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reference: document possibility of spurious CAS failures

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An operation like `box-cas!` can fail spuriously on some platforms,
such as ARM processors. Mention that in the documentation.

The `box-cas!` and similar implementations could instead retry on
spurious failure, and that would avoid potential problems with code
tested on x86, where there's no spurious failure. In this case,
though, it seems better to stick with the existing behavior and
stay closer to the machine operation.
This commit is contained in:
Matthew Flatt 2020-06-27 13:55:09 -06:00
parent a186e0070a
commit 221408cb34
1 changed files with 9 additions and 3 deletions
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12
pkgs/racket-doc/scribblings/reference/data.scrbl
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@ -120,10 +120,16 @@ boxes that are not @tech{impersonators}.
@racketblock[
(and (eq? old (unbox loc)) (set-box! loc new) #t)]
except that @racket[box-cas!] can spuriously fail on some platforms.
That is, with low probability, the result can be @racket[#f] with the
value in @racket[box] left unchanged, even if @racket[box] contains
@racket[old].
When Racket is compiled with support for @tech{futures},
@racket[box-cas!] uses a hardware @emph{compare and set} operation.
Uses of @racket[box-cas!] be performed safely in a @tech{future} (i.e.,
allowing the future thunk to continue in parallel). }
@racket[box-cas!] is guaranteed to use a hardware @emph{compare and
set} operation. Uses of @racket[box-cas!] be performed safely in a
@tech{future} (i.e., allowing the future thunk to continue in
parallel).}
@; ----------------------------------------------------------------------
@include-section["hashes.scrbl"]