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Remove the now-unused ISC_STACK

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We are using the liburcu concurrent data structures instead.
This commit is contained in:
Tony Finch
2023-05-12 12:20:42 +01:00
parent f11cc83142
commit fc770a8bd0
4 changed files with 4 additions and 229 deletions
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doc/dev/dev.md
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@@ -657,45 +657,6 @@ Items can be removed from the list using `ISC_LIST_UNLINK`:
ISC_LIST_UNLINK(foolist, foo, link);
##### Atomic stacks
In `<isc/stack.h>`, there are also similar macros for singly-linked
stacks (`ISC_STACK`) and atomic stacks (`ISC_ASTACK`).
Lock-free linked data structures have pitfalls that the `ISC_ASTACK`
macros avoid by having a restricted API. Firstly, it is difficult to
implement atomic doubly-linked lists without at least a double-width
compare-exchange, which is not a widely supported primitive, so this
stack is singly-linked. Secondly, when individual elements can be
removed from a list (whether doubly-linked or singly-linked), we run
into the ABA problem, where removes and (re-)inserts race and as a
result the links get in a tangle. Safe support for removing individual
elements requires some kind of garbage collection, or extending link
pointers with generation counters. Instead the ASTACK macros only
provide a function for emptying the entire stack in one go,
`ISC_ASTACK_TO_STACK()`. This only requires setting the `top` pointer to
`NULL`, which cannot cause `ISC_ASTACK_PUSH()` to go wrong.
The `ISC_ASTACK` macros are used the same way as the non-atomic
doubly-linked lists described above, except that not all of the
`ISC_LIST` macros have `ISC_ASTACK` equivalents, and stacks have a
`TOP` instead of a `HEAD`.
As well as read-only iteration similar to `ISC_LIST` described above,
there is an idiom for emptying an atomic stack into a regular stack,
and processing its contents:
ISC_STACK(isc_foo_t) drain = ISC_ASTACK_TO_STACK(foo_work);
while (!ISC_STACK_EMPTY(drain)) {
isc_foo_t *foo = ISC_STACK_POP(drain, link);
/* do things */
/* maybe again? ISC_ASTACK_PUSH(foo_work, foo, link) */
}
There is also an `ISC_ASTACK_ADD()` macro, which is a convenience
wrapper for pushing an element onto a stack if it has not already been
added.
#### <a name="names"></a>Names
The `dns_name` API has facilities for processing DNS names and labels,