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
105struct closure;
106struct closure_syncer;
107typedef void (closure_fn) (struct work_struct *);
108extern struct dentry *bcache_debug;
109
110struct closure_waitlist {
111	struct llist_head	list;
112};
113
114enum 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
143struct 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
163	unsigned int		magic;
164	struct list_head	all;
165	unsigned long		ip;
166	unsigned long		waiting_on;
167#endif
168};
169
170void closure_sub(struct closure *cl, int v);
171void closure_put(struct closure *cl);
172void __closure_wake_up(struct closure_waitlist *list);
173bool closure_wait(struct closure_waitlist *list, struct closure *cl);
174void __closure_sync(struct closure *cl);
175
176static inline unsigned closure_nr_remaining(struct closure *cl)
177{
178	return atomic_read(&cl->remaining) & CLOSURE_REMAINING_MASK;
179}
180
181/**
182 * closure_sync - sleep until a closure a closure has nothing left to wait on
183 *
184 * Sleeps until the refcount hits 1 - the thread that's running the closure owns
185 * the last refcount.
186 */
187static inline void closure_sync(struct closure *cl)
188{
189#ifdef CONFIG_DEBUG_CLOSURES
190	BUG_ON(closure_nr_remaining(cl) != 1 && !cl->closure_get_happened);
191#endif
192
193	if (cl->closure_get_happened)
194		__closure_sync(cl);
195}
196
197#ifdef CONFIG_DEBUG_CLOSURES
198
199void closure_debug_create(struct closure *cl);
200void closure_debug_destroy(struct closure *cl);
201
202#else
203
204static inline void closure_debug_create(struct closure *cl) {}
205static inline void closure_debug_destroy(struct closure *cl) {}
206
207#endif
208
209static inline void closure_set_ip(struct closure *cl)
210{
211#ifdef CONFIG_DEBUG_CLOSURES
212	cl->ip = _THIS_IP_;
213#endif
214}
215
216static inline void closure_set_ret_ip(struct closure *cl)
217{
218#ifdef CONFIG_DEBUG_CLOSURES
219	cl->ip = _RET_IP_;
220#endif
221}
222
223static inline void closure_set_waiting(struct closure *cl, unsigned long f)
224{
225#ifdef CONFIG_DEBUG_CLOSURES
226	cl->waiting_on = f;
227#endif
228}
229
230static inline void closure_set_stopped(struct closure *cl)
231{
232	atomic_sub(CLOSURE_RUNNING, &cl->remaining);
233}
234
235static inline void set_closure_fn(struct closure *cl, closure_fn *fn,
236				  struct workqueue_struct *wq)
237{
238	closure_set_ip(cl);
239	cl->fn = fn;
240	cl->wq = wq;
241}
242
243static inline void closure_queue(struct closure *cl)
244{
245	struct workqueue_struct *wq = cl->wq;
246	/**
247	 * Changes made to closure, work_struct, or a couple of other structs
248	 * may cause work.func not pointing to the right location.
249	 */
250	BUILD_BUG_ON(offsetof(struct closure, fn)
251		     != offsetof(struct work_struct, func));
252
253	if (wq) {
254		INIT_WORK(&cl->work, cl->work.func);
255		BUG_ON(!queue_work(wq, &cl->work));
256	} else
257		cl->fn(&cl->work);
258}
259
260/**
261 * closure_get - increment a closure's refcount
262 */
263static inline void closure_get(struct closure *cl)
264{
265	cl->closure_get_happened = true;
266
267#ifdef CONFIG_DEBUG_CLOSURES
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 */
281static inline void closure_init(struct closure *cl, struct closure *parent)
282{
283	cl->fn = NULL;
284	cl->parent = parent;
285	if (parent)
286		closure_get(parent);
287
288	atomic_set(&cl->remaining, CLOSURE_REMAINING_INITIALIZER);
289	cl->closure_get_happened = false;
290
291	closure_debug_create(cl);
292	closure_set_ip(cl);
293}
294
295static inline void closure_init_stack(struct closure *cl)
296{
297	memset(cl, 0, sizeof(struct closure));
298	atomic_set(&cl->remaining, CLOSURE_REMAINING_INITIALIZER);
299}
300
301/**
302 * closure_wake_up - wake up all closures on a wait list,
303 *		     with memory barrier
304 */
305static inline void closure_wake_up(struct closure_waitlist *list)
306{
307	/* Memory barrier for the wait list */
308	smp_mb();
309	__closure_wake_up(list);
310}
311
312#define CLOSURE_CALLBACK(name)	void name(struct work_struct *ws)
313#define closure_type(name, type, member)				\
314	struct closure *cl = container_of(ws, struct closure, work);	\
315	type *name = container_of(cl, type, member)
316
317/**
318 * continue_at - jump to another function with barrier
319 *
320 * After @cl is no longer waiting on anything (i.e. all outstanding refs have
321 * been dropped with closure_put()), it will resume execution at @fn running out
322 * of @wq (or, if @wq is NULL, @fn will be called by closure_put() directly).
