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Re: [libvirt] [PATCH 1/2] qemu: Add missing lock of virDomainObj before calling virDomainUnref

On 03/03/2011 12:47 PM, Laine Stump wrote:
> This was found while researching the root cause of:
>     https://bugzilla.redhat.com/show_bug.cgi?id=670848
> virDomainUnref should only be called with the lock held for the
> virDomainObj in question. However, when a transient qemu domain gets
> EOF on its monitor socket, it queues an event which frees the monitor,
> which unref's the virDomainObj without first locking it. If another
> thread has already locked the virDomainObj, the modification of the
> refcount could potentially be corrupted. In an extreme case, it could
> also be potentially unlocked by virDomainObjFree, thus left open to
> modification by anyone else who would have otherwise waited for the
> lock (not to mention the fact that they would be accessing freed
> data!).
> The solution is to have qemuMonitorFree lock the domain object right
> before unrefing it. Since the caller to qemuMonitorFree doesn't expect
> this lock to be held, if the refcount doesn't go all the way to 0,
> qemuMonitorFree must unlock it after the unref.
> ---
>  src/qemu/qemu_process.c |    4 +++-
>  1 files changed, 3 insertions(+), 1 deletions(-)
> diff --git a/src/qemu/qemu_process.c b/src/qemu/qemu_process.c
> index c419c75..1d67b3c 100644
> --- a/src/qemu/qemu_process.c
> +++ b/src/qemu/qemu_process.c
> @@ -596,7 +596,9 @@ static void qemuProcessHandleMonitorDestroy(qemuMonitorPtr mon,
>      qemuDomainObjPrivatePtr priv = vm->privateData;
>      if (priv->mon == mon)
>          priv->mon = NULL;
> -    virDomainObjUnref(vm);
> +    virDomainObjLock(vm);
> +    if (virDomainObjUnref(vm) > 0)
> +        virDomainObjUnlock(vm);

Phooey, this causes 'virsh save domain file' to deadlock.

I'm still looking into why, but it may be that we're going about fixing
this wrong.  Maybe we should be making sure that qemuProcessStop ensures
that the monitor callback function won't trigger if there is no domain
for it to trigger on.

Eric Blake   eblake redhat com    +1-801-349-2682
Libvirt virtualization library http://libvirt.org

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