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Mauro Carvalho Chehab2cdb54c2020-04-21 19:04:05 +02001.. SPDX-License-Identifier: GPL-2.0
2
3=================================================
4Using RCU hlist_nulls to protect list and objects
5=================================================
6
7This section describes how to use hlist_nulls to
8protect read-mostly linked lists and
9objects using SLAB_TYPESAFE_BY_RCU allocations.
10
11Please read the basics in Documentation/RCU/listRCU.rst
12
Mauro Carvalho Chehab2d9c3182020-04-21 19:04:11 +020013Using 'nulls'
14=============
15
Mauro Carvalho Chehab2cdb54c2020-04-21 19:04:05 +020016Using special makers (called 'nulls') is a convenient way
17to solve following problem :
18
19A typical RCU linked list managing objects which are
20allocated with SLAB_TYPESAFE_BY_RCU kmem_cache can
21use following algos :
22
231) Lookup algo
24--------------
25
26::
27
28 rcu_read_lock()
29 begin:
30 obj = lockless_lookup(key);
31 if (obj) {
32 if (!try_get_ref(obj)) // might fail for free objects
33 goto begin;
34 /*
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
38 */
39 if (obj->key != key) { // not the object we expected
40 put_ref(obj);
41 goto begin;
42 }
43 }
44 rcu_read_unlock();
45
46Beware that lockless_lookup(key) cannot use traditional hlist_for_each_entry_rcu()
47but a version with an additional memory barrier (smp_rmb())
48
49::
50
51 lockless_lookup(key)
52 {
53 struct hlist_node *node, *next;
54 for (pos = rcu_dereference((head)->first);
55 pos && ({ next = pos->next; smp_rmb(); prefetch(next); 1; }) &&
56 ({ tpos = hlist_entry(pos, typeof(*tpos), member); 1; });
57 pos = rcu_dereference(next))
58 if (obj->key == key)
59 return obj;
60 return NULL;
61 }
62
63And note the traditional hlist_for_each_entry_rcu() misses this smp_rmb()::
64
65 struct hlist_node *node;
66 for (pos = rcu_dereference((head)->first);
67 pos && ({ prefetch(pos->next); 1; }) &&
68 ({ tpos = hlist_entry(pos, typeof(*tpos), member); 1; });
69 pos = rcu_dereference(pos->next))
70 if (obj->key == key)
71 return obj;
72 return NULL;
73
74Quoting Corey Minyard::
75
76 "If the object is moved from one list to another list in-between the
77 time the hash is calculated and the next field is accessed, and the
78 object has moved to the end of a new list, the traversal will not
79 complete properly on the list it should have, since the object will
80 be on the end of the new list and there's not a way to tell it's on a
81 new list and restart the list traversal. I think that this can be
82 solved by pre-fetching the "next" field (with proper barriers) before
83 checking the key."
84
852) Insert algo
86--------------
87
88We need to make sure a reader cannot read the new 'obj->obj_next' value
89and previous value of 'obj->key'. Or else, an item could be deleted
90from a chain, and inserted into another chain. If new chain was empty
91before the move, 'next' pointer is NULL, and lockless reader can
92not detect it missed following items in original chain.
93
94::
95
96 /*
97 * Please note that new inserts are done at the head of list,
98 * not in the middle or end.
99 */
100 obj = kmem_cache_alloc(...);
101 lock_chain(); // typically a spin_lock()
102 obj->key = key;
103 /*
104 * we need to make sure obj->key is updated before obj->next
105 * or obj->refcnt
106 */
107 smp_wmb();
108 atomic_set(&obj->refcnt, 1);
109 hlist_add_head_rcu(&obj->obj_node, list);
110 unlock_chain(); // typically a spin_unlock()
111
112
1133) Remove algo
114--------------
115Nothing special here, we can use a standard RCU hlist deletion.
116But thanks to SLAB_TYPESAFE_BY_RCU, beware a deleted object can be reused
117very very fast (before the end of RCU grace period)
118
119::
120
121 if (put_last_reference_on(obj) {
122 lock_chain(); // typically a spin_lock()
123 hlist_del_init_rcu(&obj->obj_node);
124 unlock_chain(); // typically a spin_unlock()
125 kmem_cache_free(cachep, obj);
126 }
127
128
129
130--------------------------------------------------------------------------
131
Mauro Carvalho Chehab2d9c3182020-04-21 19:04:11 +0200132Avoiding extra smp_rmb()
133========================
134
Mauro Carvalho Chehab2cdb54c2020-04-21 19:04:05 +0200135With hlist_nulls we can avoid extra smp_rmb() in lockless_lookup()
136and extra smp_wmb() in insert function.
137
138For example, if we choose to store the slot number as the 'nulls'
139end-of-list marker for each slot of the hash table, we can detect
140a race (some writer did a delete and/or a move of an object
141to another chain) checking the final 'nulls' value if
142the lookup met the end of chain. If final 'nulls' value
143is not the slot number, then we must restart the lookup at
144the beginning. If the object was moved to the same chain,
145then the reader doesn't care : It might eventually
146scan the list again without harm.
147
148
1491) lookup algo
150--------------
151
152::
153
154 head = &table[slot];
155 rcu_read_lock();
156 begin:
157 hlist_nulls_for_each_entry_rcu(obj, node, head, member) {
158 if (obj->key == key) {
159 if (!try_get_ref(obj)) // might fail for free objects
160 goto begin;
161 if (obj->key != key) { // not the object we expected
162 put_ref(obj);
163 goto begin;
164 }
165 goto out;
166 }
167 /*
168 * if the nulls value we got at the end of this lookup is
169 * not the expected one, we must restart lookup.
170 * We probably met an item that was moved to another chain.
171 */
172 if (get_nulls_value(node) != slot)
173 goto begin;
174 obj = NULL;
175
176 out:
177 rcu_read_unlock();
178
1792) Insert function
180------------------
181
182::
183
184 /*
185 * Please note that new inserts are done at the head of list,
186 * not in the middle or end.
187 */
188 obj = kmem_cache_alloc(cachep);
189 lock_chain(); // typically a spin_lock()
190 obj->key = key;
191 /*
192 * changes to obj->key must be visible before refcnt one
193 */
194 smp_wmb();
195 atomic_set(&obj->refcnt, 1);
196 /*
197 * insert obj in RCU way (readers might be traversing chain)
198 */
199 hlist_nulls_add_head_rcu(&obj->obj_node, list);
200 unlock_chain(); // typically a spin_unlock()