It happened again so I collected some information about it.

top - 06:41:08 up 7 days, 23:05,  1 user,  load average: 6.87, 6.31, 6.22
Tasks: 129 total,   1 running, 128 sleeping,   0 stopped,   0 zombie
%Cpu(s): 64.8 us,  0.0 sy,  0.0 ni, 23.9 id,  0.0 wa,  0.0 hi,  0.0 si, 11.3 st
KiB Mem:   1012108 total,   970104 used,    42004 free,    26140 buffers
KiB Swap:   262140 total,     7480 used,   254660 free.   660712 cached Mem

  PID USER      PR  NI    VIRT    RES    SHR S  %CPU %MEM     TIME+ COMMAND                                                     
 7046 www-data  20   0 2041404  29832   5604 S 200.1  2.9   1036:38 /usr/sbin/apache2 -k start                                  
 7230 www-data  20   0 2040752  31604   4932 S 200.1  3.1   3767:03 /usr/sbin/apache2 -k start                                  
17441 www-data  20   0 2036472  20124   4828 S 100.0  2.0   2532:40 /usr/sbin/apache2 -k start                                  
22281 www-data  20   0 2032484  16444   5432 S 100.0  1.6   1067:03 /usr/sbin/apache2 -k start                                  
 5493 root      20   0   24980   3020   2516 R   1.0  0.3   0:00.28 top                                                         
    7 root      20   0       0      0      0 S   0.3  0.0  37:09.72 [rcu_sched]                                                 
  734 root      20   0       0      0      0 S   0.3  0.0   0:00.14 [kworker/u16:0]                                             
...

Stracing them does not suggest anything:

root@developers:~# strace -p 7046
Process 7046 attached
read(14, ^CProcess 7046 detached
 <detached ...>
root@developers:~# strace -p 7230
Process 7230 attached
futex(0x7f70fa7ec9d0, FUTEX_WAIT, 7257, NULL^CProcess 7230 detached
 <detached ...>
root@developers:~# strace -p 17441
Process 17441 attached
futex(0x7f70fa7ec9d0, FUTEX_WAIT, 17468, NULL^CProcess 17441 detached
 <detached ...>
root@developers:~# strace -p 22281
Process 22281 attached
futex(0x7f70fa7ec9d0, FUTEX_WAIT, 22334, NULL^CProcess 22281 detached
 <detached ...>

I waited 5-10 seconds before doing ^C.

root@developers:~# gdb /usr/sbin/apache2 7046
GNU gdb (Ubuntu 7.7-0ubuntu3.1) 7.7
Copyright (C) 2014 Free Software Foundation, Inc.
License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html>
This is free software: you are free to change and redistribute it.
There is NO WARRANTY, to the extent permitted by law.  Type "show copying"
and "show warranty" for details.
This GDB was configured as "x86_64-linux-gnu".
Type "show configuration" for configuration details.
For bug reporting instructions, please see:
<http://www.gnu.org/software/gdb/bugs/>.
Find the GDB manual and other documentation resources online at:
<http://www.gnu.org/software/gdb/documentation/>.
For help, type "help".
Type "apropos word" to search for commands related to "word"...
Reading symbols from /usr/sbin/apache2...Reading symbols from /usr/lib/debug//usr/sbin/apache2...done.
done.
Attaching to program: /usr/sbin/apache2, process 7046
Reading symbols from /lib/x86_64-linux-gnu/libpcre.so.3...(no debugging symbols found)...done.
Loaded symbols for /lib/x86_64-linux-gnu/libpcre.so.3
Reading symbols from /usr/lib/x86_64-linux-gnu/libaprutil-1.so.0...(no debugging symbols found)...done.
Loaded symbols for /usr/lib/x86_64-linux-gnu/libaprutil-1.so.0
Reading symbols from /usr/lib/x86_64-linux-gnu/libapr-1.so.0...(no debugging symbols found)...done.
Loaded symbols for /usr/lib/x86_64-linux-gnu/libapr-1.so.0
Reading symbols from /lib/x86_64-linux-gnu/libpthread.so.0...Reading symbols from /usr/lib/debug//lib/x86_64-linux-gnu/libpthread-2.19.so...done.
done.
[New LWP 7073]
[New LWP 7072]
[New LWP 7071]
[New LWP 7070]
[New LWP 7069]
[New LWP 7068]
[New LWP 7067]
[New LWP 7066]
[New LWP 7065]
[New LWP 7064]
[New LWP 7063]
[New LWP 7062]
[New LWP 7061]
[New LWP 7060]
[New LWP 7059]
[New LWP 7058]
[New LWP 7057]
[New LWP 7056]
[New LWP 7055]
[New LWP 7054]
[New LWP 7053]
[New LWP 7052]
[New LWP 7051]
[New LWP 7050]
[New LWP 7049]
[New LWP 7048]
[Thread debugging using libthread_db enabled]
Using host libthread_db library "/lib/x86_64-linux-gnu/libthread_db.so.1".
Loaded symbols for /lib/x86_64-linux-gnu/libpthread.so.0
Reading symbols from /lib/x86_64-linux-gnu/libc.so.6...Reading symbols from /usr/lib/debug//lib/x86_64-linux-gnu/libc-2.19.so...done.
done.
Loaded symbols for /lib/x86_64-linux-gnu/libc.so.6
Reading symbols from /lib/x86_64-linux-gnu/libcrypt.so.1...Reading symbols from /usr/lib/debug//lib/x86_64-linux-gnu/libcrypt-2.19.so...done.
done.
Loaded symbols for /lib/x86_64-linux-gnu/libcrypt.so.1
Reading symbols from /lib/x86_64-linux-gnu/libexpat.so.1...(no debugging symbols found)...done.
Loaded symbols for /lib/x86_64-linux-gnu/libexpat.so.1
Reading symbols from /lib/x86_64-linux-gnu/libuuid.so.1...(no debugging symbols found)...done.
Loaded symbols for /lib/x86_64-linux-gnu/libuuid.so.1
Reading symbols from /lib/x86_64-linux-gnu/libdl.so.2...Reading symbols from /usr/lib/debug//lib/x86_64-linux-gnu/libdl-2.19.so...done.
done.
Loaded symbols for /lib/x86_64-linux-gnu/libdl.so.2
Reading symbols from /lib64/ld-linux-x86-64.so.2...Reading symbols from /usr/lib/debug//lib/x86_64-linux-gnu/ld-2.19.so...done.
done.
Loaded symbols for /lib64/ld-linux-x86-64.so.2
Reading symbols from /lib/x86_64-linux-gnu/libnss_compat.so.2...Reading symbols from /usr/lib/debug//lib/x86_64-linux-gnu/libnss_compat-2.19.so...done.
done.
Loaded symbols for /lib/x86_64-linux-gnu/libnss_compat.so.2
Reading symbols from /lib/x86_64-linux-gnu/libnsl.so.1...Reading symbols from /usr/lib/debug//lib/x86_64-linux-gnu/libnsl-2.19.so...done.
done.
