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+Note that the AO_stack implementation is licensed under the GPL,
+unlike the lower level routines.
+
+The header file atomic_ops_stack.h defines a linked stack abstraction.
+Stacks may be accessed by multiple concurrent threads.  The implementation
+is 1-lock-free, i.e. it will continue to make progress if at most one
+thread becomes inactive while operating on the data structure.
+
+(The implementation can be built to be N-lock-free for any given N.  But that
+seems to rarely be useful, especially since larger N involve some slowdown.)
+
+This makes it safe to access these data structures from non-reentrant
+signal handlers, provided at most one non-signal-handler thread is
+accessing the data structure at once.  This latter condition can be
+ensured by acquiring an ordinary lock around the non-handler accesses
+to the data structure.
+
+For details see:
+
+Hans-J. Boehm, "An Almost Non-Blocking Stack", PODC 2004,
+http://portal.acm.org/citation.cfm?doid=1011767.1011774, or
+http://www.hpl.hp.com/techreports/2004/HPL-2004-105.html
+(This is not exactly the implementation described there, since the
+interface was cleaned up in the interim.  But it should perform
+very similarly.)
+
+We use a fully lock-free implementation when the underlying hardware
+makes that less expensive, i.e. when we have a double-wide compare-and-swap
+operation available.  (The fully lock-free implementation uses an AO_t-
+sized version count, and assumes it does not wrap during the time any
+given operation is active.  This seems reasonably safe on 32-bit hardware,
+and very safe on 64-bit hardware.) If a fully lock-free implementation
+is used, the macro AO_STACK_IS_LOCK_FREE will be defined.
+
+The implementation is interesting only because it allows reuse of
+existing nodes.  This is necessary, for example, to implement a memory
+allocator.
+
+Since we want to leave the precise stack node type up to the client,
+we insist only that each stack node contains a link field of type AO_t.
+When a new node is pushed on the stack, the push operation expects to be
+passed the pointer to this link field, which will then be overwritten by
+this link field.  Similarly, the pop operation returns a pointer to the
+link field of the object that previously was on the top of the stack.
+
+The cleanest way to use these routines is probably to define the stack node
+type with an initial AO_t link field, so that the conversion between the
+link-field pointer and the stack element pointer is just a compile-time
+cast.  But other possibilities exist.  (This would be cleaner in C++ with
+templates.)
+
+A stack is represented by an AO_stack_t structure.  (This is normally
+2 or 3 times the size of a pointer.)  It may be statically initialized
+by setting it to AO_STACK_INITIALIZER, or dynamically initialized to
+an empty stack with AO_stack_init.  There are only three operations for
+accessing stacks:
+
+void AO_stack_init(AO_stack_t *list);
+void AO_stack_push_release(AO_stack_t *list, AO_t *new_element);
+AO_t * AO_stack_pop_acquire(volatile AO_stack_t *list);
+
+We require that the objects pushed as list elements remain addressable
+as long as any push or pop operation are in progress.  (It is OK for an object
+to be "pop"ped off a stack and "deallocated" with a concurrent "pop" on
+the same stack still in progress, but only if "deallocation" leaves the
+object addressable.  The second "pop" may still read the object, but
+the value it reads will not matter.)
+
+We require that the headers (AO_stack objects) remain allocated and
+valid as long as any operations on them are still in-flight.
+
+We also provide macros AO_REAL_HEAD_PTR that converts an AO_stack_t
+to a pointer to the link field in the next element, and AO_REAL_NEXT_PTR
+that converts a link field to a real, dereferencable, pointer to the link field
+in the next element.  This is intended only for debugging, or to traverse
+the list after modification has ceased.  There is otherwise no guarantee that
+walking a stack using this macro will produce any kind of consistent
+picture of the data structure.