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

[Glauber Costa]

On Mon, Jun 14, 2010 at 02:58:47PM -0500, Anthony Liguori wrote:
> On 06/14/2010 02:42 PM, 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<address@hidden>
> >>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.
> 
> vm_stop() is probably just acting a glorified sleep() since it has
> to wait for each thread to stop.
No, it is not. It also makes sure no vcpus are running, and thus, not generating
new requests (or waiting for any replies, for that matter).

(Note: I am not advocating the inclusion of this, just trying to build my own
awareness)