lib/llist.c

   1 // SPDX-License-Identifier: GPL-2.0-only
   2 /*
   3  * Lock-less NULL terminated single linked list
   4  *
   5  * The basic atomic operation of this list is cmpxchg on long.  On
   6  * architectures that don't have NMI-safe cmpxchg implementation, the
   7  * list can NOT be used in NMI handlers.  So code that uses the list in
   8  * an NMI handler should depend on CONFIG_ARCH_HAVE_NMI_SAFE_CMPXCHG.
   9  *
  10  * Copyright 2010,2011 Intel Corp.
  11  *   Author: Huang Ying <ying.huang@intel.com>
  12  */
  13 #include <linux/kernel.h>
  14 #include <linux/export.h>
  15 #include <linux/llist.h>
  16
  17
  18 /**
  19  * llist_add_batch - add several linked entries in batch
  20  * @new_first:  first entry in batch to be added
  21  * @new_last:   last entry in batch to be added
  22  * @head:       the head for your lock-less list
  23  *
  24  * Return whether list is empty before adding.
  25  */
  26 bool llist_add_batch(struct llist_node *new_first, struct llist_node *new_last,
  27                      struct llist_head *head)
  28 {
  29         struct llist_node *first = READ_ONCE(head->first);
  30
  31         do {
  32                 new_last->next = first;
  33         } while (!try_cmpxchg(&head->first, &first, new_first));
  34
  35         return !first;
  36 }
  37 EXPORT_SYMBOL_GPL(llist_add_batch);
  38
  39 /**
  40  * llist_del_first - delete the first entry of lock-less list
  41  * @head:       the head for your lock-less list
  42  *
  43  * If list is empty, return NULL, otherwise, return the first entry
  44  * deleted, this is the newest added one.
  45  *
  46  * Only one llist_del_first user can be used simultaneously with
  47  * multiple llist_add users without lock.  Because otherwise
  48  * llist_del_first, llist_add, llist_add (or llist_del_all, llist_add,
  49  * llist_add) sequence in another user may change @head->first->next,
  50  * but keep @head->first.  If multiple consumers are needed, please
  51  * use llist_del_all or use lock between consumers.
  52  */
  53 struct llist_node *llist_del_first(struct llist_head *head)
  54 {
  55         struct llist_node *entry, *next;
  56
  57         entry = smp_load_acquire(&head->first);
  58         do {
  59                 if (entry == NULL)
  60                         return NULL;
  61                 next = READ_ONCE(entry->next);
  62         } while (!try_cmpxchg(&head->first, &entry, next));
  63
  64         return entry;
  65 }
  66 EXPORT_SYMBOL_GPL(llist_del_first);
  67
  68 /**
  69  * llist_del_first_this - delete given entry of lock-less list if it is first
  70  * @head:       the head for your lock-less list
  71  * @this:       a list entry.
  72  *
  73  * If head of the list is given entry, delete and return %true else
  74  * return %false.
  75  *
  76  * Multiple callers can safely call this concurrently with multiple
  77  * llist_add() callers, providing all the callers offer a different @this.
  78  */
  79 bool llist_del_first_this(struct llist_head *head,
  80                           struct llist_node *this)
  81 {
  82         struct llist_node *entry, *next;
  83
  84         /* acquire ensures orderig wrt try_cmpxchg() is llist_del_first() */
  85         entry = smp_load_acquire(&head->first);
  86         do {
  87                 if (entry != this)
  88                         return false;
  89                 next = READ_ONCE(entry->next);
  90         } while (!try_cmpxchg(&head->first, &entry, next));
  91
  92         return true;
  93 }
  94 EXPORT_SYMBOL_GPL(llist_del_first_this);
  95
  96 /**
  97  * llist_reverse_order - reverse order of a llist chain
  98  * @head:       first item of the list to be reversed
  99  *
 100  * Reverse the order of a chain of llist entries and return the
 101  * new first entry.
 102  */
 103 struct llist_node *llist_reverse_order(struct llist_node *head)
 104 {
 105         struct llist_node *new_head = NULL;
 106
 107         while (head) {
 108                 struct llist_node *tmp = head;
 109                 head = head->next;
 110                 tmp->next = new_head;
 111                 new_head = tmp;
 112         }
 113
 114         return new_head;
 115 }
 116 EXPORT_SYMBOL_GPL(llist_reverse_order);