Re: [Qemu-devel] qemu-kvm-1.0.1 - unable to exit if vcpu is in infinite loop

[Stefan Hajnoczi <stefanha@gmail.com>]

On Thu, Jun 28, 2012 at 2:05 PM, Peter Lieven <pl@dlhnet.de> wrote:
> i debugged my initial problem further and found out that the problem happens
> to be that
> the main thread is stuck in pause_all_vcpus() on reset or quit commands in
> the monitor
> if one cpu is stuck in the do-while loop kvm_cpu_exec. If I modify the
> condition from while (ret == 0)
> to while ((ret == 0) && !env->stop); it works, but is this the right fix?
> "Quit" command seems to work, but on "Reset" the VM enterns pause state.

I think I'm hitting something similar.  I installed a F17 amd64 guest
(3.5 kernel) but before booting entered the GRUB boot menu edit mode.
The guest seemed unresponsive so I switched to the monitor, which also
froze shortly afterwards.  The VNC screen ended up being all black.

qemu-kvm.git/master 3e4305694fd891b69e4450e59ec4c65420907ede
Linux 3.2.0-3-amd64 from Debian testing

$ qemu-system-x86_64 -enable-kvm -m 1024 -smp 2 -drive
if=virtio,cache=none,file=f17.img,aio=native -serial stdio

(gdb) thread apply all bt

Thread 3 (Thread 0x7f8008e23700 (LWP 367)):
#0  0x00007f800f891727 in ioctl () at ../sysdeps/unix/syscall-template.S:82
#1  0x00007f80137b92c9 in kvm_vcpu_ioctl
(env=env@entry=0x7f8015b49640, type=type@entry=44672)
    at /home/stefanha/qemu-kvm/kvm-all.c:1619
#2  0x00007f80137b93fe in kvm_cpu_exec (env=env@entry=0x7f8015b49640)
    at /home/stefanha/qemu-kvm/kvm-all.c:1506
#3  0x00007f8013766f31 in qemu_kvm_cpu_thread_fn (arg=0x7f8015b49640)
    at /home/stefanha/qemu-kvm/cpus.c:756
#4  0x00007f800fb4db50 in start_thread (arg=<optimized out>) at
pthread_create.c:304
#5  0x00007f800f8986dd in clone () at
../sysdeps/unix/sysv/linux/x86_64/clone.S:112
#6  0x0000000000000000 in ?? ()

This vcpu is still executing guest code and I've seen it successfully
dispatching I/O.  The problem is it's missing the exit_request...

Thread 2 (Thread 0x7f8008622700 (LWP 368)):
#0  pthread_cond_wait@@GLIBC_2.3.2 ()
    at ../nptl/sysdeps/unix/sysv/linux/x86_64/pthread_cond_wait.S:162
#1  0x00007f801372b229 in qemu_cond_wait (cond=<optimized out>,
    mutex=mutex@entry=0x7f80144367c0) at qemu-thread-posix.c:113
#2  0x00007f8013766eff in qemu_kvm_wait_io_event (env=<optimized out>)
    at /home/stefanha/qemu-kvm/cpus.c:724
#3  qemu_kvm_cpu_thread_fn (arg=0x7f8015b67450) at
/home/stefanha/qemu-kvm/cpus.c:761
#4  0x00007f800fb4db50 in start_thread (arg=<optimized out>) at
pthread_create.c:304
#5  0x00007f800f8986dd in clone () at
../sysdeps/unix/sysv/linux/x86_64/clone.S:112
#6  0x0000000000000000 in ?? ()

No problems here.

Thread 1 (Thread 0x7f801347b8c0 (LWP 365)):
#0  pthread_cond_wait@@GLIBC_2.3.2 ()
    at ../nptl/sysdeps/unix/sysv/linux/x86_64/pthread_cond_wait.S:162
#1  0x00007f801372b229 in qemu_cond_wait (cond=cond@entry=0x7f801402fd80,
    mutex=mutex@entry=0x7f80144367c0) at qemu-thread-posix.c:113
#2  0x00007f8013768949 in pause_all_vcpus () at
/home/stefanha/qemu-kvm/cpus.c:962
#3  0x00007f80136028c8 in main (argc=<optimized out>, argv=<optimized out>,
    envp=<optimized out>) at /home/stefanha/qemu-kvm/vl.c:3695

We're deadlocked in pause_all_vcpus(), waiting for vcpu #0 to pause.
Unfortunately vcpu #0 has ->exit_request=0 although ->stop=1.

Here are the vcpus:

(gdb) p first_cpu
$6 = (struct CPUX86State *) 0x7f8015b49640
(gdb) p first_cpu->next_cpu
$7 = (struct CPUX86State *) 0x7f8015b67450
(gdb) p first_cpu->next_cpu->next_cpu
$8 = (struct CPUX86State *) 0x0

(gdb) p first_cpu->stop
$9 = 1
(gdb) p first_cpu->stopped
$10 = 0
(gdb) p first_cpu->exit_request
$11 = 0

:(

This isn't easy to reproduce.  I tried entering the GRUB boot menu
again and there was no deadlock.

Stefan