include/linux/closure.h

   1 /* SPDX-License-Identifier: GPL-2.0 */
   2 #ifndef _LINUX_CLOSURE_H
   3 #define _LINUX_CLOSURE_H
   4
   5 #include <linux/llist.h>
   6 #include <linux/sched.h>
   7 #include <linux/sched/task_stack.h>
   8 #include <linux/workqueue.h>
   9
  10 /*
  11  * Closure is perhaps the most overused and abused term in computer science, but
  12  * since I've been unable to come up with anything better you're stuck with it
  13  * again.
  14  *
  15  * What are closures?
  16  *
  17  * They embed a refcount. The basic idea is they count "things that are in
  18  * progress" - in flight bios, some other thread that's doing something else -
  19  * anything you might want to wait on.
  20  *
  21  * The refcount may be manipulated with closure_get() and closure_put().
  22  * closure_put() is where many of the interesting things happen, when it causes
  23  * the refcount to go to 0.
  24  *
  25  * Closures can be used to wait on things both synchronously and asynchronously,
  26  * and synchronous and asynchronous use can be mixed without restriction. To
  27  * wait synchronously, use closure_sync() - you will sleep until your closure's
  28  * refcount hits 1.
  29  *
  30  * To wait asynchronously, use
  31  *   continue_at(cl, next_function, workqueue);
  32  *
  33  * passing it, as you might expect, the function to run when nothing is pending
  34  * and the workqueue to run that function out of.
  35  *
  36  * continue_at() also, critically, requires a 'return' immediately following the
  37  * location where this macro is referenced, to return to the calling function.
  38  * There's good reason for this.
  39  *
  40  * To use safely closures asynchronously, they must always have a refcount while
  41  * they are running owned by the thread that is running them. Otherwise, suppose
  42  * you submit some bios and wish to have a function run when they all complete:
  43  *
  44  * foo_endio(struct bio *bio)
  45  * {
  46  *      closure_put(cl);
  47  * }
  48  *
  49  * closure_init(cl);
  50  *
  51  * do_stuff();
  52  * closure_get(cl);
  53  * bio1->bi_endio = foo_endio;
  54  * bio_submit(bio1);
  55  *
  56  * do_more_stuff();
  57  * closure_get(cl);
  58  * bio2->bi_endio = foo_endio;
  59  * bio_submit(bio2);
  60  *
  61  * continue_at(cl, complete_some_read, system_wq);
  62  *
  63  * If closure's refcount started at 0, complete_some_read() could run before the
  64  * second bio was submitted - which is almost always not what you want! More
  65  * importantly, it wouldn't be possible to say whether the original thread or
  66  * complete_some_read()'s thread owned the closure - and whatever state it was
  67  * associated with!
  68  *
  69  * So, closure_init() initializes a closure's refcount to 1 - and when a
  70  * closure_fn is run, the refcount will be reset to 1 first.
  71  *
  72  * Then, the rule is - if you got the refcount with closure_get(), release it
  73  * with closure_put() (i.e, in a bio->bi_endio function). If you have a refcount
  74  * on a closure because you called closure_init() or you were run out of a
  75  * closure - _always_ use continue_at(). Doing so consistently will help
  76  * eliminate an entire class of particularly pernicious races.
  77  *
  78  * Lastly, you might have a wait list dedicated to a specific event, and have no
  79  * need for specifying the condition - you just want to wait until someone runs
  80  * closure_wake_up() on the appropriate wait list. In that case, just use
  81  * closure_wait(). It will return either true or false, depending on whether the
  82  * closure was already on a wait list or not - a closure can only be on one wait
  83  * list at a time.
  84  *
  85  * Parents:
  86  *
  87  * closure_init() takes two arguments - it takes the closure to initialize, and
  88  * a (possibly null) parent.
