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