mbuf: introduce routine to get private mbuf pool flags

[dpdk.git] / doc / guides / prog_guide / stack_lib.rst

diff --git a/doc/guides/prog_guide/stack_lib.rst b/doc/guides/prog_guide/stack_lib.rst
index 25a8cc3..8fe8804 100644 (file)
--- a/doc/guides/prog_guide/stack_lib.rst
+++ b/doc/guides/prog_guide/stack_lib.rst
@@ -10,7 +10,8 @@ stack of pointers.
 The stack library provides the following basic operations:
 
 *  Create a uniquely named stack of a user-specified size and using a
-   user-specified socket.
+   user-specified socket, with either standard (lock-based) or lock-free
+   behavior.
 
 *  Push and pop a burst of one or more stack objects (pointers). These function
    are multi-threading safe.
@@ -24,5 +25,59 @@ The stack library provides the following basic operations:
 Implementation
 ~~~~~~~~~~~~~~
 
-The stack consists of a contiguous array of pointers, a current index, and a
-spinlock. Accesses to the stack are made multi-thread safe by the spinlock.
+The library supports two types of stacks: standard (lock-based) and lock-free.
+Both types use the same set of interfaces, but their implementations differ.
+
+Lock-based Stack
+----------------
+
+The lock-based stack consists of a contiguous array of pointers, a current
+index, and a spinlock. Accesses to the stack are made multi-thread safe by the
+spinlock.
+
+Lock-free Stack
+------------------
+
+The lock-free stack consists of a linked list of elements, each containing a
+data pointer and a next pointer, and an atomic stack depth counter. The
+lock-free property means that multiple threads can push and pop simultaneously,
+and one thread being preempted/delayed in a push or pop operation will not
+impede the forward progress of any other thread.
+
+The lock-free push operation enqueues a linked list of pointers by pointing the
+list's tail to the current stack head, and using a CAS to swing the stack head
+pointer to the head of the list. The operation retries if it is unsuccessful
+(i.e. the list changed between reading the head and modifying it), else it
+adjusts the stack length and returns.
+
+The lock-free pop operation first reserves one or more list elements by
+adjusting the stack length, to ensure the dequeue operation will succeed
+without blocking. It then dequeues pointers by walking the list -- starting
+from the head -- then swinging the head pointer (using a CAS as well). While
+walking the list, the data pointers are recorded in an object table.
+
+The linked list elements themselves are maintained in a lock-free LIFO, and are
+allocated before stack pushes and freed after stack pops. Since the stack has a
+fixed maximum depth, these elements do not need to be dynamically created.
+
+The lock-free behavior is selected by passing the *RTE_STACK_F_LF* flag to
+rte_stack_create().
+
+Preventing the ABA Problem
+^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+To prevent the ABA problem, this algorithm stack uses a 128-bit
+compare-and-swap instruction to atomically update both the stack top pointer
+and a modification counter. The ABA problem can occur without a modification
+counter if, for example:
+
+1. Thread A reads head pointer X and stores the pointed-to list element.
+2. Other threads modify the list such that the head pointer is once again X,
+   but its pointed-to data is different than what thread A read.
+3. Thread A changes the head pointer with a compare-and-swap and succeeds.
+
+In this case thread A would not detect that the list had changed, and would
+both pop stale data and incorrect change the head pointer. By adding a
+modification counter that is updated on every push and pop as part of the
+compare-and-swap, the algorithm can detect when the list changes even if the
+head pointer remains the same.