diff options
| -rw-r--r-- | Documentation/RCU/arrayRCU.rst (renamed from Documentation/RCU/arrayRCU.txt) | 34 | ||||
| -rw-r--r-- | Documentation/RCU/index.rst | 1 |
2 files changed, 24 insertions, 11 deletions
diff --git a/Documentation/RCU/arrayRCU.txt b/Documentation/RCU/arrayRCU.rst index f05a9afb2c39..4051ea3871ef 100644 --- a/Documentation/RCU/arrayRCU.txt +++ b/Documentation/RCU/arrayRCU.rst @@ -1,19 +1,21 @@ -Using RCU to Protect Read-Mostly Arrays +.. _array_rcu_doc: +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 +1. :ref:`Hash Tables <hash_tables>` -2. Static Arrays +2. :ref:`Static Arrays <static_arrays>` -3. Resizeable Arrays +3. :ref:`Resizable Arrays <resizable_arrays>` Each of these three situations involves an RCU-protected pointer to an array that is separately indexed. It might be tempting to consider use of RCU to instead protect the index into an array, however, this use -case is -not- supported. The problem with RCU-protected indexes into +case is **not** supported. The problem with RCU-protected indexes into arrays is that compilers can play way too many optimization games with integers, which means that the rules governing handling of these indexes are far more trouble than they are worth. If RCU-protected indexes into @@ -24,16 +26,20 @@ to be safely used. That aside, each of the three RCU-protected pointer situations are described in the following sections. +.. _hash_tables: 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. +.. _static_arrays: 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, @@ -41,13 +47,17 @@ 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? +Quick Quiz: + Why is it so important that updates be rare when using seqlock? + +:ref:`Answer to Quick Quiz <answer_quick_quiz_seqlock>` +.. _resizable_arrays: -Situation 3: Resizeable Arrays +Situation 3: Resizable Arrays +------------------------------ -Use of RCU for resizeable arrays is demonstrated by the grow_ary() +Use of RCU for resizable arrays is demonstrated by the grow_ary() function formerly 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() @@ -60,7 +70,7 @@ 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. +you are running on:: static int grow_ary(struct ipc_ids* ids, int newsize) { @@ -112,7 +122,7 @@ 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. +deleted, and, if not, the pointer is returned:: struct kern_ipc_perm* ipc_lock(struct ipc_ids* ids, int id) { @@ -144,8 +154,10 @@ deleted, and, if not, the pointer is returned. return out; } +.. _answer_quick_quiz_seqlock: Answer to Quick Quiz: + Why is it so important that updates be rare when using seqlock? The reason that it is important that updates be rare when using seqlock is that frequent updates can livelock readers. diff --git a/Documentation/RCU/index.rst b/Documentation/RCU/index.rst index 5c99185710fa..8d20d44f8fd4 100644 --- a/Documentation/RCU/index.rst +++ b/Documentation/RCU/index.rst @@ -7,6 +7,7 @@ RCU concepts .. toctree:: :maxdepth: 3 + arrayRCU rcu listRCU UP |