qemu_mutex deadlock with -smp 2

["Darrick J. Wong" <djwong-r/Jw6+rmf7HQT0dZR+AlfA@public.gmane.org>]

Hi,

I'm attempting to run a KVM instance with 2 CPUs configured.  The VM
appears to get most of the way through booting Ubuntu and then kvm locks
up.  At that point there's nothing printed on stdout from the kvm
process and the monitor doesn't respond either.  The host's kernel is
2.6.24-rc7 and the kvm binary is built from git.

Here's the command line I use:

/usr/local/bin/qemu-system-x86_64 -serial telnet:0.0.0.0:7005,server,nodelay \
  -m 1024 -smp 2 -vnc :5 -std-vga \
  -net nic,macaddr=00:12:34:56:78:95,model=rtl8139 -net tap -boot c \
  -monitor telnet:0.0.0.0:8005,server,nowait,nodelay \
  -drive file=/u/abat0.img,if=scsi,boot=on -snapshot

This usually suffices to reproduce:

/usr/local/bin/qemu-system-x86_64 \
  -m 1024 -smp 2 \
  -net nic,macaddr=00:12:34:56:78:96,model=rtl8139 -net tap -boot c \
  -monitor telnet:0.0.0.0:8006,server,nowait,nodelay \
  -hda /u/abat0.img -snapshot

An strace of the process shows that it's stuck here:
futex(0x60281640, FUTEX_WAIT, 2, NULL)
and this is the gdb stack trace:

#0  0x00002ae19369eaf8 in __lll_mutex_lock_wait () from /lib/libpthread.so.0
#1  0x00002ae19369aada in _L_mutex_lock_100 () from /lib/libpthread.so.0
#2  0x00002ae19369a475 in pthread_mutex_lock () from /lib/libpthread.so.0
#3  0x000000006008c6f7 in kvm_eat_signal (env=0x60281640, timeout=-1795357456) at /usr/local/src/kvm-userspace/qemu/qemu-kvm.c:176
#4  0x000000006008c82e in kvm_eat_signals (env=0x625f15a0, timeout=10) at /usr/local/src/kvm-userspace/qemu/qemu-kvm.c:200
#5  0x000000006008c89b in kvm_main_loop_wait (env=0x625f15a0, timeout=10) at /usr/local/src/kvm-userspace/qemu/qemu-kvm.c:219
#6  0x000000006008d04d in kvm_main_loop_cpu (env=0x625f15a0) at /usr/local/src/kvm-userspace/qemu/qemu-kvm.c:335
#7  0x0000000060010b94 in main (argc=0, argv=0x7fff181eb248) at /usr/local/src/kvm-userspace/qemu/vl.c:7817

It appears that we're deadlocked on qemu_mutex; it would also appear
that disabling the network interface buys me a few more seconds until
the lockup occurs.  Most of the time it is the case that post_kvm_run()
locked qemu_mutex and then kvm_eat_signal() tries to lock it as well.

--D

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