GNU bug report logs - #55356
join-thread unusable if timeout was hit

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Message #5 received at submit <at> debbugs.gnu.org (full text, mbox):

From: angry rectangle <angryrectangle <at> cock.li>
To: bug-guile <at> gnu.org
Subject: join-thread unusable if timeout was hit
Date: Tue, 10 May 2022 20:42:03 -0400

If join-thread is called after a previous call to join-thread hits the timeout, a "mutex already locked by thread" error is thrown.

Working example:

(use-modules (ice-9 threads))
(define t (call-with-new-thread
           (lambda _
             (let r ()
               (sleep 1)
               (r)))))
(join-thread t (current-time))
(join-thread t (current-time)) ;; exception here

my guile version: latest git. cc455976813ab94de121395982435430874cbf58
my OS: guix on amd64

Message #8 received at 55356 <at> debbugs.gnu.org (full text, mbox):

From: Olivier Dion <olivier.dion <at> polymtl.ca>
To: 55356 <at> debbugs.gnu.org
Cc: Olivier Dion <olivier-dion <at> proton.me>
Subject: [PATCH] Always release thread data mutex in join-thread.
Date: Sun, 16 Oct 2022 11:36:02 -0400

From: Olivier Dion <olivier-dion <at> proton.me>

Currently the mutex is only unlocked when results are available.
However, it is not unlocked when we get a timeout from the condition
variable.

* module/ice-9/threads.scm (join-thread): Use with-mutex to ensure that
the thread data mutex is always unlocked.
---
 module/ice-9/threads.scm | 23 +++++++++++------------
 1 file changed, 11 insertions(+), 12 deletions(-)

diff --git a/module/ice-9/threads.scm b/module/ice-9/threads.scm
index c42bd266f..8993596e4 100644
--- a/module/ice-9/threads.scm
+++ b/module/ice-9/threads.scm
@@ -186,18 +186,17 @@ terminates, unless the target @var{thread} has already terminated."
   (match (thread-join-data thread)
     (#f (error "foreign thread cannot be joined" thread))
     ((cv . mutex)
-     (lock-mutex mutex)
-     (let lp ()
-       (cond
-        ((%thread-results cv)
-         => (lambda (results)
-              (unlock-mutex mutex)
-              (apply values results)))
-        ((if timeout
-             (wait-condition-variable cv mutex timeout)
-             (wait-condition-variable cv mutex))
-         (lp))
-        (else timeoutval))))))
+     (with-mutex mutex
+       (let lp ()
+         (cond
+          ((%thread-results cv)
+           => (lambda (results)
+                (apply values results)))
+          ((if timeout
+               (wait-condition-variable cv mutex timeout)
+               (wait-condition-variable cv mutex))
+           (lp))
+          (else timeoutval)))))))
 
 (define* (try-mutex mutex)
   "Try to lock @var{mutex}.  If the mutex is already locked, return
-- 
2.37.3

Message #13 received at 55356-done <at> debbugs.gnu.org (full text, mbox):

From: Ludovic Courtès <ludo <at> gnu.org>
To: Olivier Dion <olivier.dion <at> polymtl.ca>
Cc: 55356-done <at> debbugs.gnu.org, 61058-done <at> debbugs.gnu.org
Subject: Re: bug#61058: [PATCH v2] Fix asymetric mutex locking when joining
 thread.
Date: Thu, 25 Jan 2024 23:14:26 +0100

Hello,

Olivier Dion <olivier.dion <at> polymtl.ca> skribis:

> If `join-thread' timeout, the thread mutex is not unlocked, resulting in
> deadlock to the next call to it or deadlock of the thread itself when it
> terminates.
>
> Thus, always unlock the mutex.
>
> Fix: #55356
>
> * module/ice-9/threads.scm (join-thread): Always unlock thread mutex.
> * test-suite/tests/threads.test (join-thread): New test to ensure the
> mutex is released

Pushed as 455ee49f5573baa1bc5237a8d49083ce588a13ee with a ‘NEWS’ entry
and an additional comment in the test.

Thanks!

Ludo’.

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