1 #ifndef _LINUX_CLOSURE_H
2 #define _LINUX_CLOSURE_H
4 #include <linux/llist.h>
5 #include <linux/sched.h>
6 #include <linux/workqueue.h>
9 * Closure is perhaps the most overused and abused term in computer science, but
10 * since I've been unable to come up with anything better you're stuck with it
15 * They embed a refcount. The basic idea is they count "things that are in
16 * progress" - in flight bios, some other thread that's doing something else -
17 * anything you might want to wait on.
19 * The refcount may be manipulated with closure_get() and closure_put().
20 * closure_put() is where many of the interesting things happen, when it causes
21 * the refcount to go to 0.
23 * Closures can be used to wait on things both synchronously and asynchronously,
24 * and synchronous and asynchronous use can be mixed without restriction. To
25 * wait synchronously, use closure_sync() - you will sleep until your closure's
28 * To wait asynchronously, use
29 * continue_at(cl, next_function, workqueue);
31 * passing it, as you might expect, the function to run when nothing is pending
32 * and the workqueue to run that function out of.
34 * continue_at() also, critically, is a macro that returns the calling function.
35 * There's good reason for this.
37 * To use safely closures asynchronously, they must always have a refcount while
38 * they are running owned by the thread that is running them. Otherwise, suppose
39 * you submit some bios and wish to have a function run when they all complete:
41 * foo_endio(struct bio *bio, int error)
50 * bio1->bi_endio = foo_endio;
55 * bio2->bi_endio = foo_endio;
58 * continue_at(cl, complete_some_read, system_wq);
60 * If closure's refcount started at 0, complete_some_read() could run before the
61 * second bio was submitted - which is almost always not what you want! More
62 * importantly, it wouldn't be possible to say whether the original thread or
63 * complete_some_read()'s thread owned the closure - and whatever state it was
66 * So, closure_init() initializes a closure's refcount to 1 - and when a
67 * closure_fn is run, the refcount will be reset to 1 first.
69 * Then, the rule is - if you got the refcount with closure_get(), release it
70 * with closure_put() (i.e, in a bio->bi_endio function). If you have a refcount
71 * on a closure because you called closure_init() or you were run out of a
72 * closure - _always_ use continue_at(). Doing so consistently will help
73 * eliminate an entire class of particularly pernicious races.
75 * For a closure to wait on an arbitrary event, we need to introduce waitlists:
77 * struct closure_waitlist list;
78 * closure_wait_event(list, cl, condition);
79 * closure_wake_up(wait_list);
81 * These work analagously to wait_event() and wake_up() - except that instead of
82 * operating on the current thread (for wait_event()) and lists of threads, they
83 * operate on an explicit closure and lists of closures.
85 * Because it's a closure we can now wait either synchronously or
86 * asynchronously. closure_wait_event() returns the current value of the
87 * condition, and if it returned false continue_at() or closure_sync() can be
88 * used to wait for it to become true.
90 * It's useful for waiting on things when you can't sleep in the context in
91 * which you must check the condition (perhaps a spinlock held, or you might be
92 * beneath generic_make_request() - in which case you can't sleep on IO).
94 * closure_wait_event() will wait either synchronously or asynchronously,
95 * depending on whether the closure is in blocking mode or not. You can pick a
96 * mode explicitly with closure_wait_event_sync() and
97 * closure_wait_event_async(), which do just what you might expect.
99 * Lastly, you might have a wait list dedicated to a specific event, and have no
100 * need for specifying the condition - you just want to wait until someone runs
101 * closure_wake_up() on the appropriate wait list. In that case, just use
102 * closure_wait(). It will return either true or false, depending on whether the
103 * closure was already on a wait list or not - a closure can only be on one wait
108 * closure_init() takes two arguments - it takes the closure to initialize, and
109 * a (possibly null) parent.
111 * If parent is non null, the new closure will have a refcount for its lifetime;
112 * a closure is considered to be "finished" when its refcount hits 0 and the
113 * function to run is null. Hence
115 * continue_at(cl, NULL, NULL);
117 * returns up the (spaghetti) stack of closures, precisely like normal return
118 * returns up the C stack. continue_at() with non null fn is better thought of
119 * as doing a tail call.
