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Eric Dumazet536533e2008-11-16 19:41:14 -08001Using hlist_nulls to protect read-mostly linked lists and
2objects using SLAB_DESTROY_BY_RCU allocations.
3
4Please read the basics in Documentation/RCU/listRCU.txt
5
6Using special makers (called 'nulls') is a convenient way
7to solve following problem :
8
9A typical RCU linked list managing objects which are
10allocated with SLAB_DESTROY_BY_RCU kmem_cache can
11use following algos :
12
131) Lookup algo
14--------------
15rcu_read_lock()
16begin:
17obj = lockless_lookup(key);
18if (obj) {
19 if (!try_get_ref(obj)) // might fail for free objects
20 goto begin;
21 /*
22 * Because a writer could delete object, and a writer could
23 * reuse these object before the RCU grace period, we
24 * must check key after geting the reference on object
25 */
26 if (obj->key != key) { // not the object we expected
27 put_ref(obj);
28 goto begin;
29 }
30}
31rcu_read_unlock();
32
33Beware that lockless_lookup(key) cannot use traditional hlist_for_each_entry_rcu()
34but a version with an additional memory barrier (smp_rmb())
35
36lockless_lookup(key)
37{
38 struct hlist_node *node, *next;
39 for (pos = rcu_dereference((head)->first);
40 pos && ({ next = pos->next; smp_rmb(); prefetch(next); 1; }) &&
41 ({ tpos = hlist_entry(pos, typeof(*tpos), member); 1; });
42 pos = rcu_dereference(next))
43 if (obj->key == key)
44 return obj;
45 return NULL;
46
47And note the traditional hlist_for_each_entry_rcu() misses this smp_rmb() :
48
49 struct hlist_node *node;
50 for (pos = rcu_dereference((head)->first);
51 pos && ({ prefetch(pos->next); 1; }) &&
52 ({ tpos = hlist_entry(pos, typeof(*tpos), member); 1; });
53 pos = rcu_dereference(pos->next))
54 if (obj->key == key)
55 return obj;
56 return NULL;
57}
58
59Quoting Corey Minyard :
60
61"If the object is moved from one list to another list in-between the
62 time the hash is calculated and the next field is accessed, and the
63 object has moved to the end of a new list, the traversal will not
64 complete properly on the list it should have, since the object will
65 be on the end of the new list and there's not a way to tell it's on a
66 new list and restart the list traversal. I think that this can be
67 solved by pre-fetching the "next" field (with proper barriers) before
68 checking the key."
69
702) Insert algo :
71----------------
72
73We need to make sure a reader cannot read the new 'obj->obj_next' value
74and previous value of 'obj->key'. Or else, an item could be deleted
75from a chain, and inserted into another chain. If new chain was empty
76before the move, 'next' pointer is NULL, and lockless reader can
77not detect it missed following items in original chain.
78
79/*
80 * Please note that new inserts are done at the head of list,
81 * not in the middle or end.
82 */
83obj = kmem_cache_alloc(...);
84lock_chain(); // typically a spin_lock()
85obj->key = key;
86atomic_inc(&obj->refcnt);
87/*
88 * we need to make sure obj->key is updated before obj->next
89 */
90smp_wmb();
91hlist_add_head_rcu(&obj->obj_node, list);
92unlock_chain(); // typically a spin_unlock()
93
94
953) Remove algo
96--------------
97Nothing special here, we can use a standard RCU hlist deletion.
98But thanks to SLAB_DESTROY_BY_RCU, beware a deleted object can be reused
99very very fast (before the end of RCU grace period)
100
101if (put_last_reference_on(obj) {
102 lock_chain(); // typically a spin_lock()
103 hlist_del_init_rcu(&obj->obj_node);
104 unlock_chain(); // typically a spin_unlock()
105 kmem_cache_free(cachep, obj);
106}
107
108
109
110--------------------------------------------------------------------------
111With hlist_nulls we can avoid extra smp_rmb() in lockless_lookup()
112and extra smp_wmb() in insert function.
113
114For example, if we choose to store the slot number as the 'nulls'
115end-of-list marker for each slot of the hash table, we can detect
116a race (some writer did a delete and/or a move of an object
117to another chain) checking the final 'nulls' value if
118the lookup met the end of chain. If final 'nulls' value
119is not the slot number, then we must restart the lookup at
120the begining. If the object was moved to same chain,
121then the reader doesnt care : It might eventually
122scan the list again without harm.
123
124
1251) lookup algo
126
127 head = &table[slot];
128 rcu_read_lock();
129begin:
130 hlist_nulls_for_each_entry_rcu(obj, node, head, member) {
131 if (obj->key == key) {
132 if (!try_get_ref(obj)) // might fail for free objects
133 goto begin;
134 if (obj->key != key) { // not the object we expected
135 put_ref(obj);
136 goto begin;
137 }
138 goto out;
139 }
140/*
141 * if the nulls value we got at the end of this lookup is
142 * not the expected one, we must restart lookup.
143 * We probably met an item that was moved to another chain.
144 */
145 if (get_nulls_value(node) != slot)
146 goto begin;
147 obj = NULL;
148
149out:
150 rcu_read_unlock();
151
1522) Insert function :
153--------------------
154
155/*
156 * Please note that new inserts are done at the head of list,
157 * not in the middle or end.
158 */
159obj = kmem_cache_alloc(cachep);
160lock_chain(); // typically a spin_lock()
161obj->key = key;
162atomic_set(&obj->refcnt, 1);
163/*
164 * insert obj in RCU way (readers might be traversing chain)
165 */
166hlist_nulls_add_head_rcu(&obj->obj_node, list);
167unlock_chain(); // typically a spin_unlock()