Lockup with --accel tcg,thread=single

[Doug Gale]

I found a lockup in single threaded TCG, with OVMF bios, 16 CPUs.

qemu-system-x86_64 --accel tcg,thread=single -smp cpus=16 -bios
/usr/share/ovmf/OVMF.fd

Using Ubuntu 18.04 LTS, default gnome desktop. There is no guest OS,
there is no hard drive, just the OVMF firmware locks it up. (I
narrowed it down to this from a much larger repro)

Let that run for about 10 seconds or so, then attempt to Ctrl-C in the
terminal that launched qemu. You should see the hung program thing
(wait or force quit) appear on the qemu window, and the terminal just
prints ^C without interrupting qemu. DO NOT click force quit or wait.
Use killall -9 qemu-system-x86_64 to kill it if you must, gdb kill is
also okay. If you force quit you will likely freeze the entire gnome
desktop. This is what kmsg reports:
https://gist.githubusercontent.com/doug65536/810e0471c11008c97edb4aa0d86ddd66/raw/40038a355542f0edb1f14a7c7fb7c2db60da0025/end%2520of%2520syslog%2520after%2520desktop%2520lockup,
this is what syslog reports:
https://gist.githubusercontent.com/doug65536/810e0471c11008c97edb4aa0d86ddd66/raw/40038a355542f0edb1f14a7c7fb7c2db60da0025/the%2520end%2520of%2520dmesg%2520after%2520desktop%2520lockup.
Probably bugs in gnome but just warning about locking up your machines
when reproducing.

Peter Maydell helped me bisect it in IRC.
 Works fine at commit 1e8a98b53867f61
 Fails at commit 9458a9a1df1a4c7

MTTCG works fine all the way up to master.

Configure command line:

../qemu/configure --target-list=x86_64-softmmu,i386-softmmu
--audio-drv-list=pa --enable-libusb --disable-libssh
--enable-virglrenderer --enable-opengl --enable-debug

The issue occurs without --enable-debug. I didn't strip the configure
down though, it may not need all of those configure options exactly.

OVMF from ubuntu package manager, package named ovmf, exact version is
0~20180205.c0d9813c-2ubuntu0.1

Stack trace at lockup at
https://gist.githubusercontent.com/doug65536/810e0471c11008c97edb4aa0d86ddd66/raw/40038a355542f0edb1f14a7c7fb7c2db60da0025/backtrace%2520all