mythbackend main thread blocks soon after startup on CentOS 6.2

[J. Ali Harlow <ali@…>]

Running fixes/0.24 (294968bea7546d5c0d27b7326401e79a7e67764b) plus http://code.mythtv.org/trac/attachment/ticket/9704/mythtv-0.24-backport_reconnect_fixes.patch and mysql-5.1.52-1.el6_0.1.x86_64 on CentOS 6.2

Whenever I start mythbackend, I have a short window to start the frontend during which time it will connect (and appears to work fine). After that short window closes, the backend no longer accepts any connections.

I wrote a simple script/program to show what mythbackend was watching (attached). When I run this during the window, I see something like the following:

22280: poll fds=[{fd=3, events=1, revents=0} {fd=9, events=1, revents=0} {fd=38, events=1, revents=0} {fd=3, events=1, revents=0}] nfds=0x4
22285: poll fds=[{fd=7, events=1, revents=0}] nfds=0x1
22288: select nfds=0xf readfds=[12 13 14] writefds=0x0 exceptfds=0x0
22444: poll fds=[{fd=26, events=1, revents=0}] nfds=0x1
COMMAND     PID USER   FD   TYPE DEVICE SIZE/OFF   NODE NAME
mythbacke 22280 root    3r  FIFO    0,8      0t0 117605 pipe
mythbacke 22280 root    7r  FIFO    0,8      0t0 117615 pipe
mythbacke 22280 root    9u  IPv4 117616      0t0    TCP *:lds-dump (LISTEN)
mythbacke 22280 root   12u  IPv4 117619      0t0    UDP *:apc-6549 
mythbacke 22280 root   13u  IPv4 117620      0t0    UDP 239.255.255.250:ssdp 
mythbacke 22280 root   14u  IPv4 117621      0t0    UDP 255.255.255.255:ssdp 
mythbacke 22280 root   26r  FIFO    0,8      0t0 118008 pipe
mythbacke 22280 root   38u  IPv4 118014      0t0    TCP *:lds-distrib (LISTEN)

so we can see that the main thread is watching some pipe (twice) and the two listening TCP ports.

A little later, we get this:

22285: poll fds=[{fd=7, events=1, revents=0}] nfds=0x1
22288: select nfds=0xf readfds=[12 13 14] writefds=0x0 exceptfds=0x0
22444: poll fds=[{fd=26, events=1, revents=0}] nfds=0x1
COMMAND     PID USER   FD   TYPE DEVICE SIZE/OFF   NODE NAME
mythbacke 22280 root    7r  FIFO    0,8      0t0 117615 pipe
mythbacke 22280 root   12u  IPv4 117619      0t0    UDP *:apc-6549 
mythbacke 22280 root   13u  IPv4 117620      0t0    UDP 239.255.255.250:ssdp 
mythbacke 22280 root   14u  IPv4 117621      0t0    UDP 255.255.255.255:ssdp 
mythbacke 22280 root   26r  FIFO    0,8      0t0 118008 pipe

and strace tells us the main thread is blocked reading the pipe that it was watching:

# strace -p 22280
Process 22280 attached - interrupt to quit
read(3, 

A backtrace at this point is attached.

[J. Ali Harlow <ali@…>]

Backtrace

[J. Ali Harlow <ali@…>]

Script to see what mythbackend is watching

[J. Ali Harlow <ali@…>]

Helper program for myth-poll

[wagnerrp]

Duplicate of #10188

[wagnerrp]

scratch that...

[danielk]

10. Ali Harlow, this appears to be an issue with dbus. 1/ Is dbus installed? 2/ can you toggle whether it is installed and report back if that resolves the deadlock.

There probably isn't much we can do about dbus issues other than to just disable it, but it would be good to have some confirmation as to whether that is the real issue here.

PS In the future when we ask for a backtrace in mythtv land we really mean a "thread apply all bt" but in this case I don't think that is necessary.

[J. Ali Harlow <ali@…>]

Replying to danielk:

10. Ali Harlow, this appears to be an issue with dbus. 1/ Is dbus installed? 2/ can you toggle whether it is installed and report back if that resolves the deadlock.

yes, dbus-1.2.24-5.el6_1.x86_64 is installed. I can't uninstall it (just about every gnome package requires it). I tried killing the session daemon to see if I could do this temporarily to test but this just causes gnome to log me out (and then for gdm to get very confused).

PS In the future when we ask for a backtrace in mythtv land we really mean a "thread apply all bt" but in this case I don't think that is necessary.

Okay. Let me know if this would be useful and if rebuilding dbus-libs to get the debug symbols would be helpful.

Do we know what myth is doing to initiate the communication with dbus? I searched the source of mythbackend and the libs but didn't find it mentioned anywhere. (Wondering if I can disable it from there and achieve the same effect.)