Loaded symbols for /lib/x86_64-linux-gnu/libnsl.so.1
Reading symbols from /lib/x86_64-linux-gnu/libnss_nis.so.2...Reading symbols from /usr/lib/debug//lib/x86_64-linux-gnu/libnss_nis-2.19.so...done.
done.
Loaded symbols for /lib/x86_64-linux-gnu/libnss_nis.so.2
Reading symbols from /lib/x86_64-linux-gnu/libnss_files.so.2...Reading symbols from /usr/lib/debug//lib/x86_64-linux-gnu/libnss_files-2.19.so...done.
done.
Loaded symbols for /lib/x86_64-linux-gnu/libnss_files.so.2
Reading symbols from /lib/x86_64-linux-gnu/libnss_dns.so.2...Reading symbols from /usr/lib/debug//lib/x86_64-linux-gnu/libnss_dns-2.19.so...done.
done.
Loaded symbols for /lib/x86_64-linux-gnu/libnss_dns.so.2
Reading symbols from /lib/x86_64-linux-gnu/libresolv.so.2...Reading symbols from /usr/lib/debug//lib/x86_64-linux-gnu/libresolv-2.19.so...done.
done.
Loaded symbols for /lib/x86_64-linux-gnu/libresolv.so.2
Reading symbols from /usr/lib/x86_64-linux-gnu/apr-util-1/apr_dbm_db-1.so...(no debugging symbols found)...done.
Loaded symbols for /usr/lib/x86_64-linux-gnu/apr-util-1/apr_dbm_db-1.so
Reading symbols from /usr/lib/x86_64-linux-gnu/libdb-5.3.so...(no debugging symbols found)...done.
Loaded symbols for /usr/lib/x86_64-linux-gnu/libdb-5.3.so
Reading symbols from /usr/lib/apache2/modules/mod_access_compat.so...Reading symbols from /usr/lib/debug//usr/lib/apache2/modules/mod_access_compat.so...done.
done.
Loaded symbols for /usr/lib/apache2/modules/mod_access_compat.so
Reading symbols from /usr/lib/apache2/modules/mod_alias.so...Reading symbols from /usr/lib/debug//usr/lib/apache2/modules/mod_alias.so...done.
done.
Loaded symbols for /usr/lib/apache2/modules/mod_alias.so
Reading symbols from /usr/lib/apache2/modules/mod_auth_basic.so...Reading symbols from /usr/lib/debug//usr/lib/apache2/modules/mod_auth_basic.so...done.
done.
Loaded symbols for /usr/lib/apache2/modules/mod_auth_basic.so
Reading symbols from /usr/lib/apache2/modules/mod_authn_core.so...Reading symbols from /usr/lib/debug//usr/lib/apache2/modules/mod_authn_core.so...done.
done.
Loaded symbols for /usr/lib/apache2/modules/mod_authn_core.so
Reading symbols from /usr/lib/apache2/modules/mod_authn_file.so...Reading symbols from /usr/lib/debug//usr/lib/apache2/modules/mod_authn_file.so...done.
done.
Loaded symbols for /usr/lib/apache2/modules/mod_authn_file.so
Reading symbols from /usr/lib/apache2/modules/mod_authz_core.so...Reading symbols from /usr/lib/debug//usr/lib/apache2/modules/mod_authz_core.so...done.
done.
Loaded symbols for /usr/lib/apache2/modules/mod_authz_core.so
Reading symbols from /usr/lib/apache2/modules/mod_authz_host.so...Reading symbols from /usr/lib/debug//usr/lib/apache2/modules/mod_authz_host.so...done.
done.
Loaded symbols for /usr/lib/apache2/modules/mod_authz_host.so
Reading symbols from /usr/lib/apache2/modules/mod_authz_user.so...Reading symbols from /usr/lib/debug//usr/lib/apache2/modules/mod_authz_user.so...done.
done.
Loaded symbols for /usr/lib/apache2/modules/mod_authz_user.so
Reading symbols from /usr/lib/apache2/modules/mod_autoindex.so...Reading symbols from /usr/lib/debug//usr/lib/apache2/modules/mod_autoindex.so...done.
done.
Loaded symbols for /usr/lib/apache2/modules/mod_autoindex.so
Reading symbols from /usr/lib/apache2/modules/mod_deflate.so...Reading symbols from /usr/lib/debug//usr/lib/apache2/modules/mod_deflate.so...done.
done.
Loaded symbols for /usr/lib/apache2/modules/mod_deflate.so
Reading symbols from /lib/x86_64-linux-gnu/libz.so.1...(no debugging symbols found)...done.
Loaded symbols for /lib/x86_64-linux-gnu/libz.so.1
Reading symbols from /usr/lib/apache2/modules/mod_dir.so...Reading symbols from /usr/lib/debug//usr/lib/apache2/modules/mod_dir.so...done.
done.
Loaded symbols for /usr/lib/apache2/modules/mod_dir.so
Reading symbols from /usr/lib/apache2/modules/mod_env.so...Reading symbols from /usr/lib/debug//usr/lib/apache2/modules/mod_env.so...done.
done.
Loaded symbols for /usr/lib/apache2/modules/mod_env.so
Reading symbols from /usr/lib/apache2/modules/mod_filter.so...Reading symbols from /usr/lib/debug//usr/lib/apache2/modules/mod_filter.so...done.
done.
Loaded symbols for /usr/lib/apache2/modules/mod_filter.so
Reading symbols from /usr/lib/apache2/modules/mod_gnutls.so...(no debugging symbols found)...done.
Loaded symbols for /usr/lib/apache2/modules/mod_gnutls.so
Reading symbols from /usr/lib/x86_64-linux-gnu/libapr_memcache.so.0...(no debugging symbols found)...done.
Loaded symbols for /usr/lib/x86_64-linux-gnu/libapr_memcache.so.0
Reading symbols from /usr/lib/x86_64-linux-gnu/libgnutls.so.26...(no debugging symbols found)...done.
Loaded symbols for /usr/lib/x86_64-linux-gnu/libgnutls.so.26
Reading symbols from /lib/x86_64-linux-gnu/libgcrypt.so.11...(no debugging symbols found)...done.
Loaded symbols for /lib/x86_64-linux-gnu/libgcrypt.so.11
Reading symbols from /usr/lib/x86_64-linux-gnu/libtasn1.so.6...(no debugging symbols found)...done.
Loaded symbols for /usr/lib/x86_64-linux-gnu/libtasn1.so.6
Reading symbols from /usr/lib/x86_64-linux-gnu/libp11-kit.so.0...(no debugging symbols found)...done.
Loaded symbols for /usr/lib/x86_64-linux-gnu/libp11-kit.so.0
Reading symbols from /lib/x86_64-linux-gnu/libgpg-error.so.0...(no debugging symbols found)...done.
Loaded symbols for /lib/x86_64-linux-gnu/libgpg-error.so.0
Reading symbols from /usr/lib/x86_64-linux-gnu/libffi.so.6...(no debugging symbols found)...done.
