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

[Anthony Liguori <anthony@codemonkey.ws>]

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.

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?

Regards,

Anthony Liguori

> ---
>   net/tap.c |    4 ++++
>   1 files changed, 4 insertions(+), 0 deletions(-)
>
> diff --git a/net/tap.c b/net/tap.c
> index 0147dab..f34dd9c 100644
> --- a/net/tap.c
> +++ b/net/tap.c
> @@ -330,6 +330,9 @@ static int launch_script(const char *setup_script, const char *ifname, int fd)
>       sigaddset(&mask, SIGCHLD);
>       sigprocmask(SIG_BLOCK,&mask,&oldmask);
>
> +    /* make sure no cpus are running, so the apic does not
> +     * get confused */
> +    vm_stop(0);
>       /* try to launch network script */
>       pid = fork();
>       if (pid == 0) {
> @@ -350,6 +353,7 @@ static int launch_script(const char *setup_script, const char *ifname, int fd)
>           execv(setup_script, args);
>           _exit(1);
>       } else if (pid>  0) {
> +        vm_start();
>           while (waitpid(pid,&status, 0) != pid) {
>               /* loop */
>           }
>