Re: [Qemu-devel] [PATCH] stop cpus before forking.

[Avi Kivity <avi@redhat.com>]

On 06/14/2010 10:33 PM, Anthony Liguori wrote:
> On 06/14/2010 02:27 PM, Glauber Costa wrote:
>> This patch fixes a bug that happens with kvm, irqchip-in-kernel,
>> while adding a netdev. Despite the situations of reproduction being
>> specific to kvm, I believe this fix is pretty generic, and fits here.
>> Specially if we ever want to have our own irqchip in kernel too.
>>
>> The problem happens after the fork system call, and although it is not
>> 100 % reproduceable, happens pretty often. After fork, the memory where
>> the apic is mapped is present in both processes. It ends up confusing
>> the vcpus somewhere in the irq<->  ack path, and qemu hangs, with no
>> irqs being delivered at all from that point on.
>>
>> Making sure the vcpus are stopped before forking makes the problem go
>> away. Besides, this is a pretty unfrequent operation, which already 
>> hangs
>> the io-thread for a while. So it should not hurt performance.
>
> This doesn't make very much sense to me but smells like a kernel bug 
> to me.

It is, and the fix would be to create the APIC memory slot as sharable 
across forks (should be easy to fix in the kernel).

> Even if it isn't, I can't rationalize why stopping the vm like this is 
> enough to fix such a problem.  Is the problem that the KVM VCPU 
> threads get duplicated while potentially running or something like that?

I think it's COW triggering a copy on one vcpu while the other reads 
from the old page.

-- 
error compiling committee.c: too many arguments to function