Loaded symbols for /usr/lib/x86_64-linux-gnu/libffi.so.6
Reading symbols from /usr/lib/apache2/modules/mod_headers.so...Reading symbols from /usr/lib/debug//usr/lib/apache2/modules/mod_headers.so...done.
done.
Loaded symbols for /usr/lib/apache2/modules/mod_headers.so
Reading symbols from /usr/lib/apache2/modules/mod_mime.so...Reading symbols from /usr/lib/debug//usr/lib/apache2/modules/mod_mime.so...done.
done.
Loaded symbols for /usr/lib/apache2/modules/mod_mime.so
Reading symbols from /usr/lib/apache2/modules/mod_mpm_event.so...Reading symbols from /usr/lib/debug//usr/lib/apache2/modules/mod_mpm_event.so...done.
done.
Loaded symbols for /usr/lib/apache2/modules/mod_mpm_event.so
Reading symbols from /usr/lib/apache2/modules/mod_negotiation.so...Reading symbols from /usr/lib/debug//usr/lib/apache2/modules/mod_negotiation.so...done.
done.
Loaded symbols for /usr/lib/apache2/modules/mod_negotiation.so
Reading symbols from /usr/lib/apache2/modules/mod_rewrite.so...Reading symbols from /usr/lib/debug//usr/lib/apache2/modules/mod_rewrite.so...done.
done.
Loaded symbols for /usr/lib/apache2/modules/mod_rewrite.so
Reading symbols from /usr/lib/apache2/modules/mod_setenvif.so...Reading symbols from /usr/lib/debug//usr/lib/apache2/modules/mod_setenvif.so...done.
done.
Loaded symbols for /usr/lib/apache2/modules/mod_setenvif.so
Reading symbols from /usr/lib/apache2/modules/mod_status.so...Reading symbols from /usr/lib/debug//usr/lib/apache2/modules/mod_status.so...done.
done.
Loaded symbols for /usr/lib/apache2/modules/mod_status.so
Reading symbols from /lib/x86_64-linux-gnu/libgcc_s.so.1...(no debugging symbols found)...done.
Loaded symbols for /lib/x86_64-linux-gnu/libgcc_s.so.1
0x00007f71115bd3bd in read () at ../sysdeps/unix/syscall-template.S:81
81	../sysdeps/unix/syscall-template.S: No such file or directory.
(gdb) bt
#0  0x00007f71115bd3bd in read () at ../sysdeps/unix/syscall-template.S:81
#1  0x00007f71120e99f7 in read (__nbytes=1, __buf=0x7fff0691023b, __fd=<optimized out>)
    at /usr/include/x86_64-linux-gnu/bits/unistd.h:44
#2  ap_mpm_podx_check (pod=<optimized out>) at mpm_unix.c:535
#3  0x00007f710d55d0eb in child_main (child_num_arg=child_num_arg@entry=4) at event.c:2212
#4  0x00007f710d5618c2 in make_child (s=0x7f7112043de0, slot=slot@entry=4) at event.c:2299
#5  0x00007f710d561939 in startup_children (number_to_start=number_to_start@entry=1) at event.c:2325
#6  0x00007f710d5620e1 in server_main_loop (remaining_children_to_start=1) at event.c:2615
#7  event_run (_pconf=<optimized out>, plog=<optimized out>, s=<optimized out>) at event.c:2683
#8  0x00007f71120c469e in ap_run_mpm (pconf=0x7f7112073028, plog=0x7f7112047028, s=0x7f7112043de0) at mpm_common.c:96
#9  0x00007f71120bde36 in main (argc=3, argv=0x7fff06910688) at main.c:777
(gdb) bt full
#0  0x00007f71115bd3bd in read () at ../sysdeps/unix/syscall-template.S:81
No locals.
#1  0x00007f71120e99f7 in read (__nbytes=1, __buf=0x7fff0691023b, __fd=<optimized out>)
    at /usr/include/x86_64-linux-gnu/bits/unistd.h:44
No locals.
#2  ap_mpm_podx_check (pod=<optimized out>) at mpm_unix.c:535
        c = 18 '\022'
        fd = 14
        rc = <optimized out>
#3  0x00007f710d55d0eb in child_main (child_num_arg=child_num_arg@entry=4) at event.c:2212
        threads = 0x7f711387c7e0
        rv = <optimized out>
        ts = 0x7f7111fd5518
        thread_attr = 0x7f7111fd5538
        start_thread_id = 0x7f7111fd5598
#4  0x00007f710d5618c2 in make_child (s=0x7f7112043de0, slot=slot@entry=4) at event.c:2299
        pid = 0
#5  0x00007f710d561939 in startup_children (number_to_start=number_to_start@entry=1) at event.c:2325
        i = 4
#6  0x00007f710d5620e1 in server_main_loop (remaining_children_to_start=1) at event.c:2615
        status = 0
        pid = {pid = -1, in = 0x9, out = 0x7f7112073028, err = 0x7f71117dcff6}
        i = <optimized out>
        old_gen = 0
        child_slot = <optimized out>
        exitwhy = APR_PROC_EXIT
        processed_status = <optimized out>
#7  event_run (_pconf=<optimized out>, plog=<optimized out>, s=<optimized out>) at event.c:2683
        remaining_children_to_start = <optimized out>
#8  0x00007f71120c469e in ap_run_mpm (pconf=0x7f7112073028, plog=0x7f7112047028, s=0x7f7112043de0) at mpm_common.c:96
        pHook = 0x7f711200aa88
        n = 0
        rv = -512
#9  0x00007f71120bde36 in main (argc=3, argv=0x7fff06910688) at main.c:777
        c = 0 '\000'
        showcompile = 0
        showdirectives = 0
        confname = 0x7f71120fd607 "apache2.conf"
---Type <return> to continue, or q <return> to quit---
        def_server_root = 0x7f71120fd5fa "/etc/apache2"
        temp_error_log = 0x0
        error = <optimized out>
        process = 0x7f7112075118
        pconf = 0x7f7112073028
        plog = 0x7f7112047028
        ptemp = 0x7f7112039028
        pcommands = 0x7f7112051028
        opt = 0x7f7112051118
        rv = <optimized out>
        mod = 0x7f711231f160 <ap_prelinked_modules+64>
        opt_arg = 0x7f7112075028 "("
        signal_server = <optimized out>
(gdb) 

root@developers:~# gdb /usr/sbin/apache2 7230
GNU gdb (Ubuntu 7.7-0ubuntu3.1) 7.7
Copyright (C) 2014 Free Software Foundation, Inc.
License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html>
This is free software: you are free to change and redistribute it.
There is NO WARRANTY, to the extent permitted by law.  Type "show copying"
and "show warranty" for details.
This GDB was configured as "x86_64-linux-gnu".
Type "show configuration" for configuration details.
For bug reporting instructions, please see:
<http://www.gnu.org/software/gdb/bugs/>.
