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

[Jan Kiszka]

On 2012-08-17 16:36, Jan Kiszka wrote:
> On 2012-08-17 15:11, Jan Kiszka wrote:
>> On 2012-08-06 17:11, Stefan Hajnoczi wrote:
>>> On Thu, Jun 28, 2012 at 2:05 PM, Peter Lieven <address@hidden> 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
>>> (address@hidden, address@hidden)
>>>     at /home/stefanha/qemu-kvm/kvm-all.c:1619
>>> #2  0x00007f80137b93fe in kvm_cpu_exec (address@hidden)
>>>     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>,
>>>     address@hidden) 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 (address@hidden,
>>>     address@hidden) 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
>>
>> CPUState::exit_request is only set on specific synchronous events, see
>> target-i386/kvm.c.
>>
>> More interesting is CPUState::thread_kicked. If it's set, qemu_cpu_kick
>> will skip the kicking via a signal. Maybe there is some race. Let me
>> think about such possibilities again...
> 
> diff --git a/cpus.c b/cpus.c
> index e476a3c..30f3228 100644
> --- a/cpus.c
> +++ b/cpus.c
> @@ -726,6 +726,9 @@ static void qemu_kvm_wait_io_event(CPUArchState *env)
>      }
>  
>      qemu_kvm_eat_signals(env);
> +    /* Ensure that checking env->stop cannot overtake signal processing so
> +     * that we lose the latter without stopping. */
> +    smp_rmb();

rmb is nonsense. Should be a plain barrier() - if at all.

>      qemu_wait_io_event_common(env);
>  }
>  
> Can anyone imagine that such a barrier may actually be required? If it
> is currently possible that env->stop is evaluated before we called into
> sigtimedwait in qemu_kvm_eat_signals, then we could actually eat the
> signal without properly processing its reason (stop).

Jan

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