  89  *
  90  * If parent is non null, the new closure will have a refcount for its lifetime;
  91  * a closure is considered to be "finished" when its refcount hits 0 and the
  92  * function to run is null. Hence
  93  *
  94  * continue_at(cl, NULL, NULL);
  95  *
  96  * returns up the (spaghetti) stack of closures, precisely like normal return
  97  * returns up the C stack. continue_at() with non null fn is better thought of
  98  * as doing a tail call.
  99  *
 100  * All this implies that a closure should typically be embedded in a particular
 101  * struct (which its refcount will normally control the lifetime of), and that
 102  * struct can very much be thought of as a stack frame.
 103  */
 104
 105 struct closure;
 106 struct closure_syncer;
 107 typedef void (closure_fn) (struct work_struct *);
 108 extern struct dentry *bcache_debug;
 109
 110 struct closure_waitlist {
 111         struct llist_head       list;
 112 };
 113
 114 enum closure_state {
 115         /*
 116          * CLOSURE_WAITING: Set iff the closure is on a waitlist. Must be set by
 117          * the thread that owns the closure, and cleared by the thread that's
 118          * waking up the closure.
 119          *
 120          * The rest are for debugging and don't affect behaviour:
 121          *
 122          * CLOSURE_RUNNING: Set when a closure is running (i.e. by
 123          * closure_init() and when closure_put() runs then next function), and
 124          * must be cleared before remaining hits 0. Primarily to help guard
 125          * against incorrect usage and accidentally transferring references.
 126          * continue_at() and closure_return() clear it for you, if you're doing
 127          * something unusual you can use closure_set_dead() which also helps
 128          * annotate where references are being transferred.
 129          */
 130
 131         CLOSURE_BITS_START      = (1U << 26),
 132         CLOSURE_DESTRUCTOR      = (1U << 26),
 133         CLOSURE_WAITING         = (1U << 28),
 134         CLOSURE_RUNNING         = (1U << 30),
 135 };
 136
 137 #define CLOSURE_GUARD_MASK                                      \
 138         ((CLOSURE_DESTRUCTOR|CLOSURE_WAITING|CLOSURE_RUNNING) << 1)
 139
 140 #define CLOSURE_REMAINING_MASK          (CLOSURE_BITS_START - 1)
 141 #define CLOSURE_REMAINING_INITIALIZER   (1|CLOSURE_RUNNING)
 142
 143 struct closure {
 144         union {
 145                 struct {
 146                         struct workqueue_struct *wq;
 147                         struct closure_syncer   *s;
 148                         struct llist_node       list;
 149                         closure_fn              *fn;
 150                 };
 151                 struct work_struct      work;
 152         };
 153
 154         struct closure          *parent;
 155
 156         atomic_t                remaining;
 157         bool                    closure_get_happened;
 158
 159 #ifdef CONFIG_DEBUG_CLOSURES
 160 #define CLOSURE_MAGIC_DEAD      0xc054dead
 161 #define CLOSURE_MAGIC_ALIVE     0xc054a11e
 162 #define CLOSURE_MAGIC_STACK     0xc05451cc
 163
 164         unsigned int            magic;
 165         struct list_head        all;
 166         unsigned long           ip;
 167         unsigned long           waiting_on;
 168 #endif
 169 };
 170
 171 void closure_sub(struct closure *cl, int v);
 172 void closure_put(struct closure *cl);
 173 void __closure_wake_up(struct closure_waitlist *list);
 174 bool closure_wait(struct closure_waitlist *list, struct closure *cl);
 175 void __closure_sync(struct closure *cl);
 176
 177 static inline unsigned closure_nr_remaining(struct closure *cl)
 178 {
 179         return atomic_read(&cl->remaining) & CLOSURE_REMAINING_MASK;
 180 }
 181
 182 /**
 183  * closure_sync - sleep until a closure a closure has nothing left to wait on
 184  *
 185  * Sleeps until the refcount hits 1 - the thread that's running the closure owns
 186  * the last refcount.