Find the GDB manual and other documentation resources online at:
<http://www.gnu.org/software/gdb/documentation/>.
For help, type "help".
Type "apropos word" to search for commands related to "word"...
Reading symbols from /usr/sbin/apache2...Reading symbols from /usr/lib/debug//usr/sbin/apache2...done.
done.
Attaching to program: /usr/sbin/apache2, process 7230
Reading symbols from /lib/x86_64-linux-gnu/libpcre.so.3...(no debugging symbols found)...done.
Loaded symbols for /lib/x86_64-linux-gnu/libpcre.so.3
Reading symbols from /usr/lib/x86_64-linux-gnu/libaprutil-1.so.0...(no debugging symbols found)...done.
Loaded symbols for /usr/lib/x86_64-linux-gnu/libaprutil-1.so.0
Reading symbols from /usr/lib/x86_64-linux-gnu/libapr-1.so.0...(no debugging symbols found)...done.
Loaded symbols for /usr/lib/x86_64-linux-gnu/libapr-1.so.0
Reading symbols from /lib/x86_64-linux-gnu/libpthread.so.0...Reading symbols from /usr/lib/debug//lib/x86_64-linux-gnu/libpthread-2.19.so...done.
done.
[New LWP 7257]
[New LWP 7256]
[New LWP 7255]
[New LWP 7254]
[New LWP 7253]
[New LWP 7252]
[New LWP 7251]
[New LWP 7250]
[New LWP 7249]
[New LWP 7248]
[New LWP 7247]
[New LWP 7246]
[New LWP 7245]
[New LWP 7244]
[New LWP 7243]
[New LWP 7242]
[New LWP 7241]
[New LWP 7240]
[New LWP 7239]
[New LWP 7238]
[New LWP 7237]
[New LWP 7236]
[New LWP 7235]
[New LWP 7234]
[New LWP 7233]
[New LWP 7232]
[Thread debugging using libthread_db enabled]
Using host libthread_db library "/lib/x86_64-linux-gnu/libthread_db.so.1".
Loaded symbols for /lib/x86_64-linux-gnu/libpthread.so.0
Reading symbols from /lib/x86_64-linux-gnu/libc.so.6...Reading symbols from /usr/lib/debug//lib/x86_64-linux-gnu/libc-2.19.so...done.
done.
Loaded symbols for /lib/x86_64-linux-gnu/libc.so.6
Reading symbols from /lib/x86_64-linux-gnu/libcrypt.so.1...Reading symbols from /usr/lib/debug//lib/x86_64-linux-gnu/libcrypt-2.19.so...done.
done.
Loaded symbols for /lib/x86_64-linux-gnu/libcrypt.so.1
Reading symbols from /lib/x86_64-linux-gnu/libexpat.so.1...(no debugging symbols found)...done.
Loaded symbols for /lib/x86_64-linux-gnu/libexpat.so.1
Reading symbols from /lib/x86_64-linux-gnu/libuuid.so.1...(no debugging symbols found)...done.
Loaded symbols for /lib/x86_64-linux-gnu/libuuid.so.1
Reading symbols from /lib/x86_64-linux-gnu/libdl.so.2...Reading symbols from /usr/lib/debug//lib/x86_64-linux-gnu/libdl-2.19.so...done.
done.
Loaded symbols for /lib/x86_64-linux-gnu/libdl.so.2
Reading symbols from /lib64/ld-linux-x86-64.so.2...Reading symbols from /usr/lib/debug//lib/x86_64-linux-gnu/ld-2.19.so...done.
done.
Loaded symbols for /lib64/ld-linux-x86-64.so.2
Reading symbols from /lib/x86_64-linux-gnu/libnss_compat.so.2...Reading symbols from /usr/lib/debug//lib/x86_64-linux-gnu/libnss_compat-2.19.so...done.
done.
Loaded symbols for /lib/x86_64-linux-gnu/libnss_compat.so.2
Reading symbols from /lib/x86_64-linux-gnu/libnsl.so.1...Reading symbols from /usr/lib/debug//lib/x86_64-linux-gnu/libnsl-2.19.so...done.
done.
Loaded symbols for /lib/x86_64-linux-gnu/libnsl.so.1
Reading symbols from /lib/x86_64-linux-gnu/libnss_nis.so.2...Reading symbols from /usr/lib/debug//lib/x86_64-linux-gnu/libnss_nis-2.19.so...done.
done.
Loaded symbols for /lib/x86_64-linux-gnu/libnss_nis.so.2
Reading symbols from /lib/x86_64-linux-gnu/libnss_files.so.2...Reading symbols from /usr/lib/debug//lib/x86_64-linux-gnu/libnss_files-2.19.so...done.
done.
Loaded symbols for /lib/x86_64-linux-gnu/libnss_files.so.2
Reading symbols from /lib/x86_64-linux-gnu/libnss_dns.so.2...Reading symbols from /usr/lib/debug//lib/x86_64-linux-gnu/libnss_dns-2.19.so...done.
done.
Loaded symbols for /lib/x86_64-linux-gnu/libnss_dns.so.2
Reading symbols from /lib/x86_64-linux-gnu/libresolv.so.2...Reading symbols from /usr/lib/debug//lib/x86_64-linux-gnu/libresolv-2.19.so...done.
done.
Loaded symbols for /lib/x86_64-linux-gnu/libresolv.so.2
Reading symbols from /usr/lib/x86_64-linux-gnu/apr-util-1/apr_dbm_db-1.so...(no debugging symbols found)...done.
Loaded symbols for /usr/lib/x86_64-linux-gnu/apr-util-1/apr_dbm_db-1.so
Reading symbols from /usr/lib/x86_64-linux-gnu/libdb-5.3.so...(no debugging symbols found)...done.
Loaded symbols for /usr/lib/x86_64-linux-gnu/libdb-5.3.so
Reading symbols from /usr/lib/apache2/modules/mod_access_compat.so...Reading symbols from /usr/lib/debug//usr/lib/apache2/modules/mod_access_compat.so...done.
done.
Loaded symbols for /usr/lib/apache2/modules/mod_access_compat.so
Reading symbols from /usr/lib/apache2/modules/mod_alias.so...Reading symbols from /usr/lib/debug//usr/lib/apache2/modules/mod_alias.so...done.
done.
Loaded symbols for /usr/lib/apache2/modules/mod_alias.so
Reading symbols from /usr/lib/apache2/modules/mod_auth_basic.so...Reading symbols from /usr/lib/debug//usr/lib/apache2/modules/mod_auth_basic.so...done.
done.
Loaded symbols for /usr/lib/apache2/modules/mod_auth_basic.so
Reading symbols from /usr/lib/apache2/modules/mod_authn_core.so...Reading symbols from /usr/lib/debug//usr/lib/apache2/modules/mod_authn_core.so...done.
done.
Loaded symbols for /usr/lib/apache2/modules/mod_authn_core.so
Reading symbols from /usr/lib/apache2/modules/mod_authn_file.so...Reading symbols from /usr/lib/debug//usr/lib/apache2/modules/mod_authn_file.so...done.
done.
