Re: PR lib/59751: dlclose is not MT-safe depending on the libraries unloaded

Hi,

On Thursday 16 Jul 2026, at 00:03, Taylor R Campbell wrote:
> Can you try the attached patch and see if it works in your complex
> application?

Thanks ! It fixes the recursive behaviour yes.
At least with my quick testing, the application does not deadlock anymore.

Now, I'm back to my original problem though (before your changes,
ld.elf_so around Jan 22 this year), which is a segfault in dclose(),
deterministic, although the backtrace can vary depending on the code or
if I prevent some libraries from dlclos'ing. So it was looking really
similar to the 59751 issue, but it's actually not.

I don't think the application does something wrong. It works under
linux (which is not a reason, but I also inspected the code a
bit). The problem is that it's a really huge code base ...

I not sure how to trace this. Next step is probably compiling
ld.elf_so with debug and step through...

Just FYI, the backtrace of thread 1 is in unloaded code, at one can
see by printing the address _before_ the segfault:

(gdb) b dlclose
(gdb) r
Thread 1 "" hit Breakpoint 1.2, 0x00007f7ff7ef26cb in dlclose ()
   from /usr/libexec/ld.elf_so
(gdb) info sym 0x00007f7ff5c211d0
ament_index_cpp::get_resource(std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const&, std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const&, std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >&, std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >*) + 499 in section .text of /home/tho/openrobots/x86_64-netbsd11/lib/libament_index_cpp.so
(gdb) c
Continuing.

Thread 1 "" hit Breakpoint 1.2, 0x00007f7ff7ef26cb in dlclose ()
   from /usr/libexec/ld.elf_so
(gdb) c
Continuing.
[LWP 998 of process 25222 exited]

Thread 1 "" received signal SIGSEGV, Segmentation fault.
0x00007f7ff5c321d0 in ?? ()
(gdb) thr app all bt

Thread 2 (process 1493):
#0  0x00007f7ff78c126a in _sys___select50 () from /usr/lib/libc.so.12
#1  0x00007f7ff784d24a in __select50 () from /usr/lib/libpthread.so.1
#2  0x00007f7ff7e7dab9 in NotifierThreadProc () from /usr/pkg-2025Q3/lib/libtcl86.so
#3  0x00007f7ff7851383 in ?? () from /usr/lib/libpthread.so.1
#4  0x00007f7ff7917de0 in ?? () from /usr/lib/libc.so.12
#5  0x0000000000200000 in ?? ()
#6  0x0000000000000000 in ?? ()

Thread 1 (process 372):
#0  0x00007f7ff5c211d0 in ?? ()
#1  0x00007f7ff77cbe90 in ?? ()
#2  0x00007f7ff7766780 in ?? ()
#3  0x00007f7fffffd150 in ?? ()
#4  0x00007f7ff5f903e6 in std::destroy_at<rcpputils::SharedLibrary> (__location=0x7f7ff7766780) at /usr/include/g++/bits/stl_construct.h:88
Backtrace stopped: frame did not save the PC