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 typedef void (closure_fn) (struct closure *); 107 108 struct closure_waitlist { 109 struct llist_head list; 110 }; 111 112 enum closure_state { 113 /* 114 * CLOSURE_WAITING: Set iff the closure is on a waitlist. Must be set by 115 * the thread that owns the closure, and cleared by the thread that's 116 * waking up the closure. 117 * 118 * CLOSURE_SLEEPING: Must be set before a thread uses a closure to sleep 119 * - indicates that cl->task is valid and closure_put() may wake it up. 120 * Only set or cleared by the thread that owns the closure. 121 * 122 * The rest are for debugging and don't affect behaviour: 123 * 124 * CLOSURE_RUNNING: Set when a closure is running (i.e. by 125 * closure_init() and when closure_put() runs then next function), and 126 * must be cleared before remaining hits 0. Primarily to help guard 127 * against incorrect usage and accidentally transferring references. 128 * continue_at() and closure_return() clear it for you, if you're doing 129 * something unusual you can use closure_set_dead() which also helps 130 * annotate where references are being transferred. 131 * 132 * CLOSURE_STACK: Sanity check - remaining should never hit 0 on a 133 * closure with this flag set 134 */ 135 136 CLOSURE_BITS_START = (1 << 23), 137 CLOSURE_DESTRUCTOR = (1 << 23), 138 CLOSURE_WAITING = (1 << 25), 139 CLOSURE_SLEEPING = (1 << 27), 140 CLOSURE_RUNNING = (1 << 29), 141 CLOSURE_STACK = (1 << 31), 142 }; 143 144 #define CLOSURE_GUARD_MASK \ 145 ((CLOSURE_DESTRUCTOR|CLOSURE_WAITING|CLOSURE_SLEEPING| \ 146 CLOSURE_RUNNING|CLOSURE_STACK) << 1) 147 148 #define CLOSURE_REMAINING_MASK (CLOSURE_BITS_START - 1) 149 #define CLOSURE_REMAINING_INITIALIZER (1|CLOSURE_RUNNING) 150 151 struct closure { 152 union { 153 struct { 154 struct workqueue_struct *wq; 155 struct task_struct *task; 156 struct llist_node list; 157 closure_fn *fn; 158 }; 159 struct work_struct work; 160 }; 161 162 struct closure *parent; 163 164 atomic_t remaining; 165 166 #ifdef CONFIG_BCACHE_CLOSURES_DEBUG 167 #define CLOSURE_MAGIC_DEAD 0xc054dead 168 #define CLOSURE_MAGIC_ALIVE 0xc054a11e 169 170 unsigned magic; 171 struct list_head all; 172 unsigned long ip; 173 unsigned long waiting_on; 174 #endif 175 }; 176 177 void closure_sub(struct closure *cl, int v); 178 void closure_put(struct closure *cl); 179 void __closure_wake_up(struct closure_waitlist *list); 180 bool closure_wait(struct closure_waitlist *list, struct closure *cl); 181 void closure_sync(struct closure *cl); 182 183 #ifdef CONFIG_BCACHE_CLOSURES_DEBUG 184 185 void closure_debug_init(void); 186 void closure_debug_create(struct closure *cl); 187 void closure_debug_destroy(struct closure *cl); 188 189 #else 190 191 static inline void closure_debug_init(void) {} 192 static inline void closure_debug_create(struct closure *cl) {} 193 static inline void closure_debug_destroy(struct closure *cl) {} 194 195 #endif 196 197 static inline void closure_set_ip(struct closure *cl) 198 { 199 #ifdef CONFIG_BCACHE_CLOSURES_DEBUG 200 cl->ip = _THIS_IP_; 201 #endif 202 } 203 204 static inline void closure_set_ret_ip(struct closure *cl) 205 { 206 #ifdef CONFIG_BCACHE_CLOSURES_DEBUG 207 cl->ip = _RET_IP_; 208 #endif 209 } 210 211 static inline void closure_set_waiting(struct closure *cl, unsigned long f) 212 { 213 #ifdef CONFIG_BCACHE_CLOSURES_DEBUG 214 cl->waiting_on = f; 215 #endif 216 } 217 218 static inline void __closure_end_sleep(struct closure *cl) 219 { 220 __set_current_state(TASK_RUNNING); 221 222 if (atomic_read(&cl->remaining) & CLOSURE_SLEEPING) 223 atomic_sub(CLOSURE_SLEEPING, &cl->remaining); 224 } 225 226 static inline void __closure_start_sleep(struct closure *cl) 227 { 228 closure_set_ip(cl); 229 cl->task = current; 230 set_current_state(TASK_UNINTERRUPTIBLE); 231 232 if (!