lib/klist.c

   1 // SPDX-License-Identifier: GPL-2.0-only
   2 /*
   3  * klist.c - Routines for manipulating klists.
   4  *
   5  * Copyright (C) 2005 Patrick Mochel
   6  *
   7  * This klist interface provides a couple of structures that wrap around
   8  * struct list_head to provide explicit list "head" (struct klist) and list
   9  * "node" (struct klist_node) objects. For struct klist, a spinlock is
  10  * included that protects access to the actual list itself. struct
  11  * klist_node provides a pointer to the klist that owns it and a kref
  12  * reference count that indicates the number of current users of that node
  13  * in the list.
  14  *
  15  * The entire point is to provide an interface for iterating over a list
  16  * that is safe and allows for modification of the list during the
  17  * iteration (e.g. insertion and removal), including modification of the
  18  * current node on the list.
  19  *
  20  * It works using a 3rd object type - struct klist_iter - that is declared
  21  * and initialized before an iteration. klist_next() is used to acquire the
  22  * next element in the list. It returns NULL if there are no more items.
  23  * Internally, that routine takes the klist's lock, decrements the
  24  * reference count of the previous klist_node and increments the count of
  25  * the next klist_node. It then drops the lock and returns.
  26  *
  27  * There are primitives for adding and removing nodes to/from a klist.
  28  * When deleting, klist_del() will simply decrement the reference count.
  29  * Only when the count goes to 0 is the node removed from the list.
  30  * klist_remove() will try to delete the node from the list and block until
  31  * it is actually removed. This is useful for objects (like devices) that
  32  * have been removed from the system and must be freed (but must wait until
  33  * all accessors have finished).
  34  */
  35
  36 #include <linux/klist.h>
  37 #include <linux/export.h>
  38 #include <linux/sched.h>
  39
  40 /*
  41  * Use the lowest bit of n_klist to mark deleted nodes and exclude
  42  * dead ones from iteration.
  43  */
  44 #define KNODE_DEAD              1LU
  45 #define KNODE_KLIST_MASK        ~KNODE_DEAD
  46
  47 static struct klist *knode_klist(struct klist_node *knode)
  48 {
  49         return (struct klist *)
  50                 ((unsigned long)knode->n_klist & KNODE_KLIST_MASK);
  51 }
  52
  53 static bool knode_dead(struct klist_node *knode)
  54 {
  55         return (unsigned long)knode->n_klist & KNODE_DEAD;
  56 }
  57
  58 static void knode_set_klist(struct klist_node *knode, struct klist *klist)
  59 {
  60         knode->n_klist = klist;
  61         /* no knode deserves to start its life dead */
  62         WARN_ON(knode_dead(knode));
  63 }
  64
  65 static void knode_kill(struct klist_node *knode)
  66 {
  67         /* and no knode should die twice ever either, see we're very humane */
  68         WARN_ON(knode_dead(knode));
  69         *(unsigned long *)&knode->n_klist |= KNODE_DEAD;
  70 }
  71
  72 /**
  73  * klist_init - Initialize a klist structure.
  74  * @k: The klist we're initializing.
  75  * @get: The get function for the embedding object (NULL if none)
  76  * @put: The put function for the embedding object (NULL if none)
  77  *
  78  * Initialises the klist structure.  If the klist_node structures are
  79  * going to be embedded in refcounted objects (necessary for safe
  80  * deletion) then the get/put arguments are used to initialise
  81  * functions that take and release references on the embedding
  82  * objects.
  83  */
  84 void klist_init(struct klist *k, void (*get)(struct klist_node *),
  85                 void (*put)(struct klist_node *))
  86 {
  87         INIT_LIST_HEAD(&k->k_list);
  88         spin_lock_init(&k->k_lock);
  89         k->get = get;
  90         k->put = put;
  91 }
  92 EXPORT_SYMBOL_GPL(klist_init);
  93
  94 static void add_head(struct klist *k, struct klist_node *n)
  95 {
  96         spin_lock(&k->k_lock);
  97         list_add(&n->n_node, &k->k_list);
  98         spin_unlock(&k->k_lock);
  99 }
 100
 101 static void add_tail(struct klist *k, struct klist_node *n)
 102 {
 103         spin_lock(&k->k_lock);
 104         list_add_tail(&n->n_node, &k->k_list);
 105         spin_unlock(&k->k_lock);
 106 }
 107
 108 static void klist_node_init(struct klist *k, struct klist_node *n)
 109 {
 110         INIT_LIST_HEAD(&n->n_node);
 111         kref_init(&n->n_ref);
 112         knode_set_klist(n, k);
 113         if (k->get)
 114                 k->get(n);
 115 }
 116
 117 /**
 118  * klist_add_head - Initialize a klist_node and add it to front.
 119  * @n: node we're adding.
 120  * @k: klist it's going on.
