Documentation/RCU/arrayRCU.txt

Using RCU to Protect Read-Mostly Arrays


Although RCU is more commonly used to protect linked lists, it can
also be used to protect arrays.  Three situations are as follows:

1.  Hash Tables

2.  Static Arrays

3.  Resizeable Arrays

Each of these situations are discussed below.


Situation 1: Hash Tables

Hash tables are often implemented as an array, where each array entry
has a linked-list hash chain.  Each hash chain can be protected by RCU
as described in the listRCU.txt document.  This approach also applies
to other array-of-list situations, such as radix trees.


Situation 2: Static Arrays

Static arrays, where the data (rather than a pointer to the data) is
located in each array element, and where the array is never resized,
have not been used with RCU.  Rik van Riel recommends using seqlock in
this situation, which would also have minimal read-side overhead as long
as updates are rare.

Quick Quiz:  Why is it so important that updates be rare when
	     using seqlock?


Situation 3: Resizeable Arrays

Use of RCU for resizeable arrays is demonstrated by the grow_ary()
function used by the System V IPC code.  The array is used to map from
semaphore, message-queue, and shared-memory IDs to the data structure
that represents the corresponding IPC construct.  The grow_ary()
function does not acquire any locks; instead its caller must hold the
ids->sem semaphore.

The grow_ary() function, shown below, does some limit checks, allocates a
new ipc_id_ary, copies the old to the new portion of the new, initializes
the remainder of the new, updates the ids->entries pointer to point to
the new array, and invokes ipc_rcu_putref() to free up the old array.
Note that rcu_assign_pointer() is used to update the ids->entries pointer,
which includes any memory barriers required on whatever architecture
you are running on.

	static int grow_ary(struct ipc_ids* ids, int newsize)
	{
		struct ipc_id_ary* new;
		struct ipc_id_ary* old;
		int i;
		int size = ids->entries->size;

		if(newsize > IPCMNI)
			newsize = IPCMNI;
		if(newsize <= size)
			return newsize;

		new = ipc_rcu_alloc(sizeof(struct kern_ipc_perm *)*newsize +
				    sizeof(struct ipc_id_ary));
		if(new == NULL)
			return size;
		new->size = newsize;
		memcpy(new->p, ids->entries->p,
		       sizeof(struct kern_ipc_perm *)*size +
		       sizeof(struct ipc_id_ary));
		for(i=size;i<newsize;i++) {
			new->p[i] = NULL;
		}
		old = ids->entries;

		/*
		 * Use rcu_assign_pointer() to make sure the memcpyed
		 * contents of the new array are visible before the new
		 * array becomes visible.
		 */
		rcu_assign_pointer(ids->entries, new);

		ipc_rcu_putref(old);
		return newsize;
	}

The ipc_rcu_putref() function decrements the array's reference count
and then, if the reference count has dropped to zero, uses call_rcu()
to free the array after a grace period has elapsed.

The array is traversed by the ipc_lock() function.  This function
indexes into the array under the protection of rcu_read_lock(),
using rcu_dereference() to pick up the pointer to the array so
that it may later safely be dereferenced -- memory barriers are
required on the Alpha CPU.  Since the size of the array is stored
with the array itself, there can be no array-size mismatches, so
a simple check suffices.  The pointer to the structure corresponding
to the desired IPC object is placed in "out", with NULL indicating
a non-existent entry.  After acquiring "out->lock", the "out->deleted"
flag indicates whether the IPC object is in the process of being
deleted, and, if not, the pointer is returned.

	struct kern_ipc_perm* ipc_lock(struct ipc_ids* ids, int id)
	{
		struct kern_ipc_perm* out;
		int lid = id % SEQ_MULTIPLIER;
		struct ipc_id_ary* entries;

		rcu_read_lock();
		entries = rcu_dereference(ids->entries);
		if(lid >= entries->size) {
			rcu_read_unlock();
			return NULL;
		}
		out = entries->p[lid];
		if(out == NULL) {
			rcu_read_unlock();
			return NULL;
		}
		spin_lock(&out->lock);

		/* ipc_rmid() may have already freed the ID while ipc_lock
		 * was spinning: here verify that the structure is still valid
		 */
		if (out->deleted) {
			spin_unlock(&out->lock);
			rcu_read_unlock();
			return NULL;
		}
		return out;
	}


Answer to Quick Quiz:

