1.. _array_rcu_doc:
2
3Using RCU to Protect Read-Mostly Arrays
4=======================================
5
6Although RCU is more commonly used to protect linked lists, it can
7also be used to protect arrays.  Three situations are as follows:
8
91.  :ref:`Hash Tables <hash_tables>`
10
112.  :ref:`Static Arrays <static_arrays>`
12
133.  :ref:`Resizable Arrays <resizable_arrays>`
14
15Each of these three situations involves an RCU-protected pointer to an
16array that is separately indexed.  It might be tempting to consider use
17of RCU to instead protect the index into an array, however, this use
18case is **not** supported.  The problem with RCU-protected indexes into
19arrays is that compilers can play way too many optimization games with
20integers, which means that the rules governing handling of these indexes
21are far more trouble than they are worth.  If RCU-protected indexes into
22arrays prove to be particularly valuable (which they have not thus far),
23explicit cooperation from the compiler will be required to permit them
24to be safely used.
25
26That aside, each of the three RCU-protected pointer situations are
27described in the following sections.
28
29.. _hash_tables:
30
31Situation 1: Hash Tables
32------------------------
33
34Hash tables are often implemented as an array, where each array entry
35has a linked-list hash chain.  Each hash chain can be protected by RCU
36as described in listRCU.rst.  This approach also applies to other
37array-of-list situations, such as radix trees.
38
39.. _static_arrays:
40
41Situation 2: Static Arrays
42--------------------------
43
44Static arrays, where the data (rather than a pointer to the data) is
45located in each array element, and where the array is never resized,
46have not been used with RCU.  Rik van Riel recommends using seqlock in
47this situation, which would also have minimal read-side overhead as long
48as updates are rare.
49
50Quick Quiz:
51		Why is it so important that updates be rare when using seqlock?
52
53:ref:`Answer to Quick Quiz <answer_quick_quiz_seqlock>`
54
55.. _resizable_arrays:
56
57Situation 3: Resizable Arrays
58------------------------------
59
60Use of RCU for resizable arrays is demonstrated by the grow_ary()
61function formerly used by the System V IPC code.  The array is used
62to map from semaphore, message-queue, and shared-memory IDs to the data
63structure that represents the corresponding IPC construct.  The grow_ary()
64function does not acquire any locks; instead its caller must hold the
65ids->sem semaphore.
66
67The grow_ary() function, shown below, does some limit checks, allocates a
68new ipc_id_ary, copies the old to the new portion of the new, initializes
69the remainder of the new, updates the ids->entries pointer to point to
70the new array, and invokes ipc_rcu_putref() to free up the old array.
71Note that rcu_assign_pointer() is used to update the ids->entries pointer,
72which includes any memory barriers required on whatever architecture
73you are running on::
74
75	static int grow_ary(struct ipc_ids* ids, int newsize)
76	{
77		struct ipc_id_ary* new;
78		struct ipc_id_ary* old;
79		int i;
80		int size = ids->entries->size;
81
82		if(newsize > IPCMNI)
83			newsize = IPCMNI;
84		if(newsize <= size)
85			return newsize;
86
87		new = ipc_rcu_alloc(sizeof(struct kern_ipc_perm *)*newsize +
88				    sizeof(struct ipc_id_ary));
89		if(new == NULL)
90			return size;
91		new->size = newsize;
92		memcpy(new->p, ids->entries->p,
93		       sizeof(struct kern_ipc_perm *)*size +
94		       sizeof(struct ipc_id_ary));
95		for(i=size;i<newsize;i++) {
96			new->p[i] = NULL;
97		}
98		old = ids->entries;
99
100		/*
101		 * Use rcu_assign_pointer() to make sure the memcpyed
102		 * contents of the new array are visible before the new
103		 * array becomes visible.
104		 */
105		rcu_assign_pointer(ids->entries, new);
106
107		ipc_rcu_putref(old);
108		return newsize;
109	}
110
111The ipc_rcu_putref() function decrements the array's reference count
112and then, if the reference count has dropped to zero, uses call_rcu()
113to free the array after a grace period has elapsed.
114
115The array is traversed by the ipc_lock() function.  This function
116indexes into the array under the protection of rcu_read_lock(),
117using rcu_dereference() to pick up the pointer to the array so
118that it may later safely be dereferenced -- memory barriers are
119required on the Alpha CPU.  Since the size of the array is stored
120with the array itself, there can be no array-size mismatches, so
121a simple check suffices.  The pointer to the structure corresponding
122to the desired IPC object is placed in "out", with NULL indicating
123a non-existent entry.  After acquiring "out->lock", the "out->deleted"
124flag indicates whether the IPC object is in the process of being
125deleted, and, if not, the pointer is returned::
126
127	struct kern_ipc_perm* ipc_lock(struct ipc_ids* ids, int id)
128	{
129		struct kern_ipc_perm* out;
130		int lid = id % SEQ_MULTIPLIER;
131		struct ipc_id_ary* entries;
132
133		rcu_read_lock();
134		entries = rcu_dereference(ids->entries);
135		if(lid >= entries->size) {
136			rcu_read_unlock();
137			return NULL;
138		}
139		out = entries->p[lid];
140		if(out == NULL) {
141			rcu_read_unlock();
142			return NULL;
143		}
144		spin_lock(&out->lock);
145
146		/* ipc_rmid() may have already freed the ID while ipc_lock
147		 * was spinning: here verify that the structure is still valid
148		 */
149		if (out->deleted) {
150			spin_unlock(&out->lock);
151			rcu_read_unlock();
152			return NULL;
153		}
154		return out;
155	}
156
157.. _answer_quick_quiz_seqlock:
158
159Answer to Quick Quiz:
160	Why is it so important that updates be rare when using seqlock?
161
162	The reason that it is important that updates be rare when
163	using seqlock is that frequent updates can livelock readers.
164	One way to avoid this problem is to assign a seqlock for
165	each array entry rather than to the entire array.
166