 121  */
 122 void klist_add_head(struct klist_node *n, struct klist *k)
 123 {
 124         klist_node_init(k, n);
 125         add_head(k, n);
 126 }
 127 EXPORT_SYMBOL_GPL(klist_add_head);
 128
 129 /**
 130  * klist_add_tail - Initialize a klist_node and add it to back.
 131  * @n: node we're adding.
 132  * @k: klist it's going on.
 133  */
 134 void klist_add_tail(struct klist_node *n, struct klist *k)
 135 {
 136         klist_node_init(k, n);
 137         add_tail(k, n);
 138 }
 139 EXPORT_SYMBOL_GPL(klist_add_tail);
 140
 141 /**
 142  * klist_add_behind - Init a klist_node and add it after an existing node
 143  * @n: node we're adding.
 144  * @pos: node to put @n after
 145  */
 146 void klist_add_behind(struct klist_node *n, struct klist_node *pos)
 147 {
 148         struct klist *k = knode_klist(pos);
 149
 150         klist_node_init(k, n);
 151         spin_lock(&k->k_lock);
 152         list_add(&n->n_node, &pos->n_node);
 153         spin_unlock(&k->k_lock);
 154 }
 155 EXPORT_SYMBOL_GPL(klist_add_behind);
 156
 157 /**
 158  * klist_add_before - Init a klist_node and add it before an existing node
 159  * @n: node we're adding.
 160  * @pos: node to put @n after
 161  */
 162 void klist_add_before(struct klist_node *n, struct klist_node *pos)
 163 {
 164         struct klist *k = knode_klist(pos);
 165
 166         klist_node_init(k, n);
 167         spin_lock(&k->k_lock);
 168         list_add_tail(&n->n_node, &pos->n_node);
 169         spin_unlock(&k->k_lock);
 170 }
 171 EXPORT_SYMBOL_GPL(klist_add_before);
 172
 173 struct klist_waiter {
 174         struct list_head list;
 175         struct klist_node *node;
 176         struct task_struct *process;
 177         int woken;
 178 };
 179
 180 static DEFINE_SPINLOCK(klist_remove_lock);
 181 static LIST_HEAD(klist_remove_waiters);
 182
 183 static void klist_release(struct kref *kref)
 184 {
 185         struct klist_waiter *waiter, *tmp;
 186         struct klist_node *n = container_of(kref, struct klist_node, n_ref);
 187
 188         WARN_ON(!knode_dead(n));
 189         list_del(&n->n_node);
 190         spin_lock(&klist_remove_lock);
 191         list_for_each_entry_safe(waiter, tmp, &klist_remove_waiters, list) {
 192                 if (waiter->node != n)
 193                         continue;
 194
 195                 list_del(&waiter->list);
 196                 waiter->woken = 1;
 197                 mb();
 198                 wake_up_process(waiter->process);
 199         }
 200         spin_unlock(&klist_remove_lock);
 201         knode_set_klist(n, NULL);
 202 }
 203
 204 static int klist_dec_and_del(struct klist_node *n)
 205 {
 206         return kref_put(&n->n_ref, klist_release);
 207 }
 208
 209 static void klist_put(struct klist_node *n, bool kill)
 210 {
 211         struct klist *k = knode_klist(n);
 212         void (*put)(struct klist_node *) = k->put;
 213
 214         spin_lock(&k->k_lock);
 215         if (kill)
 216                 knode_kill(n);
 217         if (!klist_dec_and_del(n))
 218                 put = NULL;
 219         spin_unlock(&k->k_lock);
 220         if (put)
 221                 put(n);
 222 }
 223
 224 /**
 225  * klist_del - Decrement the reference count of node and try to remove.
 226  * @n: node we're deleting.
 227  */
 228 void klist_del(struct klist_node *n)
 229 {
 230         klist_put(n, true);
 231 }
 232 EXPORT_SYMBOL_GPL(klist_del);
 233
 234 /**
 235  * klist_remove - Decrement the refcount of node and wait for it to go away.
 236  * @n: node we're removing.
 237  */
 238 void klist_remove(struct klist_node *n)
 239 {
 240         struct klist_waiter waiter;
 241
 242         waiter.node = n;
 243         waiter.process = current;
 244         waiter.woken = 0;
 245         spin_lock(&klist_remove_lock);
 246         list_add(&waiter.list, &klist_remove_waiters);
 247         spin_unlock(&klist_remove_lock);
 248
 249         klist_del(n);
 250
 251         for (;;) {
 252                 set_current_state(TASK_UNINTERRUPTIBLE);
 253                 if (waiter.woken)
 254                         break;
 255                 schedule();
 256         }
 257         __set_current_state(TASK_RUNNING);
 258 }
 259 EXPORT_SYMBOL_GPL(klist_remove);
 260
 261 /**
 262  * klist_node_attached - Say whether a node is bound to a list or not.
 263  * @n: Node that we're testing.
 264  */
 265 int klist_node_attached(struct klist_node *n)
 266 {
 267         return (n->n_klist != NULL);
 268 }
 269 EXPORT_SYMBOL_GPL(klist_node_attached);
 270
 271 /**
 272  * klist_iter_init_node - Initialize a klist_iter structure.
