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 closure *); 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; 157 158 #ifdef CONFIG_DEBUG_CLOSURES 159 #define CLOSURE_MAGIC_DEAD 0xc054dead 160 #define CLOSURE_MAGIC_ALIVE 0xc054a11e 161 162 unsigned int magic; 163 struct list_head all; 164 unsigned long ip; 165 unsigned long waiting_on; 166 #endif 167 }; 168 169 void closure_sub(struct closure *cl, int v); 170 void closure_put(struct closure *cl); 171 void __closure_wake_up(struct closure_waitlist *list); 172 bool closure_wait(struct closure_waitlist *list, struct closure *cl); 173 void __closure_sync(struct closure *cl); 174 175 static inline unsigned closure_nr_remaining(struct closure *cl) 176 { 177 return atomic_read(&cl->remaining) & CLOSURE_REMAINING_MASK; 178 } 179 180 /** 181 * closure_sync - sleep until a closure a closure has nothing left to wait on 182 * 183 * Sleeps until the refcount hits 1 - the thread that's running the closure owns 184 * the last refcount. 185 */ 186 static inline void closure_sync(struct closure *cl) 187 { 188 if (closure_nr_remaining(cl) != 1) 189 __closure_sync(cl); 190 } 191 192 #ifdef CONFIG_DEBUG_CLOSURES 193 194 void closure_debug_create(struct closure *cl); 195 void closure_debug_destroy(struct closure *cl); 196 197 #else 198 199 static inline void closure_debug_create(struct closure *cl) {} 200 static inline void closure_debug_destroy(struct closure *cl) {} 201 202 #endif 203 204 static inline void closure_set_ip(struct closure *cl) 205 { 206 #ifdef CONFIG_DEBUG_CLOSURES 207 cl->ip = _THIS_IP_; 208 #endif 209 } 210 211 static inline void closure_set_ret_ip(struct closure *cl) 212 { 213 #ifdef CONFIG_DEBUG_CLOSURES 214 cl->ip = _RET_IP_; 215 #endif 216 } 217 218 static inline void closure_set_waiting(struct closure *cl, unsigned long f) 219 { 220 #ifdef CONFIG_DEBUG_CLOSURES 221 cl->waiting_on = f; 222 #endif 223 } 224 225 static inline void closure_set_stopped(struct closure *cl) 226 { 227 atomic_sub(CLOSURE_RUNNING, &cl->remaining); 228 } 229 230 static inline void set_closure_fn(struct closure *cl, closure_fn *fn, 231 struct workqueue_struct *wq) 232 { 233 closure_set_ip(cl); 234 cl->fn = fn; 235 cl->wq = wq; 236 /* between atomic_dec() in closure_put() */ 237 smp_mb__before_atomic(); 238 } 239 240 static inline void closure_queue(struct closure *cl) 241 { 242 struct workqueue_struct *wq = cl->wq; 243 /** 244 * Changes made to closure, work_struct, or a couple of other structs 245 * may cause work.func not pointing to the right location. 246 */ 247 BUILD_BUG_ON(offsetof(struct closure, fn) 248 != offsetof(struct work_struct, func)); 249 250 if (wq) { 251 INIT_WORK(&cl->work, cl->work.func); 252 BUG_ON(!queue_work(wq, &cl->work)); 253 } else 254 cl->fn(cl); 255 } 256 257 /** 258 * closure_get - increment a closure's refcount 259 */ 260 static inline void closure_get(struct closure *cl) 261 { 262 #ifdef CONFIG_DEBUG_CLOSURES 263 BUG_ON((atomic_inc_return(&cl->remaining) & 264 CLOSURE_REMAINING_MASK) <= 1); 265 #else 266 atomic_inc(&cl->remaining); 267 #endif 268 } 269 270 /** 271 * closure_init - Initialize a closure, setting the refcount to 1 272 * @cl: closure to initialize 273 * @parent: parent of the new closure. cl will take a refcount on it for its 274 * lifetime; may be NULL. 275 */ 276 static inline void closure_init(struct closure *cl, struct closure *parent) 277 { 278 cl->fn = NULL; 279 cl->parent = parent; 280 if (parent) 281 closure_get(parent); 282 283 atomic_set(&cl->remaining, CLOSURE_REMAINING_INITIALIZER); 284 285 closure_debug_create(cl); 286 closure_set_ip(cl); 287 } 288 289 static inline void closure_init_stack(struct closure *cl) 290 { 291 memset(cl, 0, sizeof(struct closure)); 292 