drivers/md/bcache/closure.h

   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 */