 273  * @k: klist we're iterating.
 274  * @i: klist_iter we're filling.
 275  * @n: node to start with.
 276  *
 277  * Similar to klist_iter_init(), but starts the action off with @n,
 278  * instead of with the list head.
 279  */
 280 void klist_iter_init_node(struct klist *k, struct klist_iter *i,
 281                           struct klist_node *n)
 282 {
 283         i->i_klist = k;
 284         i->i_cur = NULL;
 285         if (n && kref_get_unless_zero(&n->n_ref))
 286                 i->i_cur = n;
 287 }
 288 EXPORT_SYMBOL_GPL(klist_iter_init_node);
 289
 290 /**
 291  * klist_iter_init - Iniitalize a klist_iter structure.
 292  * @k: klist we're iterating.
 293  * @i: klist_iter structure we're filling.
 294  *
 295  * Similar to klist_iter_init_node(), but start with the list head.
 296  */
 297 void klist_iter_init(struct klist *k, struct klist_iter *i)
 298 {
 299         klist_iter_init_node(k, i, NULL);
 300 }
 301 EXPORT_SYMBOL_GPL(klist_iter_init);
 302
 303 /**
 304  * klist_iter_exit - Finish a list iteration.
 305  * @i: Iterator structure.
 306  *
 307  * Must be called when done iterating over list, as it decrements the
 308  * refcount of the current node. Necessary in case iteration exited before
 309  * the end of the list was reached, and always good form.
 310  */
 311 void klist_iter_exit(struct klist_iter *i)
 312 {
 313         if (i->i_cur) {
 314                 klist_put(i->i_cur, false);
 315                 i->i_cur = NULL;
 316         }
 317 }
 318 EXPORT_SYMBOL_GPL(klist_iter_exit);
 319
 320 static struct klist_node *to_klist_node(struct list_head *n)
 321 {
 322         return container_of(n, struct klist_node, n_node);
 323 }
 324
 325 /**
 326  * klist_prev - Ante up prev node in list.
 327  * @i: Iterator structure.
 328  *
 329  * First grab list lock. Decrement the reference count of the previous
 330  * node, if there was one. Grab the prev node, increment its reference
 331  * count, drop the lock, and return that prev node.
 332  */
 333 struct klist_node *klist_prev(struct klist_iter *i)
 334 {
 335         void (*put)(struct klist_node *) = i->i_klist->put;
 336         struct klist_node *last = i->i_cur;
 337         struct klist_node *prev;
 338         unsigned long flags;
 339
 340         spin_lock_irqsave(&i->i_klist->k_lock, flags);
 341
 342         if (last) {
 343                 prev = to_klist_node(last->n_node.prev);
 344                 if (!klist_dec_and_del(last))
 345                         put = NULL;
 346         } else
 347                 prev = to_klist_node(i->i_klist->k_list.prev);
 348
 349         i->i_cur = NULL;
 350         while (prev != to_klist_node(&i->i_klist->k_list)) {
 351                 if (likely(!knode_dead(prev))) {
 352                         kref_get(&prev->n_ref);
 353                         i->i_cur = prev;
 354                         break;
 355                 }
 356                 prev = to_klist_node(prev->n_node.prev);
 357         }
 358
 359         spin_unlock_irqrestore(&i->i_klist->k_lock, flags);
 360
 361         if (put && last)
 362                 put(last);
 363         return i->i_cur;
 364 }
 365 EXPORT_SYMBOL_GPL(klist_prev);
 366
 367 /**
 368  * klist_next - Ante up next node in list.
 369  * @i: Iterator structure.
 370  *
 371  * First grab list lock. Decrement the reference count of the previous
 372  * node, if there was one. Grab the next node, increment its reference
 373  * count, drop the lock, and return that next node.
 374  */
 375 struct klist_node *klist_next(struct klist_iter *i)
 376 {
 377         void (*put)(struct klist_node *) = i->i_klist->put;
 378         struct klist_node *last = i->i_cur;
 379         struct klist_node *next;
 380         unsigned long flags;
 381
 382         spin_lock_irqsave(&i->i_klist->k_lock, flags);
 383
 384         if (last) {
 385                 next = to_klist_node(last->n_node.next);
 386                 if (!klist_dec_and_del(last))
 387                         put = NULL;
 388         } else
 389                 next = to_klist_node(i->i_klist->k_list.next);
 390
 391         i->i_cur = NULL;
 392         while (next != to_klist_node(&i->i_klist->k_list)) {
 393                 if (likely(!knode_dead(next))) {
 394                         kref_get(&next->n_ref);
 395                         i->i_cur = next;
 396                         break;
 397                 }
 398                 next = to_klist_node(next->n_node.next);
 399         }
 400
 401         spin_unlock_irqrestore(&i->i_klist->k_lock, flags);
 402
 403         if (put && last)
 404                 put(last);
 405         return i->i_cur;
 406 }
 407 EXPORT_SYMBOL_GPL(klist_next);