full copy of the text of the CDDL should have accompanied this
  10  * source. A copy of the CDDL is also available via the Internet at
  11  * http://www.illumos.org/license/CDDL.
  12  *
  13  * CDDL HEADER END
  14  */
  15
  16 /*
  17  * Copyright (c) 2017 by Delphix. All rights reserved.
  18  */
  19
  20 /*
  21  * ZTHR Infrastructure
  22  * ===================
  23  *
  24  * ZTHR threads are used for isolated operations that span multiple txgs
  25  * within a SPA. They generally exist from SPA creation/loading and until
  26  * the SPA is exported/destroyed. The ideal requirements for an operation
  27  * to be modeled with a zthr are the following:
  28  *
  29  * 1] The operation needs to run over multiple txgs.
  30  * 2] There is be a single point of reference in memory or on disk that
  31  *    indicates whether the operation should run/is running or is
  32  *    stopped.
  33  *
  34  * If the operation satisfies the above then the following rules guarantee
  35  * a certain level of correctness:
  36  *
  37  * 1] Any thread EXCEPT the zthr changes the work indicator from stopped
  38  *    to running but not the opposite.
  39  * 2] Only the zthr can change the work indicator from running to stopped
  40  *    (e.g. when it is done) but not the opposite.
  41  *
  42  * This way a normal zthr cycle should go like this:
  43  *
  44  * 1] An external thread changes the work indicator from stopped to
  45  *    running and wakes up the zthr.
  46  * 2] The zthr wakes up, checks the indicator and starts working.
  47  * 3] When the zthr is done, it changes the indicator to stopped, allowing
  48  *    a new cycle to start.
  49  *
  50  * == ZTHR creation
  51  *
  52  * Every zthr needs three inputs to start running:
  53  *
  54  * 1] A user-defined checker function (checkfunc) that decides whether
  55  *    the zthr should start working or go to sleep. The function should
  56  *    return TRUE when the zthr needs to work or FALSE to let it sleep,
  57  *    and should adhere to the following signature:
  58  *    boolean_t checkfunc_name(void *args, zthr_t *t);
  59  *
  60  * 2] A user-defined ZTHR function (func) which the zthr executes when
  61  *    it is not sleeping. The function should adhere to the following
  62  *    signature type:
  63  *    int func_name(void *args, zthr_t *t);
  64  *
  65  * 3] A void args pointer that will be passed to checkfunc and func
  66  *    implicitly by the infrastructure.
  67  *
  68  * The reason why the above API needs two different functions,
  69  * instead of one that both checks and does the work, has to do with
  70  * the zthr's internal lock (zthr_lock) and the allowed cancellation
  71  * windows. We want to hold the zthr_lock while running checkfunc
  72  * but not while running func. This way the zthr can be cancelled
  73  * while doing work and not while checking for work.
  74  *
  75  * To start a zthr:
  76  *     zthr_t *zthr_pointer = zthr_create(checkfunc, func, args);
  77  *
  78  * After that you should be able to wakeup, cancel, and resume the
  79  * zthr from another thread using zthr_pointer.
  80  *
  81  * NOTE: ZTHR threads could potentially wake up spuriously and the
  82  * user should take this into account when writing a checkfunc.
  83  * [see ZTHR state transitions]
  84  *
  85  * == ZTHR cancellation
  86  *
  87  * ZTHR threads must be cancelled when their SPA is being exported
  88  * or when they need to be paused so they don't interfere with other
  89  * operations.
  90  *
  91  * To cancel a zthr:
  92  *     zthr_cancel(zthr_pointer);
  93  *
  94  * To resume it:
  95  *     zthr_resume(zthr_pointer);
  96  *
  97  * A zthr will implicitly check if it has received a cancellation
  98  * signal every time func returns and everytime it wakes up [see ZTHR
  99  * state transitions below].