Loaded symbols for /usr/lib/apache2/modules/mod_authn_file.so
Reading symbols from /usr/lib/apache2/modules/mod_authz_core.so...Reading symbols from /usr/lib/debug//usr/lib/apache2/modules/mod_authz_core.so...done.
done.
Loaded symbols for /usr/lib/apache2/modules/mod_authz_core.so
Reading symbols from /usr/lib/apache2/modules/mod_authz_host.so...Reading symbols from /usr/lib/debug//usr/lib/apache2/modules/mod_authz_host.so...done.
done.
Loaded symbols for /usr/lib/apache2/modules/mod_authz_host.so
Reading symbols from /usr/lib/apache2/modules/mod_authz_user.so...Reading symbols from /usr/lib/debug//usr/lib/apache2/modules/mod_authz_user.so...done.
done.
Loaded symbols for /usr/lib/apache2/modules/mod_authz_user.so
Reading symbols from /usr/lib/apache2/modules/mod_autoindex.so...Reading symbols from /usr/lib/debug//usr/lib/apache2/modules/mod_autoindex.so...done.
done.
Loaded symbols for /usr/lib/apache2/modules/mod_autoindex.so
Reading symbols from /usr/lib/apache2/modules/mod_deflate.so...Reading symbols from /usr/lib/debug//usr/lib/apache2/modules/mod_deflate.so...done.
done.
Loaded symbols for /usr/lib/apache2/modules/mod_deflate.so
Reading symbols from /lib/x86_64-linux-gnu/libz.so.1...(no debugging symbols found)...done.
Loaded symbols for /lib/x86_64-linux-gnu/libz.so.1
Reading symbols from /usr/lib/apache2/modules/mod_dir.so...Reading symbols from /usr/lib/debug//usr/lib/apache2/modules/mod_dir.so...done.
done.
Loaded symbols for /usr/lib/apache2/modules/mod_dir.so
Reading symbols from /usr/lib/apache2/modules/mod_env.so...Reading symbols from /usr/lib/debug//usr/lib/apache2/modules/mod_env.so...done.
done.
Loaded symbols for /usr/lib/apache2/modules/mod_env.so
Reading symbols from /usr/lib/apache2/modules/mod_filter.so...Reading symbols from /usr/lib/debug//usr/lib/apache2/modules/mod_filter.so...done.
done.
Loaded symbols for /usr/lib/apache2/modules/mod_filter.so
Reading symbols from /usr/lib/apache2/modules/mod_gnutls.so...(no debugging symbols found)...done.
Loaded symbols for /usr/lib/apache2/modules/mod_gnutls.so
Reading symbols from /usr/lib/x86_64-linux-gnu/libapr_memcache.so.0...(no debugging symbols found)...done.
Loaded symbols for /usr/lib/x86_64-linux-gnu/libapr_memcache.so.0
Reading symbols from /usr/lib/x86_64-linux-gnu/libgnutls.so.26...(no debugging symbols found)...done.
Loaded symbols for /usr/lib/x86_64-linux-gnu/libgnutls.so.26
Reading symbols from /lib/x86_64-linux-gnu/libgcrypt.so.11...(no debugging symbols found)...done.
Loaded symbols for /lib/x86_64-linux-gnu/libgcrypt.so.11
Reading symbols from /usr/lib/x86_64-linux-gnu/libtasn1.so.6...(no debugging symbols found)...done.
Loaded symbols for /usr/lib/x86_64-linux-gnu/libtasn1.so.6
Reading symbols from /usr/lib/x86_64-linux-gnu/libp11-kit.so.0...(no debugging symbols found)...done.
Loaded symbols for /usr/lib/x86_64-linux-gnu/libp11-kit.so.0
Reading symbols from /lib/x86_64-linux-gnu/libgpg-error.so.0...(no debugging symbols found)...done.
Loaded symbols for /lib/x86_64-linux-gnu/libgpg-error.so.0
Reading symbols from /usr/lib/x86_64-linux-gnu/libffi.so.6...(no debugging symbols found)...done.
Loaded symbols for /usr/lib/x86_64-linux-gnu/libffi.so.6
Reading symbols from /usr/lib/apache2/modules/mod_headers.so...Reading symbols from /usr/lib/debug//usr/lib/apache2/modules/mod_headers.so...done.
done.
Loaded symbols for /usr/lib/apache2/modules/mod_headers.so
Reading symbols from /usr/lib/apache2/modules/mod_mime.so...Reading symbols from /usr/lib/debug//usr/lib/apache2/modules/mod_mime.so...done.
done.
Loaded symbols for /usr/lib/apache2/modules/mod_mime.so
Reading symbols from /usr/lib/apache2/modules/mod_mpm_event.so...Reading symbols from /usr/lib/debug//usr/lib/apache2/modules/mod_mpm_event.so...done.
done.
Loaded symbols for /usr/lib/apache2/modules/mod_mpm_event.so
Reading symbols from /usr/lib/apache2/modules/mod_negotiation.so...Reading symbols from /usr/lib/debug//usr/lib/apache2/modules/mod_negotiation.so...done.
done.
Loaded symbols for /usr/lib/apache2/modules/mod_negotiation.so
Reading symbols from /usr/lib/apache2/modules/mod_rewrite.so...Reading symbols from /usr/lib/debug//usr/lib/apache2/modules/mod_rewrite.so...done.
done.
Loaded symbols for /usr/lib/apache2/modules/mod_rewrite.so
Reading symbols from /usr/lib/apache2/modules/mod_setenvif.so...Reading symbols from /usr/lib/debug//usr/lib/apache2/modules/mod_setenvif.so...done.
done.
Loaded symbols for /usr/lib/apache2/modules/mod_setenvif.so
Reading symbols from /usr/lib/apache2/modules/mod_status.so...Reading symbols from /usr/lib/debug//usr/lib/apache2/modules/mod_status.so...done.
done.
Loaded symbols for /usr/lib/apache2/modules/mod_status.so
Reading symbols from /lib/x86_64-linux-gnu/libgcc_s.so.1...(no debugging symbols found)...done.
Loaded symbols for /lib/x86_64-linux-gnu/libgcc_s.so.1
0x00007f71115b766b in pthread_join (threadid=140123215677184, thread_return=0x7fff06910208) at pthread_join.c:92
92	pthread_join.c: No such file or directory.
(gdb) bt full
#0  0x00007f71115b766b in pthread_join (threadid=140123215677184, thread_return=0x7fff06910208) at pthread_join.c:92
        _tid = 7257
        _buffer = {__routine = 0x7f71115b75a0 <cleanup>, __arg = 0x7f70fa7ecd28, __canceltype = 291206560, __prev = 0x0}
        oldtype = 0
        pd = 0x7f70fa7ec700
        self = 0x7f7112077780
        result = 0
#1  0x00007f71117f15fd in apr_thread_join () from /usr/lib/x86_64-linux-gnu/libapr-1.so.0
No symbol table info available.