121 * All this implies that a closure should typically be embedded in a particular
122 * struct (which its refcount will normally control the lifetime of), and that
123 * struct can very much be thought of as a stack frame.
127 * Closures are based on work items but they can be thought of as more like
128 * threads - in that like threads and unlike work items they have a well
129 * defined lifetime; they are created (with closure_init()) and eventually
130 * complete after a continue_at(cl, NULL, NULL).
132 * Suppose you've got some larger structure with a closure embedded in it that's
133 * used for periodically doing garbage collection. You only want one garbage
134 * collection happening at a time, so the natural thing to do is protect it with
135 * a lock. However, it's difficult to use a lock protecting a closure correctly
136 * because the unlock should come after the last continue_to() (additionally, if
137 * you're using the closure asynchronously a mutex won't work since a mutex has
138 * to be unlocked by the same process that locked it).
140 * So to make it less error prone and more efficient, we also have the ability
141 * to use closures as locks:
143 * closure_init_unlocked();
146 * That's all we need for trylock() - the last closure_put() implicitly unlocks
147 * it for you. But for closure_lock(), we also need a wait list:
149 * struct closure_with_waitlist frobnicator_cl;
151 * closure_init_unlocked(&frobnicator_cl);
152 * closure_lock(&frobnicator_cl);
154 * A closure_with_waitlist embeds a closure and a wait list - much like struct
155 * delayed_work embeds a work item and a timer_list. The important thing is, use
156 * it exactly like you would a regular closure and closure_put() will magically
157 * handle everything for you.
161 typedef void (closure_fn) (struct closure *);
163 struct closure_waitlist {
164 struct llist_head list;
169 TYPE_closure_with_waitlist = 1,
170 MAX_CLOSURE_TYPE = 1,
175 * CLOSURE_WAITING: Set iff the closure is on a waitlist. Must be set by
176 * the thread that owns the closure, and cleared by the thread that's
177 * waking up the closure.
179 * CLOSURE_SLEEPING: Must be set before a thread uses a closure to sleep
180 * - indicates that cl->task is valid and closure_put() may wake it up.
181 * Only set or cleared by the thread that owns the closure.
183 * The rest are for debugging and don't affect behaviour:
185 * CLOSURE_RUNNING: Set when a closure is running (i.e. by
186 * closure_init() and when closure_put() runs then next function), and
187 * must be cleared before remaining hits 0. Primarily to help guard
188 * against incorrect usage and accidentally transferring references.
189 * continue_at() and closure_return() clear it for you, if you're doing
190 * something unusual you can use closure_set_dead() which also helps
191 * annotate where references are being transferred.
193 * CLOSURE_STACK: Sanity check - remaining should never hit 0 on a
194 * closure with this flag set
197 CLOSURE_BITS_START = (1 << 23),
198 CLOSURE_DESTRUCTOR = (1 << 23),
199 CLOSURE_WAITING = (1 << 25),
200 CLOSURE_SLEEPING = (1 << 27),
201 CLOSURE_RUNNING = (1 << 29),
202 CLOSURE_STACK = (1 << 31),
205 #define CLOSURE_GUARD_MASK \
206 ((CLOSURE_DESTRUCTOR|CLOSURE_WAITING|CLOSURE_SLEEPING| \
207 CLOSURE_RUNNING|CLOSURE_STACK) << 1)
209 #define CLOSURE_REMAINING_MASK (CLOSURE_BITS_START - 1)
210 #define CLOSURE_REMAINING_INITIALIZER (1|CLOSURE_RUNNING)
215 struct workqueue_struct *wq;
216 struct task_struct *task;
217 struct llist_node list;
220 struct work_struct work;
223 struct closure *parent;
227 enum closure_type type;
