[racket-dev] BUG: busy-waiting

From: Marc Burns (m4burns at happierface.convextech.ca)
Date: Wed Sep 3 21:02:06 EDT 2014

The offending function in my case seems to be peek-bytes-avail!

The busy wait is entered on line 284 of collects/file/gunzip.rkt. Here's
what happens when I add some tracing to gunzip.rkt and then try to
inflate a Racket pipe:

+           (displayln "before read-bytes!")
            (read-bytes! buffer input-port 0 (max 0 (- buf-max MAX-LOOKAHEAD)))
+           (displayln "after read-bytes!")
            ;; Even though we won't actually use bytes that we "unwind",
            ;; setting `buf-pos' to the number of unwound bytes lets us
            ;; keep track of how much to not actually read at the end.
            (set! buf-pos (min MAX-LOOKAHEAD buf-max)))
          ;; Peek (not read) some available bytes:
+         (displayln "before peek-bytes-avail!")
          (let ([got (peek-bytes-avail! buffer buf-pos #f input-port buf-pos BUFFER-SIZE)])
+           (displayln "after peek-bytes-avail!")
+  (trace READBITS)


before read-bytes!
after read-bytes!
before peek-bytes-avail!

That's it. Racket is busy-waiting in peek-bytes-avail!

On Wed, Sep 03, 2014 at 10:52:18PM +0200, Jan Dvořák wrote:
> Hello,
> I am hitting a rather uncomfortable bug that causes runtime to start
> internal busy-waiting at around:
> #0  scheme_block_until(_f=<syncing_ready>,
> fdf=<scheme_syncing_needs_wakeup>)
>    at ../src/thread.c:5199
> #1  do_sync (name="sync", with_break=0, with_timeout=0)
>    at ../src/thread.c:7109
> It goes through ../src/thread.c:5190, which Matthew mentions in one of his
> recent patches.
> The code that manages to trigger this have been written under NDA and cannot
> be published. I have not yet managed to reproduce the issue separately, but
> it seems that this might not be the only instance:
>    (09:28:36 PM) m4burns: Mordae: I'm having a similar problem somewhere in
>                           `inflate`. Haven't applied a debugger yet, but
>                           there's certainly no busy waiting in the script.
> I am stuck and would like to ask for your help.
> Best regards,
> Jan Dvorak
> _________________________
>  Racket Developers list:
>  http://lists.racket-lang.org/dev

Posted on the dev mailing list.