#2  0x00007f710d55cde7 in join_workers (listener=0x7f7111fe8948, threads=threads@entry=0x7f711384a6a0) at event.c:2053
        iter = 1
        i = <optimized out>
        rv = <optimized out>
        thread_rv = 32625
#3  0x00007f710d55d13f in child_main (child_num_arg=child_num_arg@entry=1) at event.c:2242
        threads = 0x7f711384a6a0
        rv = <optimized out>
        ts = 0x7f7111fe7518
        thread_attr = 0x7f7111fe7538
        start_thread_id = 0x7f7111fe7598
#4  0x00007f710d5618c2 in make_child (s=0x7f711204bde0, slot=1) at event.c:2299
        pid = 0
#5  0x00007f710d56208d in perform_idle_server_maintenance () at event.c:2496
        j = <optimized out>
        free_slots = {1, 0, 5, 0, 302300640, 32625, 301777144, 32625, 301410240, 32625, 1, 0, 302300640, 32625, 301410200, 
          32625, 302460968, 32625, 293458811, 32625, 301410240, 32625, 293458026, 32625, 0, 0, 20, 0, 302247976, 32625, 0, 0}
        total_non_dead = <optimized out>
        active_thread_count = <optimized out>
        idle_thread_count = <optimized out>
        free_length = <optimized out>
        i = <optimized out>
        ws = <optimized out>
        ps = <optimized out>
        totally_free_length = <optimized out>
        last_non_dead = <optimized out>
#6  server_main_loop (remaining_children_to_start=0) at event.c:2624
        status = 9
        pid = {pid = -1, in = 0x9, out = 0x7f7112073028, err = 0x7f71117dcff6}
---Type <return> to continue, or q <return> to quit---
        i = <optimized out>
        old_gen = 0
        child_slot = <optimized out>
        exitwhy = APR_PROC_SIGNAL
        processed_status = <optimized out>
#7  event_run (_pconf=<optimized out>, plog=<optimized out>, s=<optimized out>) at event.c:2683
        remaining_children_to_start = <optimized out>
#8  0x00007f71120c469e in ap_run_mpm (pconf=0x7f7112073028, plog=0x7f7112047028, s=0x7f711204bde0) at mpm_common.c:96
        pHook = 0x7f7111f75fe0
        n = 0
        rv = -512
#9  0x00007f71120bde36 in main (argc=3, argv=0x7fff06910688) at main.c:777
        c = 0 '\000'
        showcompile = 0
        showdirectives = 0
        confname = 0x7f71120fd607 "apache2.conf"
        def_server_root = 0x7f71120fd5fa "/etc/apache2"
        temp_error_log = 0x0
        error = <optimized out>
        process = 0x7f7112075118
        pconf = 0x7f7112073028
        plog = 0x7f7112047028
        ptemp = 0x7f711203f028
        pcommands = 0x7f7112051028
        opt = 0x7f7112051118
        rv = <optimized out>
        mod = 0x7f711231f160 <ap_prelinked_modules+64>
        opt_arg = 0x7f7112075028 "("
        signal_server = <optimized out>
(gdb) 

root@developers:~# gdb /usr/sbin/apache2 17441
GNU gdb (Ubuntu 7.7-0ubuntu3.1) 7.7
Copyright (C) 2014 Free Software Foundation, Inc.
License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html>
This is free software: you are free to change and redistribute it.
There is NO WARRANTY, to the extent permitted by law.  Type "show copying"
and "show warranty" for details.
This GDB was configured as "x86_64-linux-gnu".
Type "show configuration" for configuration details.
For bug reporting instructions, please see:
<http://www.gnu.org/software/gdb/bugs/>.
Find the GDB manual and other documentation resources online at:
<http://www.gnu.org/software/gdb/documentation/>.
For help, type "help".
Type "apropos word" to search for commands related to "word"...
Reading symbols from /usr/sbin/apache2...Reading symbols from /usr/lib/debug//usr/sbin/apache2...done.
done.
Attaching to program: /usr/sbin/apache2, process 17441
Reading symbols from /lib/x86_64-linux-gnu/libpcre.so.3...(no debugging symbols found)...done.
Loaded symbols for /lib/x86_64-linux-gnu/libpcre.so.3
Reading symbols from /usr/lib/x86_64-linux-gnu/libaprutil-1.so.0...(no debugging symbols found)...done.
Loaded symbols for /usr/lib/x86_64-linux-gnu/libaprutil-1.so.0
Reading symbols from /usr/lib/x86_64-linux-gnu/libapr-1.so.0...(no debugging symbols found)...done.
Loaded symbols for /usr/lib/x86_64-linux-gnu/libapr-1.so.0
Reading symbols from /lib/x86_64-linux-gnu/libpthread.so.0...Reading symbols from /usr/lib/debug//lib/x86_64-linux-gnu/libpthread-2.19.so...done.
done.
[New LWP 17468]
[New LWP 17467]
[New LWP 17466]
[New LWP 17465]
[New LWP 17464]
[New LWP 17463]
[New LWP 17462]
[New LWP 17461]
[New LWP 17460]
[New LWP 17459]
[New LWP 17458]
[New LWP 17457]
[New LWP 17456]
[New LWP 17455]
[New LWP 17454]
[New LWP 17453]
[New LWP 17452]
[New LWP 17451]
[New LWP 17450]
[New LWP 17449]
[New LWP 17448]
[New LWP 17447]
[New LWP 17446]
[New LWP 17445]
[New LWP 17444]
[New LWP 17443]
[Thread debugging using libthread_db enabled]
Using host libthread_db library "/lib/x86_64-linux-gnu/libthread_db.so.1".
Loaded symbols for /lib/x86_64-linux-gnu/libpthread.so.0
Reading symbols from /lib/x86_64-linux-gnu/libc.so.6...Reading symbols from /usr/lib/debug//lib/x86_64-linux-gnu/libc-2.19.so...done.
done.
Loaded symbols for /lib/x86_64-linux-gnu/libc.so.6
Reading symbols from /lib/x86_64-linux-gnu/libcrypt.so.1...Reading symbols from /usr/lib/debug//lib/x86_64-linux-gnu/libcrypt-2.19.so...done.
done.
Loaded symbols for /lib/x86_64-linux-gnu/libcrypt.so.1
Reading symbols from /lib/x86_64-linux-gnu/libexpat.so.1...(no debugging symbols found)...done.
Loaded symbols for /lib/x86_64-linux-gnu/libexpat.so.1
Reading symbols from /lib/x86_64-linux-gnu/libuuid.so.1...(no debugging symbols found)...done.
Loaded symbols for /lib/x86_64-linux-gnu/libuuid.so.1
Reading symbols from /lib/x86_64-linux-gnu/libdl.so.2...Reading symbols from /usr/lib/debug//lib/x86_64-linux-gnu/libdl-2.19.so...done.
done.