229 #ifdef CONFIG_BCACHE_CLOSURES_DEBUG
230 #define CLOSURE_MAGIC_DEAD 0xc054dead
231 #define CLOSURE_MAGIC_ALIVE 0xc054a11e
234 struct list_head all;
236 unsigned long waiting_on;
240 struct closure_with_waitlist {
242 struct closure_waitlist wait;
245 extern unsigned invalid_closure_type(void);
247 #define __CLOSURE_TYPE(cl, _t) \
248 __builtin_types_compatible_p(typeof(cl), struct _t) \
251 #define __closure_type(cl) \
253 __CLOSURE_TYPE(cl, closure) \
254 __CLOSURE_TYPE(cl, closure_with_waitlist) \
255 invalid_closure_type() \
258 void closure_sub(struct closure *cl, int v);
259 void closure_put(struct closure *cl);
260 void __closure_wake_up(struct closure_waitlist *list);
261 bool closure_wait(struct closure_waitlist *list, struct closure *cl);
262 void closure_sync(struct closure *cl);
264 bool closure_trylock(struct closure *cl, struct closure *parent);
265 void __closure_lock(struct closure *cl, struct closure *parent,
266 struct closure_waitlist *wait_list);
268 #ifdef CONFIG_BCACHE_CLOSURES_DEBUG
270 void closure_debug_init(void);
271 void closure_debug_create(struct closure *cl);
272 void closure_debug_destroy(struct closure *cl);
276 static inline void closure_debug_init(void) {}
277 static inline void closure_debug_create(struct closure *cl) {}
278 static inline void closure_debug_destroy(struct closure *cl) {}
282 static inline void closure_set_ip(struct closure *cl)
284 #ifdef CONFIG_BCACHE_CLOSURES_DEBUG
289 static inline void closure_set_ret_ip(struct closure *cl)
291 #ifdef CONFIG_BCACHE_CLOSURES_DEBUG
296 static inline void closure_get(struct closure *cl)
298 #ifdef CONFIG_BCACHE_CLOSURES_DEBUG
299 BUG_ON((atomic_inc_return(&cl->remaining) &
300 CLOSURE_REMAINING_MASK) <= 1);
302 atomic_inc(&cl->remaining);
306 static inline void closure_set_stopped(struct closure *cl)
308 atomic_sub(CLOSURE_RUNNING, &cl->remaining);
311 static inline bool closure_is_unlocked(struct closure *cl)
313 return atomic_read(&cl->remaining) == -1;
316 static inline void do_closure_init(struct closure *cl, struct closure *parent,
324 closure_debug_create(cl);
325 atomic_set(&cl->remaining, CLOSURE_REMAINING_INITIALIZER);
327 atomic_set(&cl->remaining, -1);
333 * Hack to get at the embedded closure if there is one, by doing an unsafe cast:
334 * the result of __closure_type() is thrown away, it's used merely for type
337 #define __to_internal_closure(cl) \
339 BUILD_BUG_ON(__closure_type(*cl) > MAX_CLOSURE_TYPE); \
340 (struct closure *) cl; \
343 #define closure_init_type(cl, parent, running) \
345 struct closure *_cl = __to_internal_closure(cl); \
346 _cl->type = __closure_type(*(cl)); \
347 do_closure_init(_cl, parent, running); \
351 * __closure_init() - Initialize a closure, skipping the memset()
353 * May be used instead of closure_init() when memory has already been zeroed.
355 #define __closure_init(cl, parent) \
356 closure_init_type(cl, parent, true)
359 * closure_init() - Initialize a closure, setting the refcount to 1
360 * @cl: closure to initialize
361 * @parent: parent of the new closure. cl will take a refcount on it for its
362 * lifetime; may be NULL.
364 #define closure_init(cl, parent) \
366 memset((cl), 0, sizeof(*(cl))); \
367 __closure_init(cl, parent); \
370 static inline void closure_init_stack(struct closure *cl)
372 memset(cl, 0, sizeof(struct closure));
373 atomic_set(&cl->remaining, CLOSURE_REMAINING_INITIALIZER|CLOSURE_STACK);
377 * closure_init_unlocked() - Initialize a closure but leave it unlocked.
378 * @cl: closure to initialize
380 * For when the closure will be used as a lock. The closure may not be used
381 * until after a closure_lock() or closure_trylock().
