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Re: Possible issue with mutexes

From: Herwig Hochleitner
Subject: Re: Possible issue with mutexes
Date: Wed, 10 Apr 2019 18:15:24 +0200

On 4/10/19 5:11 PM, Eli Zaretskii wrote:

[Why private email?]
I hit the wrong reply button, sorry.
From: Herwig Hochleitner <address@hidden>
Date: Wed, 10 Apr 2019 17:02:35 +0200

Waiting on a condition var or joining a thread on the main thread can
freeze emacs totally dead. Like kill -9 dead.

Is this a known issue or should I investigate further?
Not sure if I understand correctly the situation you describe.  Would
it be possible to post a simple reproducer?

Try evaluating this snippet (maybe not in your main emacs) and try
quitting out of the eval:

(let* ((m (make-mutex))
         (c (make-condition-variable m)))
    (with-mutex m
      (condition-wait c)))
How is the above "joining a thread"?

Joining a thread shows similar behavior, where you can't quit via C-g anymore.

  (lambda ()
    (let* ((m (make-mutex))
           (c (make-condition-variable m)))
      (with-mutex m
        (condition-wait c))))))

Anyway, I think what you did is wrote code that deliberately
deadlocks, so why are you surprised it does?

That's because this is a minimal example. I'm not surprised by it anymore ;-)

I hit this, when working on somewhat more involved code https://github.com/NixOS/nixpkgs/blob/f4d71836af5dd99b5af32ace6d1775b91213858e/pkgs/applications/editors/emacs-modes/update-melpa.el

Theres no other thread
to signal the condition variable or signal the waiting thread.

IOW, this is the expected behavior.
Yes, pardon, I'm so used to event-driven programming, I forgot that in Unix, everything is pull

What I'm trying to say: It would be nice, if C-g could also release pending condition-waits and thread-joins.

I understand, that this might be kind of a hard thing to ask, due to the lack of a separate input thread, as you say, but I'm not familiar with the internals, so I'm asking it.

kind regards

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