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

[Jan Kiszka]

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();
     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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