Loaded symbols for /lib/x86_64-linux-gnu/libdl.so.2
Reading symbols from /lib64/ld-linux-x86-64.so.2...Reading symbols from /usr/lib/debug//lib/x86_64-linux-gnu/ld-2.19.so...done.
done.
Loaded symbols for /lib64/ld-linux-x86-64.so.2
Reading symbols from /lib/x86_64-linux-gnu/libnss_compat.so.2...Reading symbols from /usr/lib/debug//lib/x86_64-linux-gnu/libnss_compat-2.19.so...done.
done.
Loaded symbols for /lib/x86_64-linux-gnu/libnss_compat.so.2
Reading symbols from /lib/x86_64-linux-gnu/libnsl.so.1...Reading symbols from /usr/lib/debug//lib/x86_64-linux-gnu/libnsl-2.19.so...done.
done.
Loaded symbols for /lib/x86_64-linux-gnu/libnsl.so.1
Reading symbols from /lib/x86_64-linux-gnu/libnss_nis.so.2...Reading symbols from /usr/lib/debug//lib/x86_64-linux-gnu/libnss_nis-2.19.so...done.
done.
Loaded symbols for /lib/x86_64-linux-gnu/libnss_nis.so.2
Reading symbols from /lib/x86_64-linux-gnu/libnss_files.so.2...Reading symbols from /usr/lib/debug//lib/x86_64-linux-gnu/libnss_files-2.19.so...done.
done.
Loaded symbols for /lib/x86_64-linux-gnu/libnss_files.so.2
Reading symbols from /lib/x86_64-linux-gnu/libnss_dns.so.2...Reading symbols from /usr/lib/debug//lib/x86_64-linux-gnu/libnss_dns-2.19.so...done.
done.
Loaded symbols for /lib/x86_64-linux-gnu/libnss_dns.so.2
Reading symbols from /lib/x86_64-linux-gnu/libresolv.so.2...Reading symbols from /usr/lib/debug//lib/x86_64-linux-gnu/libresolv-2.19.so...done.
done.
Loaded symbols for /lib/x86_64-linux-gnu/libresolv.so.2
Reading symbols from /usr/lib/x86_64-linux-gnu/apr-util-1/apr_dbm_db-1.so...(no debugging symbols found)...done.
Loaded symbols for /usr/lib/x86_64-linux-gnu/apr-util-1/apr_dbm_db-1.so
Reading symbols from /usr/lib/x86_64-linux-gnu/libdb-5.3.so...(no debugging symbols found)...done.
Loaded symbols for /usr/lib/x86_64-linux-gnu/libdb-5.3.so
Reading symbols from /usr/lib/apache2/modules/mod_access_compat.so...Reading symbols from /usr/lib/debug//usr/lib/apache2/modules/mod_access_compat.so...done.
done.
Loaded symbols for /usr/lib/apache2/modules/mod_access_compat.so
Reading symbols from /usr/lib/apache2/modules/mod_alias.so...Reading symbols from /usr/lib/debug//usr/lib/apache2/modules/mod_alias.so...done.
done.
Loaded symbols for /usr/lib/apache2/modules/mod_alias.so
Reading symbols from /usr/lib/apache2/modules/mod_auth_basic.so...Reading symbols from /usr/lib/debug//usr/lib/apache2/modules/mod_auth_basic.so...done.
done.
Loaded symbols for /usr/lib/apache2/modules/mod_auth_basic.so
Reading symbols from /usr/lib/apache2/modules/mod_authn_core.so...Reading symbols from /usr/lib/debug//usr/lib/apache2/modules/mod_authn_core.so...done.
done.
Loaded symbols for /usr/lib/apache2/modules/mod_authn_core.so
Reading symbols from /usr/lib/apache2/modules/mod_authn_file.so...Reading symbols from /usr/lib/debug//usr/lib/apache2/modules/mod_authn_file.so...done.
done.
Loaded symbols for /usr/lib/apache2/modules/mod_authn_file.so
Reading symbols from /usr/lib/apache2/modules/mod_authz_core.so...Reading symbols from /usr/lib/debug//usr/lib/apache2/modules/mod_authz_core.so...done.
done.
Loaded symbols for /usr/lib/apache2/modules/mod_authz_core.so
Reading symbols from /usr/lib/apache2/modules/mod_authz_host.so...Reading symbols from /usr/lib/debug//usr/lib/apache2/modules/mod_authz_host.so...done.
done.
Loaded symbols for /usr/lib/apache2/modules/mod_authz_host.so
Reading symbols from /usr/lib/apache2/modules/mod_authz_user.so...Reading symbols from /usr/lib/debug//usr/lib/apache2/modules/mod_authz_user.so...done.
done.
Loaded symbols for /usr/lib/apache2/modules/mod_authz_user.so
Reading symbols from /usr/lib/apache2/modules/mod_autoindex.so...Reading symbols from /usr/lib/debug//usr/lib/apache2/modules/mod_autoindex.so...done.
done.
Loaded symbols for /usr/lib/apache2/modules/mod_autoindex.so
Reading symbols from /usr/lib/apache2/modules/mod_deflate.so...Reading symbols from /usr/lib/debug//usr/lib/apache2/modules/mod_deflate.so...done.
done.
Loaded symbols for /usr/lib/apache2/modules/mod_deflate.so
Reading symbols from /lib/x86_64-linux-gnu/libz.so.1...(no debugging symbols found)...done.
Loaded symbols for /lib/x86_64-linux-gnu/libz.so.1
Reading symbols from /usr/lib/apache2/modules/mod_dir.so...Reading symbols from /usr/lib/debug//usr/lib/apache2/modules/mod_dir.so...done.
done.
Loaded symbols for /usr/lib/apache2/modules/mod_dir.so
Reading symbols from /usr/lib/apache2/modules/mod_env.so...Reading symbols from /usr/lib/debug//usr/lib/apache2/modules/mod_env.so...done.
done.
Loaded symbols for /usr/lib/apache2/modules/mod_env.so
Reading symbols from /usr/lib/apache2/modules/mod_filter.so...Reading symbols from /usr/lib/debug//usr/lib/apache2/modules/mod_filter.so...done.
done.
Loaded symbols for /usr/lib/apache2/modules/mod_filter.so
Reading symbols from /usr/lib/apache2/modules/mod_gnutls.so...(no debugging symbols found)...done.
Loaded symbols for /usr/lib/apache2/modules/mod_gnutls.so
Reading symbols from /usr/lib/x86_64-linux-gnu/libapr_memcache.so.0...(no debugging symbols found)...done.
Loaded symbols for /usr/lib/x86_64-linux-gnu/libapr_memcache.so.0
Reading symbols from /usr/lib/x86_64-linux-gnu/libgnutls.so.26...(no debugging symbols found)...done.
Loaded symbols for /usr/lib/x86_64-linux-gnu/libgnutls.so.26
Reading symbols from /lib/x86_64-linux-gnu/libgcrypt.so.11...(no debugging symbols found)...done.