383 #define closure_init_unlocked(cl) \
385 memset((cl), 0, sizeof(*(cl))); \
386 closure_init_type(cl, NULL, false); \
390 * closure_lock() - lock and initialize a closure.
391 * @cl: the closure to lock
392 * @parent: the new parent for this closure
394 * The closure must be of one of the types that has a waitlist (otherwise we
395 * wouldn't be able to sleep on contention).
397 * @parent has exactly the same meaning as in closure_init(); if non null, the
398 * closure will take a reference on @parent which will be released when it is
401 #define closure_lock(cl, parent) \
402 __closure_lock(__to_internal_closure(cl), parent, &(cl)->wait)
404 static inline void __closure_end_sleep(struct closure *cl)
406 __set_current_state(TASK_RUNNING);
408 if (atomic_read(&cl->remaining) & CLOSURE_SLEEPING)
409 atomic_sub(CLOSURE_SLEEPING, &cl->remaining);
412 static inline void __closure_start_sleep(struct closure *cl)
416 set_current_state(TASK_UNINTERRUPTIBLE);
418 if (!(atomic_read(&cl->remaining) & CLOSURE_SLEEPING))
419 atomic_add(CLOSURE_SLEEPING, &cl->remaining);
423 * closure_wake_up() - wake up all closures on a wait list.
425 static inline void closure_wake_up(struct closure_waitlist *list)
428 __closure_wake_up(list);
432 * Wait on an event, synchronously or asynchronously - analogous to wait_event()
435 * The loop is oddly structured so as to avoid a race; we must check the
436 * condition again after we've added ourself to the waitlist. We know if we were
437 * already on the waitlist because closure_wait() returns false; thus, we only
438 * schedule or break if closure_wait() returns false. If it returns true, we
439 * just loop again - rechecking the condition.
441 * The __closure_wake_up() is necessary because we may race with the event
442 * becoming true; i.e. we see event false -> wait -> recheck condition, but the
443 * thread that made the event true may have called closure_wake_up() before we
444 * added ourself to the wait list.
446 * We have to call closure_sync() at the end instead of just
447 * __closure_end_sleep() because a different thread might've called
448 * closure_wake_up() before us and gotten preempted before they dropped the
449 * refcount on our closure. If this was a stack allocated closure, that would be
452 #define closure_wait_event(list, cl, condition) \
454 typeof(condition) ret; \
459 __closure_wake_up(list); \
464 __closure_start_sleep(cl); \
466 if (!closure_wait(list, cl)) \
473 static inline void closure_queue(struct closure *cl)
475 struct workqueue_struct *wq = cl->wq;
477 INIT_WORK(&cl->work, cl->work.func);
478 BUG_ON(!queue_work(wq, &cl->work));
483 static inline void set_closure_fn(struct closure *cl, closure_fn *fn,
484 struct workqueue_struct *wq)
486 BUG_ON(object_is_on_stack(cl));
490 /* between atomic_dec() in closure_put() */
491 smp_mb__before_atomic_dec();
494 #define continue_at(_cl, _fn, _wq) \
496 set_closure_fn(_cl, _fn, _wq); \
497 closure_sub(_cl, CLOSURE_RUNNING + 1); \
501 #define closure_return(_cl) continue_at((_cl), NULL, NULL)
503 #define continue_at_nobarrier(_cl, _fn, _wq) \
505 set_closure_fn(_cl, _fn, _wq); \
506 closure_queue(_cl); \
510 #define closure_return_with_destructor(_cl, _destructor) \
512 set_closure_fn(_cl, _destructor, NULL); \
513 closure_sub(_cl, CLOSURE_RUNNING - CLOSURE_DESTRUCTOR + 1); \
517 static inline void closure_call(struct closure *cl, closure_fn fn,
518 struct workqueue_struct *wq,
519 struct closure *parent)
521 closure_init(cl, parent);
522 continue_at_nobarrier(cl, fn, wq);
525 static inline void closure_trylock_call(struct closure *cl, closure_fn fn,
526 struct workqueue_struct *wq,
527 struct closure *parent)
529 if (closure_trylock(cl, parent))
530 continue_at_nobarrier(cl, fn, wq);
533 #endif /* _LINUX_CLOSURE_H */