 100  *
 101  * At times, waiting for the zthr's func to finish its job may take
 102  * time. This may be very time-consuming for some operations that
 103  * need to cancel the SPA's zthrs (e.g spa_export). For this scenario
 104  * the user can explicitly make their ZTHR function aware of incoming
 105  * cancellation signals using zthr_iscancelled(). A common pattern for
 106  * that looks like this:
 107  *
 108  * int
 109  * func_name(void *args, zthr_t *t)
 110  * {
 111  *     ... <unpack args> ...
 112  *     while (!work_done && !zthr_iscancelled(t)) {
 113  *         ... <do more work> ...
 114  *     }
 115  *     return (0);
 116  * }
 117  *
 118  * == ZTHR exit
 119  *
 120  * For the rare cases where the zthr wants to stop running voluntarily
 121  * while running its ZTHR function (func), we provide zthr_exit().
 122  * When a zthr has voluntarily stopped running, it can be resumed with
 123  * zthr_resume(), just like it would if it was cancelled by some other
 124  * thread.
 125  *
 126  * == ZTHR cleanup
 127  *
 128  * Cancelling a zthr doesn't clean up its metadata (internal locks,
 129  * function pointers to func and checkfunc, etc..). This is because
 130  * we want to keep them around in case we want to resume the execution
 131  * of the zthr later. Similarly for zthrs that exit themselves.
 132  *
 133  * To completely cleanup a zthr, cancel it first to ensure that it
 134  * is not running and then use zthr_destroy().
 135  *
 136  * == ZTHR state transitions
 137  *
 138  *    zthr creation
 139  *      +
 140  *      |
 141  *      |      woke up
 142  *      |   +--------------+ sleep
 143  *      |   |                  ^
 144  *      |   |                  |
 145  *      |   |                  | FALSE
 146  *      |   |                  |
 147  *      v   v     FALSE        +
 148  *   cancelled? +---------> checkfunc?
 149  *      +   ^                  +
 150  *      |   |                  |
 151  *      |   |                  | TRUE
 152  *      |   |                  |
 153  *      |   |  func returned   v
 154  *      |   +---------------+ func
 155  *      |
 156  *      | TRUE
 157  *      |
 158  *      v
 159  *   zthr stopped running
 160  *
 161  */
 162
 163 #include <sys/zfs_context.h>
 164 #include <sys/zthr.h>
 165
 166 void
 167 zthr_exit(zthr_t *t, int rc)
 168 {
 169         ASSERT3P(t->zthr_thread, ==, curthread);
 170         mutex_enter(&t->zthr_lock);
 171         t->zthr_thread = NULL;
 172         t->zthr_rc = rc;
 173         cv_broadcast(&t->zthr_cv);
 174         mutex_exit(&t->zthr_lock);
 175         thread_exit();
 176 }
 177
 178 static void
 179 zthr_procedure(void *arg)
 180 {
 181         zthr_t *t = arg;
 182         int rc = 0;
 183
 184         mutex_enter(&t->zthr_lock);
 185         while (!t->zthr_cancel) {
 186                 if (t->zthr_checkfunc(t->zthr_arg, t)) {
 187                         mutex_exit(&t->zthr_lock);
 188                         rc = t->zthr_func(t->zthr_arg, t);
 189                         mutex_enter(&t->zthr_lock);
 190                 } else {
 191                         /* go to sleep */
 192                         cv_wait_sig(&t->zthr_cv, &t->zthr_lock);
 193                 }
 194         }
 195         mutex_exit(&t->zthr_lock);
 196
 197         zthr_exit(t, rc);
 198 }
 199
 200 zthr_t *
 201 zthr_create(zthr_checkfunc_t *checkfunc, zthr_func_t *func, void *arg)
 202 {
 203         zthr_t *t = kmem_zalloc(sizeof (*t), KM_SLEEP);
 204         mutex_init(&t->zthr_lock, NULL, MUTEX_DEFAULT, NULL);
 205         cv_init(&t->zthr_cv, NULL, CV_DEFAULT, NULL);
 206
 207         mutex_enter(&t->zthr_lock);
 208         t->zthr_checkfunc = checkfunc;
 209         t->zthr_func = func;
 210         t->zthr_arg = arg;
 211
 212         t->zthr_thread = thread_create(NULL, 0, zthr_procedure, t,
 213             0, &p0, TS_RUN, minclsyspri);
 214         mutex_exit(&t->zthr_lock);
 215
 216         return (t);
 217 }
 218
 219 void
 220 zthr_destroy(zthr_t *t)
 221 {
 222         VERIFY3P(t->zthr_thread, ==, NULL);
 223         mutex_destroy(&t->zthr_lock);
 224         cv_destroy(&t->zthr_cv);
 225         kmem_free(t, sizeof (*t));
 226 }
 227
 228 /*
 229  * Note: If the zthr is not sleeping and misses the wakeup
 230  * (e.g it is running its ZTHR function), it will check if
 231  * there is work to do before going to sleep using its checker
 232  * function [see ZTHR state transition in ZTHR block comment].
