elisp test-and-set?

elisp test-and-set?

[Stephen Leake]

In ada-mode, I'm using a parser running in an external process to
compute faces and indentation.

Since faces are computed by font-lock, which runs on a timer in the
background, it can try to run when indent is running in the foreground.

So in the code that deals with the external process, I need mutual
exclusion; prevent font-lock from starting a new parse if a parse is
already running.

The code I currently have for that is:

  (if (wisi-process--parser-busy parser)
      (error "%s parse abandoned; parser busy" wisi--parse-action)

    (setf (wisi-process--parser-busy parser) t)
    ... more parsing stuff
    )

There is a tiny window of time when this can fail; if font-lock and
foreground both execute the "if" condition 'simultaneously' (ie, before
the other sets busy t), both will go on to parse.

In other contexts I'd use a "test and set" instruction for this; read
the flag and set it true in one uninterruptible action. I don't see
anything like that in elisp, and searching in the elisp manual for
"mutual" and "exclusion" is no help.

I also don't see "font-lock-inhibit", which could be useful here, but would
probably have the same race condition.

Is there a way to do this right?

-- 
-- Stephe

Re: elisp test-and-set?

[Noam Postavsky]

On 25 May 2018 at 17:00, Stephen Leake <stephen_leake@stephe-leake.org> wrote:

> Since faces are computed by font-lock, which runs on a timer in the
> background, it can try to run when indent is running in the foreground.

Only if the foreground indent function calls `accept-process-output',
`sit-for', or something like that.

> In other contexts I'd use a "test and set" instruction for this; read
> the flag and set it true in one uninterruptible action. I don't see
> anything like that in elisp, and searching in the elisp manual for
> "mutual" and "exclusion" is no help.

There is `(elisp) Mutexes' (as of Emacs 26), but even with threads
enabled, only a single Lisp thread runs at one time, so there's not
really a need for it at the moment.

Re: elisp test-and-set?

[Eli Zaretskii]

> From: Stephen Leake <stephen_leake@stephe-leake.org>
> Date: Fri, 25 May 2018 16:00:04 -0500
> 
> In ada-mode, I'm using a parser running in an external process to
> compute faces and indentation.
> 
> Since faces are computed by font-lock, which runs on a timer in the
> background, it can try to run when indent is running in the foreground.

Do you have evidence of this?  Emacs is single-threaded, so only one
Lisp function can run at any given time.  I'm guessing you saw
something that led you to above conclusion, but I think the conclusion
is at least inaccurate if not incorrect.  So please tell more detail
about what you see, and probably about the implementation of that
parser.  E.g., are you running the parser from font-lock, or are you
running it from some other hook?

Re: elisp test-and-set?

[Stephen Leake]

Eli Zaretskii <eliz@gnu.org> writes:

>> From: Stephen Leake <stephen_leake@stephe-leake.org>
>> Date: Fri, 25 May 2018 16:00:04 -0500
>>
>> In ada-mode, I'm using a parser running in an external process to
>> compute faces and indentation.
>>
>> Since faces are computed by font-lock, which runs on a timer in the
>> background, it can try to run when indent is running in the foreground.
>
> Do you have evidence of this?

I thought I did, but it turned out there was a different problem.

> Emacs is single-threaded, so only one Lisp function can run at any
> given time.

Ok, I'll add a comment to this effect in the code.

Thanks.

--
-- Stephe

Re: elisp test-and-set?

[Stefan Monnier]

> Do you have evidence of this?  Emacs is single-threaded, so only one
> Lisp function can run at any given time.  I'm guessing you saw

And even with the new concurrency support, while it's not
quite single-threaded any more, its threading is cooperative.


        Stefan