[Ken Dreyer <ktdreyer@…>]

I've been trying 0.23, 0.24.1, and 0.24-fixes on CentOS 6, all with very similar symptoms to this bug report.

I've found that if I disable EIT scanning (or just remove all tuners altogether), and don't connect any frontends, then mythbackend will respond on the HTTP status port (6544). After starting up the backend, as soon as I connect the first frontend client, then the backend immediately stops responding on 6544, and after that first client disconnects I cannot get a response on 6543 either. (I'm just using watch -n 0.5 'wget -O - http://localhost:6544' to test.) The backend is hosed from that point until I restart it.

Ali, does that match your symptoms?

[Ken Dreyer <ktdreyer@…>]

Actually yes, my backtrace in gdb is identical to yours. Looks like I need to install the debuginfo RPMs for QT / DBus to see the last part of the stack trace.

[kkuphal]

I would just like to add in case it is needed for debugging that I'm running my backend and frontend on CentOS 6.0, kernel version 2.6.32-71 and an earlier version of trunk from around August 1st and I do not have this issue currently. I also have dbus installed at 1.2.24-3.

[Ken Dreyer <ktdreyer@…>]

backtrace with debugging info (this was on 0.23, but it is basically the same as Ali's)

[Ken Dreyer <ktdreyer@…>]

Replying to kkuphal: Thanks, I'm on a fully-updated 6.2 system. I downgraded my dbus, qt, and glibc back to CentOS 6's earliest versions (dbus-1.2.24-3, qt-4.6.2-16, glibc-2.12-1.7) and rebooted, but it had no effect.

The problem seems to be related to GDM's interaction with dbus. When I log into Xfce through GDM, I see mythbackend hang upon the first connection. Then when I run "init 3", then "startx", I'm able to launch mythfrontend repeatedly, and mythbackend doesn't hang.

[ktdreyer@…]

As an update, the problem occurs whenever the "messagebus" service is running, or more specifically, "dbus-daemon" with the "--system" flag. Disabling the messagebus service allows mythbackend to run without hanging.

[yoshihara@…]

I've encountered the same issue as this and been able to recover from it with disabling "messagebus" daemon starting. My environment is as follows:

OS: Scientific Linux release 6.1 (Carbon) Kernel: 2.6.32-220.4.1.el6.i686 Mythtv version: 0.24.1 TV Capture card: Earth-soft PT2

[yoshihara@…]

After observation for a while, I found the above solution had so much influences on my environment, for example, X-Window couldn't start. So, I'm waiting for the provision of another one.

[ktdreyer@…]

Here's the workaround I'm using (posted to the users' mailing list earlier)

I set up a minimal CentOS 6 chroot within /chroot, then installed mythtv-backend into that. When I run mythbackend inside the chroot, it is unable to connect with the systemwide dbus daemon, but it can still communicate with mysqld and mythfrontend over TCP/IP.

Regarding disk space, I created a 4GB logical volume for the chroot, but everything is currently only taking up 850 MB, so I could have probably gone for 2GB without problems. (I have the mythtv "recordings" directory on an altogether separate logical volume that I also mount within the chroot.)

It's certainly not optimal, but I'm not a DBus/Qt hacker so I don't know what else to do.

[Github]

Move fix from 7866a616c into MythCoreContext?.

This moves the GetMasterHostName? fix from changeset 7866a616c into MythCoreContext?, in the event this is the cause of other similar stalls in the backend protocol server.

Refs #10265

Branch: master Changeset: a0c9d003dee59ed03e52ea564d2c0a00787cb96b

[yoshihara@…]

I've downloaded and installed the latest version of mythtv through Github, but in vain...

[brian.sanders@…]

If it helps any, I just started a MythTV install on Centos 6 fully updtaed. Pulled .24-fixes from git, and have run into this problem. I basically can't run mythtv on CentOS at this time.

Just to test, I shut down messagebus and I was able to get a connection to finally be accepted. So this does seem to be the same bug. It is very easy to replicate, just install from a current CentOS build and you can't avoid it.

[Jonathan Martens <jonathan@…>]

I have the same issues with current trunk as well, see #10302. I would love to investigate this further, but I am relatively new to this. Perhaps some of you can give some pointers on where to start?

[Andrey Zhunev <a-j@…>]

I discovered that downgrading qtwebkit from qtwebkit-2.1.1-1.el6 to qtwebkit-2.0-3.el6 solves the mythbackend blocking issue on CentOS 6.2. Not sure how to dig it further though... But with qtwebkit-2.0-3 installed, mythbackend is perfectly stable already for several days in a row (working 24/7).

