153 lines
5.1 KiB
Text
153 lines
5.1 KiB
Text
Using RCU to Protect Read-Mostly Arrays
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Although RCU is more commonly used to protect linked lists, it can
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also be used to protect arrays. Three situations are as follows:
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1. Hash Tables
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2. Static Arrays
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3. Resizeable Arrays
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Each of these three situations involves an RCU-protected pointer to an
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array that is separately indexed. It might be tempting to consider use
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of RCU to instead protect the index into an array, however, this use
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case is -not- supported. The problem with RCU-protected indexes into
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arrays is that compilers can play way too many optimization games with
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integers, which means that the rules governing handling of these indexes
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are far more trouble than they are worth. If RCU-protected indexes into
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arrays prove to be particularly valuable (which they have not thus far),
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explicit cooperation from the compiler will be required to permit them
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to be safely used.
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That aside, each of the three RCU-protected pointer situations are
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described in the following sections.
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Situation 1: Hash Tables
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Hash tables are often implemented as an array, where each array entry
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has a linked-list hash chain. Each hash chain can be protected by RCU
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as described in the listRCU.txt document. This approach also applies
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to other array-of-list situations, such as radix trees.
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Situation 2: Static Arrays
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Static arrays, where the data (rather than a pointer to the data) is
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located in each array element, and where the array is never resized,
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have not been used with RCU. Rik van Riel recommends using seqlock in
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this situation, which would also have minimal read-side overhead as long
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as updates are rare.
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Quick Quiz: Why is it so important that updates be rare when
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using seqlock?
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Situation 3: Resizeable Arrays
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Use of RCU for resizeable arrays is demonstrated by the grow_ary()
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function formerly used by the System V IPC code. The array is used
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to map from semaphore, message-queue, and shared-memory IDs to the data
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structure that represents the corresponding IPC construct. The grow_ary()
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function does not acquire any locks; instead its caller must hold the
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ids->sem semaphore.
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The grow_ary() function, shown below, does some limit checks, allocates a
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new ipc_id_ary, copies the old to the new portion of the new, initializes
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the remainder of the new, updates the ids->entries pointer to point to
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the new array, and invokes ipc_rcu_putref() to free up the old array.
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Note that rcu_assign_pointer() is used to update the ids->entries pointer,
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which includes any memory barriers required on whatever architecture
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you are running on.
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static int grow_ary(struct ipc_ids* ids, int newsize)
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{
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struct ipc_id_ary* new;
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struct ipc_id_ary* old;
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int i;
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int size = ids->entries->size;
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if(newsize > IPCMNI)
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newsize = IPCMNI;
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if(newsize <= size)
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return newsize;
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new = ipc_rcu_alloc(sizeof(struct kern_ipc_perm *)*newsize +
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sizeof(struct ipc_id_ary));
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if(new == NULL)
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return size;
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new->size = newsize;
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memcpy(new->p, ids->entries->p,
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sizeof(struct kern_ipc_perm *)*size +
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sizeof(struct ipc_id_ary));
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for(i=size;i<newsize;i++) {
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new->p[i] = NULL;
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}
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old = ids->entries;
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/*
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* Use rcu_assign_pointer() to make sure the memcpyed
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* contents of the new array are visible before the new
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* array becomes visible.
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*/
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rcu_assign_pointer(ids->entries, new);
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ipc_rcu_putref(old);
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return newsize;
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}
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The ipc_rcu_putref() function decrements the array's reference count
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and then, if the reference count has dropped to zero, uses call_rcu()
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to free the array after a grace period has elapsed.
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The array is traversed by the ipc_lock() function. This function
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indexes into the array under the protection of rcu_read_lock(),
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using rcu_dereference() to pick up the pointer to the array so
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that it may later safely be dereferenced -- memory barriers are
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required on the Alpha CPU. Since the size of the array is stored
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with the array itself, there can be no array-size mismatches, so
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a simple check suffices. The pointer to the structure corresponding
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to the desired IPC object is placed in "out", with NULL indicating
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a non-existent entry. After acquiring "out->lock", the "out->deleted"
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flag indicates whether the IPC object is in the process of being
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deleted, and, if not, the pointer is returned.
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struct kern_ipc_perm* ipc_lock(struct ipc_ids* ids, int id)
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{
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struct kern_ipc_perm* out;
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int lid = id % SEQ_MULTIPLIER;
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struct ipc_id_ary* entries;
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rcu_read_lock();
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entries = rcu_dereference(ids->entries);
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if(lid >= entries->size) {
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rcu_read_unlock();
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return NULL;
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}
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out = entries->p[lid];
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if(out == NULL) {
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rcu_read_unlock();
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return NULL;
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}
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spin_lock(&out->lock);
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/* ipc_rmid() may have already freed the ID while ipc_lock
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* was spinning: here verify that the structure is still valid
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*/
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if (out->deleted) {
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spin_unlock(&out->lock);
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rcu_read_unlock();
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return NULL;
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}
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return out;
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}
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Answer to Quick Quiz:
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The reason that it is important that updates be rare when
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using seqlock is that frequent updates can livelock readers.
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One way to avoid this problem is to assign a seqlock for
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each array entry rather than to the entire array.
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