Re: [Qemu-devel] [PATCH] qom/cpu: Remove cpu->exit_request from reset state

[Peter Maydell]

On 11 March 2014 23:58, Andreas Färber <address@hidden> wrote:
> Am 20.02.2014 16:58, schrieb Peter Maydell:
>> On 16 February 2014 02:07, Edgar E. Iglesias <address@hidden> wrote:
>>> On Sat, Feb 15, 2014 at 03:42:56PM +0000, Peter Maydell wrote:
>>>> On 13 February 2014 05:07,  <address@hidden> wrote:
>>>>> From: "Edgar E. Iglesias" <address@hidden>
>>>>>
>>>>> cpu->exit_request is part of the execution environment and should
>>>>> not be cleared when a CPU resets.
>>>>>
>>>>> Otherwise, we might deadlock QEMU if a CPU resets while there is
>>>>> I/O going on.
>>>>>
>>>>> Signed-off-by: Edgar E. Iglesias <address@hidden>
>>>>> ---
>>>>>  qom/cpu.c | 1 -
>>>>>  1 file changed, 1 deletion(-)
>>>>>
>>>>> diff --git a/qom/cpu.c b/qom/cpu.c
>>>>> index 9d62479..40d82dd 100644
>>>>> --- a/qom/cpu.c
>>>>> +++ b/qom/cpu.c
>>>>> @@ -195,7 +195,6 @@ static void cpu_common_reset(CPUState *cpu)
>>>>>          log_cpu_state(cpu, cc->reset_dump_flags);
>>>>>      }
>>>>>
>>>>> -    cpu->exit_request = 0;
>>>>>      cpu->interrupt_request = 0;
>>>>>      cpu->current_tb = NULL;
>>>>>      cpu->halted = 0;
>>>>
>>>> This looks kind of odd to me. What's the situation you see where
>>>> this matters -- is the CPU resetting itself, or is some other device
>>>> in another thread triggering the CPU reset? TCG or KVM?
>>>
>>> Seeing this in TCG. The CPU gets signaled by the IO thread while the
>>> CPU is resetting itself. If the CPU looses the race, it clears its
>>> exit_request leaving the IO thread waiting for the global lock
>>> potentially forever.
>>>
>>> The CPU actually exits generated code but goes right back in because
>>> there is no exit_request pending.
>>
>> Yes, having looked at the code I agree with you, so:
>>
>> Reviewed-by: Peter Maydell <address@hidden>
>
> Thanks, applied to qom-cpu (with clarified commit message):
> https://github.com/afaerber/qemu-cpu/commits/qom-cpu

I'd forgotten about this, but it's a bugfix for a hang, right?
Seems to me like we ought to put it into 2.0 -- were you
planning to submit it via qom-cpu for 2.0?

thanks
-- PMM