[yoshihara@…]

Replying to Andrey Zhunev <a-j@…>:

I discovered that downgrading qtwebkit from qtwebkit-2.1.1-1.el6 to qtwebkit-2.0-3.el6 solves the mythbackend blocking issue on CentOS 6.2. Not sure how to dig it further though... But with qtwebkit-2.0-3 installed, mythbackend is perfectly stable already for several days in a row (working 24/7).

Hi Andrey,

I tried the work-around you discovered on my own environment consists of Scientific Linux 6.2 and CentOS 6.2.

The former had no problem, but the latter did one with start of mythfrontend showing seg fault error of its latest kernel version.

Thus, personally I think the work-around is generally effective, but I would like dev team of mythTV to fix the root cause of this issue.

Regards, Yoshii

[yoshihara@…]

I tried further since then and the problem on Scientific Linux's side was due to corruption of related libraries' installation structure for mythTV. I completely removed it and re-installed, succeeded to watch TV programs on it.

As a result, did qtwebkit cause this issue although I didn't obviously know its root cause?

Regards, Yoshii

[yoshihara@…]

Sorry for my typo.

(Wrong) Scientific Linux's side

(Correct) CentOS's side

Regards, Yoshii

[ktdreyer@…]

Yoshii: what does "rpm -qv qtwebkit" say?

[yoshihara@…]

Please see the following description:

(Scientific Linux 6.2) [root@scientific6 ~]# rpm -qv qtwebkit qtwebkit-2.0-3.el6.i686

(CentOS 6.2) [root@centos6 ~]# rpm -qv qtwebkit qtwebkit-2.0-3.el6.x86_64

I downloaded and installed each package from ATrpms repository.

Regards, Yoshii

[ktdreyer@…]

I have no idea how qtwebkit could be affecting this, but to me next step is to bisect the commits between qtwebkit 2.0 to 2.1.1, in order to understand what exactly could be causing the lockup.

[gronslet@…]

Hi, I also have this issue - getting

E  MythSocket(1676920:36): readStringList: Error, timed out after 7000 ms.   

from mythfrontend.

My setup: CentOS 6.2 Various kernels (eg. kernel-2.6.32-220.23.1.el6.centos.plus.x86_64 and patched vanilla 3.4.4) MythTV fixes/0.25 compiled from git (2012-07-13), and prebuild binaries (0.25-6.el6.x86_64). I am running a combined FE/BE (frontend/backend).

I was not able to get mythfrontend talk to the backend before I did the workaround suggested in earlier comments, ie.

# wget http://dl.atrpms.net/all/qtwebkit-2.0-3.el6.x86_64.rpm
# wget http://dl.atrpms.net/all/qtwebkit-devel-2.0-3.el6.x86_64.rpm
# yum downgrade qtwebkit-2.0-3.el6.x86_64.rpm qtwebkit-devel-2.0-3.el6.x86_64.rpm

However, I ran into the problem again after restarting some services and/or mythfrontend/backend. So it seems it is not a permanent solution.

Big thanks for having this bugtracker - I have spent numerous of hours on this, and this bug report certainly saved me some time. :)

[karlcz@…]

I just encountered this on two up to date CentOS 6.3 + EPEL + RPM Fusion machines using the mythtv 0.25 packages from RPM fusion.

A workaround that seems to work for now is to enable all of dbus, mythbackend, and GDM with auto-login as usual (for mythfrontend). Then in rc.local add this hack to get mythbackend started without dbus being available before restarting it for the sake of GDM and friends:

service messagebus stop
service mythbackend stop
sleep 5
service mythbackend start
sleep 5
service messagebus start

Does anybody know how to tell mythbackend not to use dbus?

[ktdreyer@…]

From what I can tell, the D-Bus interaction is coming from Qt event loop, not necessarily unique to MythTV. Again, I'm not a Qt hacker, but I'm not sure there's even a way to disable the D-Bus interaction via Qt's API.

[hobbes1069@…]

Well I tried updating QT from 4.6.2 to 4.6.4 in my CentOS 6.3 virtual machine hoping that the problem was fixed upstream but no luck.

[anonymous@…]

I don't think the issue is QT, but rather the rather ancient level of glib2 supplied with Centos/RHEL. In the 2.22.5 version of gmain.c, the wakeup pipe is created blocking; in later versions the wakeup code is moved into a new gwakeup.c module and uses non-blocking pipes.

A possible circumvention (haven't tried this yet) would be to modify QT src/corelib/kernel/qsocketnotifier.cpp to force registered file descriptors non-blocking.

[hobbes1069@…]

Well if someone can come up with a patch I'll try it in my CentOS 6.3 virtual machine...

[kenni]

Please use the mythtv-users mailing list for discussions. There're way too many comments in this ticket, which doesn't add any useful information at all. If the discussions on the mailing list lead to any conclusions, let's add the conclusions here afterwards.