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

(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.