Loaded symbols for /lib/x86_64-linux-gnu/libgcrypt.so.11
Reading symbols from /usr/lib/x86_64-linux-gnu/libtasn1.so.6...(no debugging symbols found)...done.
Loaded symbols for /usr/lib/x86_64-linux-gnu/libtasn1.so.6
Reading symbols from /usr/lib/x86_64-linux-gnu/libp11-kit.so.0...(no debugging symbols found)...done.
Loaded symbols for /usr/lib/x86_64-linux-gnu/libp11-kit.so.0
Reading symbols from /lib/x86_64-linux-gnu/libgpg-error.so.0...(no debugging symbols found)...done.
Loaded symbols for /lib/x86_64-linux-gnu/libgpg-error.so.0
Reading symbols from /usr/lib/x86_64-linux-gnu/libffi.so.6...(no debugging symbols found)...done.
Loaded symbols for /usr/lib/x86_64-linux-gnu/libffi.so.6
Reading symbols from /usr/lib/apache2/modules/mod_headers.so...Reading symbols from /usr/lib/debug//usr/lib/apache2/modules/mod_headers.so...done.
done.
Loaded symbols for /usr/lib/apache2/modules/mod_headers.so
Reading symbols from /usr/lib/apache2/modules/mod_mime.so...Reading symbols from /usr/lib/debug//usr/lib/apache2/modules/mod_mime.so...done.
done.
Loaded symbols for /usr/lib/apache2/modules/mod_mime.so
Reading symbols from /usr/lib/apache2/modules/mod_mpm_event.so...Reading symbols from /usr/lib/debug//usr/lib/apache2/modules/mod_mpm_event.so...done.
done.
Loaded symbols for /usr/lib/apache2/modules/mod_mpm_event.so
Reading symbols from /usr/lib/apache2/modules/mod_negotiation.so...Reading symbols from /usr/lib/debug//usr/lib/apache2/modules/mod_negotiation.so...done.
done.
Loaded symbols for /usr/lib/apache2/modules/mod_negotiation.so
Reading symbols from /usr/lib/apache2/modules/mod_rewrite.so...Reading symbols from /usr/lib/debug//usr/lib/apache2/modules/mod_rewrite.so...done.
done.
Loaded symbols for /usr/lib/apache2/modules/mod_rewrite.so
Reading symbols from /usr/lib/apache2/modules/mod_setenvif.so...Reading symbols from /usr/lib/debug//usr/lib/apache2/modules/mod_setenvif.so...done.
done.
Loaded symbols for /usr/lib/apache2/modules/mod_setenvif.so
Reading symbols from /usr/lib/apache2/modules/mod_status.so...Reading symbols from /usr/lib/debug//usr/lib/apache2/modules/mod_status.so...done.
done.
Loaded symbols for /usr/lib/apache2/modules/mod_status.so
Reading symbols from /lib/x86_64-linux-gnu/libgcc_s.so.1...(no debugging symbols found)...done.
Loaded symbols for /lib/x86_64-linux-gnu/libgcc_s.so.1
0x00007f71115b766b in pthread_join (threadid=140123215677184, thread_return=0x7fff06910208) at pthread_join.c:92
92	pthread_join.c: No such file or directory.
(gdb) bt full
#0  0x00007f71115b766b in pthread_join (threadid=140123215677184, thread_return=0x7fff06910208) at pthread_join.c:92
        _tid = 17468
        _buffer = {__routine = 0x7f71115b75a0 <cleanup>, __arg = 0x7f70fa7ecd28, __canceltype = 291206560, __prev = 0x0}
        oldtype = 0
        pd = 0x7f70fa7ec700
        self = 0x7f7112077780
        result = 0
#1  0x00007f71117f15fd in apr_thread_join () from /usr/lib/x86_64-linux-gnu/libapr-1.so.0
No symbol table info available.
#2  0x00007f710d55cde7 in join_workers (listener=0x7f7111fe8948, threads=threads@entry=0x7f711384a6a0) at event.c:2053
        iter = 1
        i = <optimized out>
        rv = <optimized out>
        thread_rv = 32625
#3  0x00007f710d55d13f in child_main (child_num_arg=child_num_arg@entry=0) at event.c:2242
        threads = 0x7f711384a6a0
        rv = <optimized out>
        ts = 0x7f7111fe7518
        thread_attr = 0x7f7111fe7538
        start_thread_id = 0x7f7111fe7598
#4  0x00007f710d5618c2 in make_child (s=0x7f711204bde0, slot=0) at event.c:2299
        pid = 0
#5  0x00007f710d56208d in perform_idle_server_maintenance () at event.c:2496
        j = <optimized out>
        free_slots = {0, 0, 5, 0, 302300640, 32625, 301777144, 32625, 301410240, 32625, 1, 0, 302300640, 32625, 301410200, 
          32625, 302460968, 32625, 293458811, 32625, 301410240, 32625, 293458026, 32625, 0, 0, 20, 0, 302247976, 32625, 0, 0}
        total_non_dead = <optimized out>
        active_thread_count = <optimized out>
        idle_thread_count = <optimized out>
        free_length = <optimized out>
        i = <optimized out>
        ws = <optimized out>
        ps = <optimized out>
        totally_free_length = <optimized out>
        last_non_dead = <optimized out>
#6  server_main_loop (remaining_children_to_start=0) at event.c:2624
        status = 9
        pid = {pid = -1, in = 0x9, out = 0x7f7112073028, err = 0x7f71117dcff6}
---Type <return> to continue, or q <return> to quit---
        i = <optimized out>
        old_gen = 0
        child_slot = <optimized out>
        exitwhy = APR_PROC_SIGNAL
        processed_status = <optimized out>
#7  event_run (_pconf=<optimized out>, plog=<optimized out>, s=<optimized out>) at event.c:2683
        remaining_children_to_start = <optimized out>
#8  0x00007f71120c469e in ap_run_mpm (pconf=0x7f7112073028, plog=0x7f7112047028, s=0x7f711204bde0) at mpm_common.c:96
        pHook = 0x7f7111f75fe0
        n = 0
        rv = -512
#9  0x00007f71120bde36 in main (argc=3, argv=0x7fff06910688) at main.c:777
        c = 0 '\000'
        showcompile = 0
        showdirectives = 0
        confname = 0x7f71120fd607 "apache2.conf"
        def_server_root = 0x7f71120fd5fa "/etc/apache2"
        temp_error_log = 0x0
        error = <optimized out>
        process = 0x7f7112075118
        pconf = 0x7f7112073028
        plog = 0x7f7112047028
        ptemp = 0x7f711203f028
        pcommands = 0x7f7112051028
        opt = 0x7f7112051118
        rv = <optimized out>
        mod = 0x7f711231f160 <ap_prelinked_modules+64>
        opt_arg = 0x7f7112075028 "("
        signal_server = <optimized out>
(gdb) 

Must admit that there is nothing mod-gnutls specific here, but we use very identical setups on many machines, and it is only the machines with mod-gnutls that exhibits this problem, and it is fairly predictable -- always happens after a couple of days.