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  1Using RCU to Protect Read-Mostly Arrays
  4Although RCU is more commonly used to protect linked lists, it can
  5also be used to protect arrays.  Three situations are as follows:
  71.  Hash Tables
  92.  Static Arrays
 113.  Resizeable Arrays
 13Each of these situations are discussed below.
 16Situation 1: Hash Tables
 18Hash tables are often implemented as an array, where each array entry
 19has a linked-list hash chain.  Each hash chain can be protected by RCU
 20as described in the listRCU.txt document.  This approach also applies
 21to other array-of-list situations, such as radix trees.
 24Situation 2: Static Arrays
 26Static arrays, where the data (rather than a pointer to the data) is
 27located in each array element, and where the array is never resized,
 28have not been used with RCU.  Rik van Riel recommends using seqlock in
 29this situation, which would also have minimal read-side overhead as long
 30as updates are rare.
 32Quick Quiz:  Why is it so important that updates be rare when
 33	     using seqlock?
 36Situation 3: Resizeable Arrays
 38Use of RCU for resizeable arrays is demonstrated by the grow_ary()
 39function used by the System V IPC code.  The array is used to map from
 40semaphore, message-queue, and shared-memory IDs to the data structure
 41that represents the corresponding IPC construct.  The grow_ary()
 42function does not acquire any locks; instead its caller must hold the
 43ids->sem semaphore.
 45The grow_ary() function, shown below, does some limit checks, allocates a
 46new ipc_id_ary, copies the old to the new portion of the new, initializes
 47the remainder of the new, updates the ids->entries pointer to point to
 48the new array, and invokes ipc_rcu_putref() to free up the old array.
 49Note that rcu_assign_pointer() is used to update the ids->entries pointer,
 50which includes any memory barriers required on whatever architecture
 51you are running on.
 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;
 60		if(newsize > IPCMNI)
 61			newsize = IPCMNI;
 62		if(newsize <= size)
 63			return newsize;
 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;
 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);
 85		ipc_rcu_putref(old);
 86		return newsize;
 87	}
 89The ipc_rcu_putref() function decrements the array's reference count
 90and then, if the reference count has dropped to zero, uses call_rcu()
 91to free the array after a grace period has elapsed.
 93The array is traversed by the ipc_lock() function.  This function
 94indexes into the array under the protection of rcu_read_lock(),
 95using rcu_dereference() to pick up the pointer to the array so
 96that it may later safely be dereferenced -- memory barriers are
 97required on the Alpha CPU.  Since the size of the array is stored
 98with the array itself, there can be no array-size mismatches, so
 99a simple check suffices.  The pointer to the structure corresponding
100to the desired IPC object is placed in "out", with NULL indicating
101a non-existent entry.  After acquiring "out->lock", the "out->deleted"
102flag indicates whether the IPC object is in the process of being
103deleted, and, if not, the pointer is returned.
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;
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);
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	}
136Answer to Quick Quiz:
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.