1 .. SPDX-License-Identifier: GPL-2.0
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4 Using RCU hlist_nulls to protect list and objects
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7 This section describes how to use hlist_nulls to
8 protect read-mostly linked lists and
9 objects using SLAB_TYPESAFE_BY_RCU allocations.
11 Please read the basics in listRCU.rst.
16 Using special makers (called 'nulls') is a convenient way
17 to solve following problem.
19 Without 'nulls', a typical RCU linked list managing objects which are
20 allocated with SLAB_TYPESAFE_BY_RCU kmem_cache can use the following
30 obj = lockless_lookup(key);
32 if (!try_get_ref(obj)) // might fail for free objects
35 * Because a writer could delete object, and a writer could
36 * reuse these object before the RCU grace period, we
37 * must check key after getting the reference on object
39 if (obj->key != key) { // not the object we expected
47 Beware that lockless_lookup(key) cannot use traditional hlist_for_each_entry_rcu()
48 but a version with an additional memory barrier (smp_rmb())
54 struct hlist_node *node, *next;
55 for (pos = rcu_dereference((head)->first);
56 pos && ({ next = pos->next; smp_rmb(); prefetch(next); 1; }) &&
57 ({ tpos = hlist_entry(pos, typeof(*tpos), member); 1; });
58 pos = rcu_dereference(next))
64 And note the traditional hlist_for_each_entry_rcu() misses this smp_rmb()::
66 struct hlist_node *node;
67 for (pos = rcu_dereference((head)->first);
68 pos && ({ prefetch(pos->next); 1; }) &&
69 ({ tpos = hlist_entry(pos, typeof(*tpos), member); 1; });
70 pos = rcu_dereference(pos->next))
75 Quoting Corey Minyard::
77 "If the object is moved from one list to another list in-between the
78 time the hash is calculated and the next field is accessed, and the
79 object has moved to the end of a new list, the traversal will not
80 complete properly on the list it should have, since the object will
81 be on the end of the new list and there's not a way to tell it's on a
82 new list and restart the list traversal. I think that this can be
83 solved by pre-fetching the "next" field (with proper barriers) before
86 2) Insertion algorithm
87 ----------------------
89 We need to make sure a reader cannot read the new 'obj->obj_next' value
90 and previous value of 'obj->key'. Otherwise, an item could be deleted
91 from a chain, and inserted into another chain. If new chain was empty
92 before the move, 'next' pointer is NULL, and lockless reader can not
93 detect the fact that it missed following items in original chain.
98 * Please note that new inserts are done at the head of list,
99 * not in the middle or end.
101 obj = kmem_cache_alloc(...);
102 lock_chain(); // typically a spin_lock()
104 atomic_set_release(&obj->refcnt, 1); // key before refcnt
105 hlist_add_head_rcu(&obj->obj_node, list);
106 unlock_chain(); // typically a spin_unlock()
112 Nothing special here, we can use a standard RCU hlist deletion.
113 But thanks to SLAB_TYPESAFE_BY_RCU, beware a deleted object can be reused
114 very very fast (before the end of RCU grace period)
118 if (put_last_reference_on(obj) {
119 lock_chain(); // typically a spin_lock()
120 hlist_del_init_rcu(&obj->obj_node);
121 unlock_chain(); // typically a spin_unlock()
122 kmem_cache_free(cachep, obj);
127 --------------------------------------------------------------------------
129 Avoiding extra smp_rmb()
130 ========================
132 With hlist_nulls we can avoid extra smp_rmb() in lockless_lookup()
133 and extra _release() in insert function.
135 For example, if we choose to store the slot number as the 'nulls'
136 end-of-list marker for each slot of the hash table, we can detect
137 a race (some writer did a delete and/or a move of an object
138 to another chain) checking the final 'nulls' value if
139 the lookup met the end of chain. If final 'nulls' value
140 is not the slot number, then we must restart the lookup at
141 the beginning. If the object was moved to the same chain,
142 then the reader doesn't care: It might occasionally
143 scan the list again without harm.
154 hlist_nulls_for_each_entry_rcu(obj, node, head, member) {
155 if (obj->key == key) {
156 if (!try_get_ref(obj)) { // might fail for free objects
160 if (obj->key != key) { // not the object we expected
169 // If the nulls value we got at the end of this lookup is
170 // not the expected one, we must restart lookup.
171 // We probably met an item that was moved to another chain.
172 if (get_nulls_value(node) != slot) {
188 * Please note that new inserts are done at the head of list,
189 * not in the middle or end.
191 obj = kmem_cache_alloc(cachep);
192 lock_chain(); // typically a spin_lock()
194 atomic_set_release(&obj->refcnt, 1); // key before refcnt
196 * insert obj in RCU way (readers might be traversing chain)
198 hlist_nulls_add_head_rcu(&obj->obj_node, list);
199 unlock_chain(); // typically a spin_unlock()