 233  * Thus, missing the wakeup still yields the expected behavior.
 234  */
 235 void
 236 zthr_wakeup(zthr_t *t)
 237 {
 238         mutex_enter(&t->zthr_lock);
 239         cv_broadcast(&t->zthr_cv);
 240         mutex_exit(&t->zthr_lock);
 241 }
 242
 243 /*
 244  * Note: If the zthr is not running (e.g. has been cancelled
 245  * already), this is a no-op.
 246  */
 247 int
 248 zthr_cancel(zthr_t *t)
 249 {
 250         int rc = 0;
 251
 252         mutex_enter(&t->zthr_lock);
 253
 254         /* broadcast in case the zthr is sleeping */
 255         cv_broadcast(&t->zthr_cv);
 256
 257         t->zthr_cancel = B_TRUE;
 258         while (t->zthr_thread != NULL)
 259                 cv_wait(&t->zthr_cv, &t->zthr_lock);
 260         t->zthr_cancel = B_FALSE;
 261         rc = t->zthr_rc;
 262         mutex_exit(&t->zthr_lock);
 263
 264         return (rc);
 265 }
 266
 267 void
 268 zthr_resume(zthr_t *t)
 269 {
 270         ASSERT3P(t->zthr_thread, ==, NULL);
 271
 272         mutex_enter(&t->zthr_lock);
 273
 274         ASSERT3P(&t->zthr_checkfunc, !=, NULL);
 275         ASSERT3P(&t->zthr_func, !=, NULL);
 276         ASSERT(!t->zthr_cancel);
 277
 278         t->zthr_thread = thread_create(NULL, 0, zthr_procedure, t,
 279             0, &p0, TS_RUN, minclsyspri);
 280
 281         mutex_exit(&t->zthr_lock);
 282 }
 283
 284 /*
 285  * This function is intended to be used by the zthr itself
 286  * to check if another thread has signal it to stop running.
 287  *
 288  * returns TRUE if we are in the middle of trying to cancel
 289  *     this thread.
 290  *
 291  * returns FALSE otherwise.
 292  */
 293 boolean_t
 294 zthr_iscancelled(zthr_t *t)
 295 {
 296         boolean_t cancelled;
 297
 298         ASSERT3P(t->zthr_thread, ==, curthread);
 299
 300         mutex_enter(&t->zthr_lock);
 301         cancelled = t->zthr_cancel;
 302         mutex_exit(&t->zthr_lock);
 303
 304         return (cancelled);
 305 }
 306
 307 boolean_t
 308 zthr_isrunning(zthr_t *t)
 309 {
 310         boolean_t running;
 311
 312         mutex_enter(&t->zthr_lock);
 313         running = (t->zthr_thread != NULL);
 314         mutex_exit(&t->zthr_lock);
 315
 316         return (running);
 317 }