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Subject: [Bug 285612] Qt thread deadlocks on destruction after fork()
Date: Sun, 23 Mar 2025 17:24:42 +0000
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https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=3D285612
Bug ID: 285612
Summary: Qt thread deadlocks on destruction after fork()
Product: Base System
Version: CURRENT
Hardware: Any
OS: Any
Status: New
Severity: Affects Only Me
Priority: ---
Component: threads
Assignee: threads@FreeBSD.org
Reporter: arrowd@FreeBSD.org
Created attachment 258950
--> https://bugs.freebsd.org/bugzilla/attachment.cgi?id=3D258950&action=
=3Dedit
Testcase program
This bug was haunting me since Qt 5, but only now I was able to derive a
minimal testcase. I tried to replicate what Qt does to minimize even more, =
but
failed to do that - too much complex stuff contributes to this bug: forking,
threads, thread-local storage and __cxa_atexit.
When the test program is ran as
./a.out fork stuff
the program successfully terminates. However, running it as
./a.out stuff fork
makes the child hang in QDBusConnectionManager::~QDBusConnectionManager [1]=
. A
bit of debugging lead me to [2], which is an exceptionally complex piece of
code. By putting prints here and there I found out that the function passed
into pthread_cleanup_push in [3] does not get called in the 'stuff-fork' ca=
se.
Not sure if it's a problem or a symptom.
'fork-stuff':
set_thread_data(non-null)
destroy_current_thread_data
set_thread_data(non-null)
pthread_cleanup
destroy_current_thread_data
destroy_current_thread_data
set_thread_data(null)
'stuff-fork':
set_thread_data(non-null)
set_thread_data(non-null)
Since debugging this issue would require building Qt with debugging enabled,
here's a recipe on how to get a debugging env with Poudriere:
echo WITH_DEBUG_PORTS=3Ddevel/qt6-base >> /usr/local/etc/poudriere.d/make.c=
onf
poudriere testport -j yourjail -p yourports -i devel/qt6-base
Then inside the jail:
fetch https://arrowd.name/t.cpp
head t.cpp (to get a c++ command line to compile the program
To apply some changes to Qt itself:
$EDITOR /wrkdirs/usr/ports/devel/qt6-base/work/...
make WITH_DEBUG=3Dyes restage reinstall
[1]
https://github.com/qt/qtbase/blob/v6.8.2/src/dbus/qdbusconnectionmanager.cp=
p#L117
[2]
https://github.com/qt/qtbase/blob/v6.8.2/src/corelib/thread/qthread_unix.cp=
p#L82
[3]
https://github.com/qt/qtbase/blob/v6.8.2/src/corelib/thread/qthread_unix.cp=
p#L338
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