 187  */
 188 static inline void closure_sync(struct closure *cl)
 189 {
 190 #ifdef CONFIG_DEBUG_CLOSURES
 191         BUG_ON(closure_nr_remaining(cl) != 1 && !cl->closure_get_happened);
 192 #endif
 193
 194         if (cl->closure_get_happened)
 195                 __closure_sync(cl);
 196 }
 197
 198 int __closure_sync_timeout(struct closure *cl, unsigned long timeout);
 199
 200 static inline int closure_sync_timeout(struct closure *cl, unsigned long timeout)
 201 {
 202 #ifdef CONFIG_DEBUG_CLOSURES
 203         BUG_ON(closure_nr_remaining(cl) != 1 && !cl->closure_get_happened);
 204 #endif
 205         return cl->closure_get_happened
 206                 ? __closure_sync_timeout(cl, timeout)
 207                 : 0;
 208 }
 209
 210 #ifdef CONFIG_DEBUG_CLOSURES
 211
 212 void closure_debug_create(struct closure *cl);
 213 void closure_debug_destroy(struct closure *cl);
 214
 215 #else
 216
 217 static inline void closure_debug_create(struct closure *cl) {}
 218 static inline void closure_debug_destroy(struct closure *cl) {}
 219
 220 #endif
 221
 222 static inline void closure_set_ip(struct closure *cl)
 223 {
 224 #ifdef CONFIG_DEBUG_CLOSURES
 225         cl->ip = _THIS_IP_;
 226 #endif
 227 }
 228
 229 static inline void closure_set_ret_ip(struct closure *cl)
 230 {
 231 #ifdef CONFIG_DEBUG_CLOSURES
 232         cl->ip = _RET_IP_;
 233 #endif
 234 }
 235
 236 static inline void closure_set_waiting(struct closure *cl, unsigned long f)
 237 {
 238 #ifdef CONFIG_DEBUG_CLOSURES
 239         cl->waiting_on = f;
 240 #endif
 241 }
 242
 243 static inline void closure_set_stopped(struct closure *cl)
 244 {
 245         atomic_sub(CLOSURE_RUNNING, &cl->remaining);
 246 }
 247
 248 static inline void set_closure_fn(struct closure *cl, closure_fn *fn,
 249                                   struct workqueue_struct *wq)
 250 {
 251         closure_set_ip(cl);
 252         cl->fn = fn;
 253         cl->wq = wq;
 254 }
 255
 256 static inline void closure_queue(struct closure *cl)
 257 {
 258         struct workqueue_struct *wq = cl->wq;
 259         /**
 260          * Changes made to closure, work_struct, or a couple of other structs
 261          * may cause work.func not pointing to the right location.
 262          */
 263         BUILD_BUG_ON(offsetof(struct closure, fn)
 264                      != offsetof(struct work_struct, func));
 265
 266         if (wq) {
 267                 INIT_WORK(&cl->work, cl->work.func);
 268                 BUG_ON(!queue_work(wq, &cl->work));
 269         } else
 270                 cl->fn(&cl->work);
 271 }
 272
 273 /**
 274  * closure_get - increment a closure's refcount
 275  */
 276 static inline void closure_get(struct closure *cl)
 277 {
 278         cl->closure_get_happened = true;
 279
 280 #ifdef CONFIG_DEBUG_CLOSURES
 281         BUG_ON((atomic_inc_return(&cl->remaining) &
 282                 CLOSURE_REMAINING_MASK) <= 1);
 283 #else
 284         atomic_inc(&cl->remaining);
 285 #endif
 286 }
 287
 288 /**
 289  * closure_get_not_zero
 290  */
 291 static inline bool closure_get_not_zero(struct closure *cl)
 292 {
 293         unsigned old = atomic_read(&cl->remaining);
 294         do {
 295                 if (!(old & CLOSURE_REMAINING_MASK))
 296                         return false;
 297
 298         } while (!atomic_try_cmpxchg_acquire(&cl->remaining, &old, old + 1));
 299
 300         return true;
 301 }
 302
 303 /**
 304  * closure_init - Initialize a closure, setting the refcount to 1
 305  * @cl:         closure to initialize
 306  * @parent:     parent of the new closure. cl will take a refcount on it for its
 307  *              lifetime; may be NULL.
