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 #endif /* _LINUX_CLOSURE_H */
458
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