323 *
324 * This is because after calling continue_at() you no longer have a ref on @cl,
325 * and whatever @cl owns may be freed out from under you - a running closure fn
326 * has a ref on its own closure which continue_at() drops.
327 *
328 * Note you are expected to immediately return after using this macro.
329 */
330#define continue_at(_cl, _fn, _wq)					\
331do {									\
332	set_closure_fn(_cl, _fn, _wq);					\
333	closure_sub(_cl, CLOSURE_RUNNING + 1);				\
334} while (0)
335
336/**
337 * closure_return - finish execution of a closure
338 *
339 * This is used to indicate that @cl is finished: when all outstanding refs on
340 * @cl have been dropped @cl's ref on its parent closure (as passed to
341 * closure_init()) will be dropped, if one was specified - thus this can be
342 * thought of as returning to the parent closure.
343 */
344#define closure_return(_cl)	continue_at((_cl), NULL, NULL)
345
346/**
347 * continue_at_nobarrier - jump to another function without barrier
348 *
349 * Causes @fn to be executed out of @cl, in @wq context (or called directly if
350 * @wq is NULL).
351 *
352 * The ref the caller of continue_at_nobarrier() had on @cl is now owned by @fn,
353 * thus it's not safe to touch anything protected by @cl after a
354 * continue_at_nobarrier().
355 */
356#define continue_at_nobarrier(_cl, _fn, _wq)				\
357do {									\
358	set_closure_fn(_cl, _fn, _wq);					\
359	closure_queue(_cl);						\
360} while (0)
361
362/**
363 * closure_return_with_destructor - finish execution of a closure,
364 *				    with destructor
365 *
366 * Works like closure_return(), except @destructor will be called when all
367 * outstanding refs on @cl have been dropped; @destructor may be used to safely
368 * free the memory occupied by @cl, and it is called with the ref on the parent
369 * closure still held - so @destructor could safely return an item to a
370 * freelist protected by @cl's parent.
371 */
372#define closure_return_with_destructor(_cl, _destructor)		\
373do {									\
374	set_closure_fn(_cl, _destructor, NULL);				\
375	closure_sub(_cl, CLOSURE_RUNNING - CLOSURE_DESTRUCTOR + 1);	\
376} while (0)
377
378/**
379 * closure_call - execute @fn out of a new, uninitialized closure
380 *
381 * Typically used when running out of one closure, and we want to run @fn
382 * asynchronously out of a new closure - @parent will then wait for @cl to
383 * finish.
384 */
385static inline void closure_call(struct closure *cl, closure_fn fn,
386				struct workqueue_struct *wq,
387				struct closure *parent)
388{
389	closure_init(cl, parent);
390	continue_at_nobarrier(cl, fn, wq);
391}
392
393#define __closure_wait_event(waitlist, _cond)				\
394do {									\
395	struct closure cl;						\
396									\
397	closure_init_stack(&cl);					\
398									\
399	while (1) {							\
400		closure_wait(waitlist, &cl);				\
401		if (_cond)						\
402			break;						\
403		closure_sync(&cl);					\
404	}								\
405	closure_wake_up(waitlist);					\
406	closure_sync(&cl);						\
407} while (0)
408
409#define closure_wait_event(waitlist, _cond)				\
410do {									\
411	if (!(_cond))							\
412		__closure_wait_event(waitlist, _cond);			\
413} while (0)
414
415#endif /* _LINUX_CLOSURE_H */
416