--- /dev/null
+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.
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