 308  */
 309 static inline void closure_init(struct closure *cl, struct closure *parent)
 310 {
 311         cl->fn = NULL;
 312         cl->parent = parent;
 313         if (parent)
 314                 closure_get(parent);
 315
 316         atomic_set(&cl->remaining, CLOSURE_REMAINING_INITIALIZER);
 317         cl->closure_get_happened = false;
 318
 319         closure_debug_create(cl);
 320         closure_set_ip(cl);
 321 }
 322
 323 static inline void closure_init_stack(struct closure *cl)
 324 {
 325         memset(cl, 0, sizeof(struct closure));
 326         atomic_set(&cl->remaining, CLOSURE_REMAINING_INITIALIZER);
 327 #ifdef CONFIG_DEBUG_CLOSURES
 328         cl->magic = CLOSURE_MAGIC_STACK;
 329 #endif
 330 }
 331
 332 static inline void closure_init_stack_release(struct closure *cl)
 333 {
 334         memset(cl, 0, sizeof(struct closure));
 335         atomic_set_release(&cl->remaining, CLOSURE_REMAINING_INITIALIZER);
 336 #ifdef CONFIG_DEBUG_CLOSURES
 337         cl->magic = CLOSURE_MAGIC_STACK;
 338 #endif
 339 }
 340
 341 /**
 342  * closure_wake_up - wake up all closures on a wait list,
 343  *                   with memory barrier
 344  */
 345 static inline void closure_wake_up(struct closure_waitlist *list)
 346 {
 347         /* Memory barrier for the wait list */
 348         smp_mb();
 349         __closure_wake_up(list);
 350 }
 351
 352 #define CLOSURE_CALLBACK(name)  void name(struct work_struct *ws)
 353 #define closure_type(name, type, member)                                \
 354         struct closure *cl = container_of(ws, struct closure, work);    \
 355         type *name = container_of(cl, type, member)
 356
 357 /**
 358  * continue_at - jump to another function with barrier
 359  *
 360  * After @cl is no longer waiting on anything (i.e. all outstanding refs have
 361  * been dropped with closure_put()), it will resume execution at @fn running out
 362  * of @wq (or, if @wq is NULL, @fn will be called by closure_put() directly).
 363  *
 364  * This is because after calling continue_at() you no longer have a ref on @cl,
 365  * and whatever @cl owns may be freed out from under you - a running closure fn
 366  * has a ref on its own closure which continue_at() drops.
 367  *
 368  * Note you are expected to immediately return after using this macro.
 369  */
 370 #define continue_at(_cl, _fn, _wq)                                      \
 371 do {                                                                    \
 372         set_closure_fn(_cl, _fn, _wq);                                  \
 373         closure_sub(_cl, CLOSURE_RUNNING + 1);                          \
 374 } while (0)
 375
 376 /**
 377  * closure_return - finish execution of a closure
 378  *
 379  * This is used to indicate that @cl is finished: when all outstanding refs on
 380  * @cl have been dropped @cl's ref on its parent closure (as passed to
 381  * closure_init()) will be dropped, if one was specified - thus this can be
 382  * thought of as returning to the parent closure.
 383  */
 384 #define closure_return(_cl)     continue_at((_cl), NULL, NULL)
 385
 386 void closure_return_sync(struct closure *cl);
 387
 388 /**
 389  * continue_at_nobarrier - jump to another function without barrier
 390  *
 391  * Causes @fn to be executed out of @cl, in @wq context (or called directly if
 392  * @wq is NULL).
 393  *
 394  * The ref the caller of continue_at_nobarrier() had on @cl is now owned by @fn,
 395  * thus it's not safe to touch anything protected by @cl after a
 396  * continue_at_nobarrier().