atomic_set(&cl->remaining, CLOSURE_REMAINING_INITIALIZER); 293 } 294 295 /** 296 * closure_wake_up - wake up all closures on a wait list, 297 * with memory barrier 298 */ 299 static inline void closure_wake_up(struct closure_waitlist *list) 300 { 301 /* Memory barrier for the wait list */ 302 smp_mb(); 303 __closure_wake_up(list); 304 } 305 306 /** 307 * continue_at - jump to another function with barrier 308 * 309 * After @cl is no longer waiting on anything (i.e. all outstanding refs have 310 * been dropped with closure_put()), it will resume execution at @fn running out 311 * of @wq (or, if @wq is NULL, @fn will be called by closure_put() directly). 312 * 313 * This is because after calling continue_at() you no longer have a ref on @cl, 314 * and whatever @cl owns may be freed out from under you - a running closure fn 315 * has a ref on its own closure which continue_at() drops. 316 * 317 * Note you are expected to immediately return after using this macro. 318 */ 319 #define continue_at(_cl, _fn, _wq) \ 320 do { \ 321 set_closure_fn(_cl, _fn, _wq); \ 322 closure_sub(_cl, CLOSURE_RUNNING + 1); \ 323 } while (0) 324 325 /** 326 * closure_return - finish execution of a closure 327 * 328 * This is used to indicate that @cl is finished: when all outstanding refs on 329 * @cl have been dropped @cl's ref on its parent closure (as passed to 330 * closure_init()) will be dropped, if one was specified - thus this can be 331 * thought of as returning to the parent closure. 332 */ 333 #define closure_return(_cl) continue_at((_cl), NULL, NULL) 334 335 /** 336 * continue_at_nobarrier - jump to another function without barrier 337 * 338 * Causes @fn to be executed out of @cl, in @wq context (or called directly if 339 * @wq is NULL). 340 * 341 * The ref the caller of continue_at_nobarrier() had on @cl is now owned by @fn, 342 * thus it's not safe to touch anything protected by @cl after a 343 * continue_at_nobarrier(). 344 */ 345 #define continue_at_nobarrier(_cl, _fn, _wq) \ 346 do { \ 347 set_closure_fn(_cl, _fn, _wq); \ 348 closure_queue(_cl); \ 349 } while (0) 350 351 /** 352 * closure_return_with_destructor - finish execution of a closure, 353 * with destructor 354 * 355 * Works like closure_return(), except @destructor will be called when all 356 * outstanding refs on @cl have been dropped; @destructor may be used to safely 357 * free the memory occupied by @cl, and it is called with the ref on the parent 358 * closure still held - so @destructor could safely return an item to a 359 * freelist protected by @cl's parent. 360 */ 361 #define closure_return_with_destructor(_cl, _destructor) \ 362 do { \ 363 set_closure_fn(_cl, _destructor, NULL); \ 364 closure_sub(_cl, CLOSURE_RUNNING - CLOSURE_DESTRUCTOR + 1); \ 365 } while (0) 366 367 /** 368 * closure_call - execute @fn out of a new, uninitialized closure 369 * 370 * Typically used when running out of one closure, and we want to run @fn 371 * asynchronously out of a new closure - @parent will then wait for @cl to 372 * finish. 373 */ 374 static inline void closure_call(struct closure *cl, closure_fn fn, 375 struct workqueue_struct *wq, 376 struct closure *parent) 377 { 378 closure_init(cl, parent); 379 continue_at_nobarrier(cl, fn, wq); 380 } 381 382 #define __closure_wait_event(waitlist, _cond) \ 383 do { \ 384 struct closure cl; \ 385 \ 386 closure_init_stack(&cl); \ 387 \ 388 while (1) { \ 389 closure_wait(waitlist, &cl); \ 390 if (_cond) \ 391 break; \ 392 closure_sync(&cl); \ 393 } \ 394 closure_wake_up(waitlist); \ 395 closure_sync(&cl); \ 396 } while (0) 397 398 #define closure_wait_event(waitlist, _cond) \ 399 do { \ 400 if (!(_cond)) \ 401 __closure_wait_event(waitlist, _cond); \ 402 } while (0) 403 404 #endif /* _LINUX_CLOSURE_H */ 405