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+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.