	The reason that it is important that updates be rare when
	using seqlock is that frequent updates can livelock readers.
	One way to avoid this problem is to assign a seqlock for
	each array entry rather than to the entire array.
1	niro	628	Using RCU to Protect Read-Mostly Arrays
2
3
4			Although RCU is more commonly used to protect linked lists, it can
5			also be used to protect arrays. Three situations are as follows:
6
7			1. Hash Tables
8
9			2. Static Arrays
10
11			3. Resizeable Arrays
12
13			Each of these situations are discussed below.
14
15
16			Situation 1: Hash Tables
17
18			Hash tables are often implemented as an array, where each array entry
19			has a linked-list hash chain. Each hash chain can be protected by RCU
20			as described in the listRCU.txt document. This approach also applies
21			to other array-of-list situations, such as radix trees.
22
23
24			Situation 2: Static Arrays
25
26			Static arrays, where the data (rather than a pointer to the data) is
27			located in each array element, and where the array is never resized,
28			have not been used with RCU. Rik van Riel recommends using seqlock in
29			this situation, which would also have minimal read-side overhead as long
30			as updates are rare.
31
32			Quick Quiz: Why is it so important that updates be rare when
33			using seqlock?
34
35
36			Situation 3: Resizeable Arrays
37
38			Use of RCU for resizeable arrays is demonstrated by the grow_ary()
39			function used by the System V IPC code. The array is used to map from
40			semaphore, message-queue, and shared-memory IDs to the data structure
41			that represents the corresponding IPC construct. The grow_ary()
42			function does not acquire any locks; instead its caller must hold the
43			ids->sem semaphore.
44
45			The grow_ary() function, shown below, does some limit checks, allocates a
46			new ipc_id_ary, copies the old to the new portion of the new, initializes
47			the remainder of the new, updates the ids->entries pointer to point to
48			the new array, and invokes ipc_rcu_putref() to free up the old array.
49			Note that rcu_assign_pointer() is used to update the ids->entries pointer,
50			which includes any memory barriers required on whatever architecture
51			you are running on.
52
53			static int grow_ary(struct ipc_ids* ids, int newsize)
54			{
55			struct ipc_id_ary* new;
56			struct ipc_id_ary* old;
57			int i;
58			int size = ids->entries->size;
59
60			if(newsize > IPCMNI)
61			newsize = IPCMNI;
62			if(newsize <= size)
63			return newsize;
64
65			new = ipc_rcu_alloc(sizeof(struct kern_ipc_perm )newsize +
66			sizeof(struct ipc_id_ary));
67			if(new == NULL)
68			return size;
69			new->size = newsize;
70			memcpy(new->p, ids->entries->p,
71			sizeof(struct kern_ipc_perm )size +
72			sizeof(struct ipc_id_ary));
73			for(i=size;i<newsize;i++) {
74			new->p[i] = NULL;
75			}
76			old = ids->entries;
77
78			/*
79			* Use rcu_assign_pointer() to make sure the memcpyed
80			* contents of the new array are visible before the new
81			* array becomes visible.
82			*/
83			rcu_assign_pointer(ids->entries, new);
84
85			ipc_rcu_putref(old);
86			return newsize;
87			}
88
89			The ipc_rcu_putref() function decrements the array's reference count
90			and then, if the reference count has dropped to zero, uses call_rcu()
91			to free the array after a grace period has elapsed.
92
93			The array is traversed by the ipc_lock() function. This function
94			indexes into the array under the protection of rcu_read_lock(),
95			using rcu_dereference() to pick up the pointer to the array so
96			that it may later safely be dereferenced -- memory barriers are
97			required on the Alpha CPU. Since the size of the array is stored
98			with the array itself, there can be no array-size mismatches, so
99			a simple check suffices. The pointer to the structure corresponding
100			to the desired IPC object is placed in "out", with NULL indicating
101			a non-existent entry. After acquiring "out->lock", the "out->deleted"
102			flag indicates whether the IPC object is in the process of being
103			deleted, and, if not, the pointer is returned.
104
105			struct kern_ipc_perm* ipc_lock(struct ipc_ids* ids, int id)
106			{
107			struct kern_ipc_perm* out;
108			int lid = id % SEQ_MULTIPLIER;
109			struct ipc_id_ary* entries;
110
111			rcu_read_lock();
112			entries = rcu_dereference(ids->entries);
113			if(lid >= entries->size) {
114			rcu_read_unlock();
115			return NULL;
116			}
117			out = entries->p[lid];
118			if(out == NULL) {
119			rcu_read_unlock();
120			return NULL;
121			}
122			spin_lock(&out->lock);
123
124			/* ipc_rmid() may have already freed the ID while ipc_lock
125			* was spinning: here verify that the structure is still valid
126			*/
127			if (out->deleted) {
128			spin_unlock(&out->lock);
129			rcu_read_unlock();
130			return NULL;
131			}
132			return out;
133			}
134
135
136			Answer to Quick Quiz:
137
138			The reason that it is important that updates be rare when
139			using seqlock is that frequent updates can livelock readers.
140			One way to avoid this problem is to assign a seqlock for
141			each array entry rather than to the entire array.