(atomic_read(&cl->remaining) & CLOSURE_SLEEPING)) 233 atomic_add(CLOSURE_SLEEPING, &cl->remaining); 234 } 235 236 static inline void closure_set_stopped(struct closure *cl) 237 { 238 atomic_sub(CLOSURE_RUNNING, &cl->remaining); 239 } 240 241 static inline void set_closure_fn(struct closure *cl, closure_fn *fn, 242 struct workqueue_struct *wq) 243 { 244 BUG_ON(object_is_on_stack(cl)); 245 closure_set_ip(cl); 246 cl->fn = fn; 247 cl->wq = wq; 248 /* between atomic_dec() in closure_put() */ 249 smp_mb__before_atomic(); 250 } 251 252 static inline void closure_queue(struct closure *cl) 253 { 254 struct workqueue_struct *wq = cl->wq; 255 if (wq) { 256 INIT_WORK(&cl->work, cl->work.func); 257 BUG_ON(!queue_work(wq, &cl->work)); 258 } else 259 cl->fn(cl); 260 } 261 262 /** 263 * closure_get - increment a closure's refcount 264 */ 265 static inline void closure_get(struct closure *cl) 266 { 267 #ifdef CONFIG_BCACHE_CLOSURES_DEBUG 268 BUG_ON((atomic_inc_return(&cl->remaining) & 269 CLOSURE_REMAINING_MASK) <= 1); 270 #else 271 atomic_inc(&cl->remaining); 272 #endif 273 } 274 275 /** 276 * closure_init - Initialize a closure, setting the refcount to 1 277 * @cl: closure to initialize 278 * @parent: parent of the new closure. cl will take a refcount on it for its 279 * lifetime; may be NULL. 280 */ 281 static inline void closure_init(struct closure *cl, struct closure *parent) 282 { 283 memset(cl, 0, sizeof(struct closure)); 284 cl->parent = parent; 285 if (parent) 286 closure_get(parent); 287 288 atomic_set(&cl->remaining, CLOSURE_REMAINING_INITIALIZER); 289 290 closure_debug_create(cl); 291 closure_set_ip(cl); 292 } 293 294 static inline void closure_init_stack(struct closure *cl) 295 { 296 memset(cl, 0, sizeof(struct closure)); 297 atomic_set(&cl->remaining, CLOSURE_REMAINING_INITIALIZER|CLOSURE_STACK); 298 } 299 300 /** 301 * closure_wake_up - wake up all closures on a wait list. 302 */ 303 static inline void closure_wake_up(struct closure_waitlist *list) 304 { 305 smp_mb(); 306 __closure_wake_up(list); 307 } 308 309 /** 310 * continue_at - jump to another function with barrier 311 * 312 * After @cl is no longer waiting on anything (i.e. all outstanding refs have 313 * been dropped with closure_put()), it will resume execution at @fn running out 314 * of @wq (or, if @wq is NULL, @fn will be called by closure_put() directly). 315 * 316 * This is because after calling continue_at() you no longer have a ref on @cl, 317 * and whatever @cl owns may be freed out from under you - a running closure fn 318 * has a ref on its own closure which continue_at() drops. 319 */ 320 #define continue_at(_cl, _fn, _wq) \ 321 do { \ 322 set_closure_fn(_cl, _fn, _wq); \ 323 closure_sub(_cl, CLOSURE_RUNNING + 1); \ 324 } while (0) 325 326 /** 327 * closure_return - finish execution of a closure 328 * 329 * This is used to indicate that @cl is finished: when all outstanding refs on 330 * @cl have been dropped @cl's ref on its parent closure (as passed to 331 * closure_init()) will be dropped, if one was specified - thus this can be 332 * thought of as returning to the parent closure. 333 */ 334 #define closure_return(_cl) continue_at((_cl), NULL, NULL) 335 336 /** 337 * continue_at_nobarrier - jump to another function without barrier 338 * 339 * Causes @fn to be executed out of @cl, in @wq context (or called directly if 340 * @wq is NULL). 341 * 342 * The ref the caller of continue_at_nobarrier() had on @cl is now owned by @fn, 343 * thus it's not safe to touch anything protected by @cl after a 344 * continue_at_nobarrier(). 345 */ 346 #define continue_at_nobarrier(_cl, _fn, _wq) \ 347 do { \ 348 set_closure_fn(_cl, _fn, _wq); \ 349 closure_queue(_cl); \ 350 } while (0) 351 352 /** 353 * closure_return - finish execution of a closure, 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 #endif /* _LINUX_CLOSURE_H */ 383