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

["Daniel P. Berrange" <berrange@redhat.com>]

On Mon, Jun 14, 2010 at 04:42:37PM -0300, Glauber Costa wrote:
> On Mon, Jun 14, 2010 at 02:33:00PM -0500, 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.
> > >
> > >Signed-off-by: Glauber Costa<glommer@redhat.com>
> > 
> > This doesn't make very much sense to me but smells like a kernel bug to me.
> My interpretation is that by doing that, we make sure no in-flight
> requests are happening. Actually, a sleep(x), with x sufficiently big
> is enough to make this problem go away, but that is too hacky.
> 
> I do agree that this is most likely a kernel bug. But as with any other
> kernel bugs, I believe this is a easy workaround to have things working
> even in older kernels until we fix it.
> 
> > 
> > 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 doubt fork is duplicating the vcpu threads. More than that, this
> bug does not happen with userspace irqchip.

Hmm, could this problem be hitting other places where we fork() besides
the netdev helper script ? I have seen a intermittent bug[1] with migration, 
where using exec: migration will freeze the entire guest and setting
-no-kvm-irqchip fixes the problem

Regards,
Daniel

[1] https://bugzilla.redhat.com/show_bug.cgi?id=585195
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