 397  */
 398 #define continue_at_nobarrier(_cl, _fn, _wq)                            \
 399 do {                                                                    \
 400         set_closure_fn(_cl, _fn, _wq);                                  \
 401         closure_queue(_cl);                                             \
 402 } while (0)
 403
 404 /**
 405  * closure_return_with_destructor - finish execution of a closure,
 406  *                                  with destructor
 407  *
 408  * Works like closure_return(), except @destructor will be called when all
 409  * outstanding refs on @cl have been dropped; @destructor may be used to safely
 410  * free the memory occupied by @cl, and it is called with the ref on the parent
 411  * closure still held - so @destructor could safely return an item to a
 412  * freelist protected by @cl's parent.
 413  */
 414 #define closure_return_with_destructor(_cl, _destructor)                \
 415 do {                                                                    \
 416         set_closure_fn(_cl, _destructor, NULL);                         \
 417         closure_sub(_cl, CLOSURE_RUNNING - CLOSURE_DESTRUCTOR + 1);     \
 418 } while (0)
 419
 420 /**
 421  * closure_call - execute @fn out of a new, uninitialized closure
 422  *
 423  * Typically used when running out of one closure, and we want to run @fn
 424  * asynchronously out of a new closure - @parent will then wait for @cl to
 425  * finish.
 426  */
 427 static inline void closure_call(struct closure *cl, closure_fn fn,
 428                                 struct workqueue_struct *wq,
 429                                 struct closure *parent)
 430 {
 431         closure_init(cl, parent);
 432         continue_at_nobarrier(cl, fn, wq);
 433 }
 434
 435 #define __closure_wait_event(waitlist, _cond)                           \
 436 do {                                                                    \
 437         struct closure cl;                                              \
 438                                                                         \
 439         closure_init_stack(&cl);                                        \
 440                                                                         \
 441         while (1) {                                                     \
 442                 closure_wait(waitlist, &cl);                            \
 443                 if (_cond)                                              \
 444                         break;                                          \
 445                 closure_sync(&cl);                                      \
 446         }                                                               \
 447         closure_wake_up(waitlist);                                      \
 448         closure_sync(&cl);                                              \
 449 } while (0)
 450
 451 #define closure_wait_event(waitlist, _cond)                             \
 452 do {                                                                    \
 453         if (!(_cond))                                                   \
 454                 __closure_wait_event(waitlist, _cond);                  \
 455 } while (0)
 456
 457 #define __closure_wait_event_timeout(waitlist, _cond, _until)           \
 458 ({                                                                      \
 459         struct closure cl;                                              \
 460         long _t;                                                        \
 461                                                                         \
 462         closure_init_stack(&cl);                                        \
 463                                                                         \
 464         while (1) {                                                     \
 465                 closure_wait(waitlist, &cl);                            \
 466                 if (_cond) {                                            \
 467                         _t = max_t(long, 1L, _until - jiffies);         \
 468                         break;                                          \
 469                 }                                                       \
 470                 _t = max_t(long, 0L, _until - jiffies);                 \
 471                 if (!_t)                                                \
 472                         break;                                          \
 473                 closure_sync_timeout(&cl, _t);                          \
 474         }                                                               \
 475         closure_wake_up(waitlist);                                      \
 476         closure_sync(&cl);                                              \
 477         _t;                                                             \
 478 })
 479
 480 /*
 481  * Returns 0 if timeout expired, remaining time in jiffies (at least 1) if
 482  * condition became true
 483  */
 484 #define closure_wait_event_timeout(waitlist, _cond, _timeout)           \
 485 ({                                                                      \
 486         unsigned long _until = jiffies + _timeout;                      \
 487         (_cond)                                                         \
 488                 ? max_t(long, 1L, _until - jiffies)                     \
 489                 : __closure_wait_event_timeout(waitlist, _cond, _until);\
 490 })
 491
 492 #endif /* _LINUX_CLOSURE_H */