Re: [PATCH 1/1] vhost: Provide a kernel warning if mutex is held whilst clean-up in progress

[Stefano Garzarella]

On Thu, Mar 03, 2022 at 04:01:06PM -0500, Michael S. Tsirkin wrote:
> On Thu, Mar 03, 2022 at 09:14:36PM +0200, Leon Romanovsky wrote:
> > On Thu, Mar 03, 2022 at 03:19:29PM +0000, Lee Jones wrote:
> > > All workers/users should be halted before any clean-up should take place.
> > >
> > > Suggested-by:  Michael S. Tsirkin <mst@xxxxxxxxxx>
> > > Signed-off-by: Lee Jones <lee.jones@xxxxxxxxxx>
> > > ---
> > >  drivers/vhost/vhost.c | 3 +++
> > >  1 file changed, 3 insertions(+)
> > >
> > > diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c
> > > index bbaff6a5e21b8..d935d2506963f 100644
> > > --- a/drivers/vhost/vhost.c
> > > +++ b/drivers/vhost/vhost.c
> > > @@ -693,6 +693,9 @@ void vhost_dev_cleanup(struct vhost_dev *dev)
> > >  	int i;
> > >
> > >  	for (i = 0; i < dev->nvqs; ++i) {
> > > +		/* Ideally all workers should be stopped prior to clean-up */
> > > +		WARN_ON(mutex_is_locked(&dev->vqs[i]->mutex));
> > > +
> > >  		mutex_lock(&dev->vqs[i]->mutex);
> >
> > I know nothing about vhost, but this construction and patch looks
> > strange to me.
> >
> > If all workers were stopped, you won't need mutex_lock(). The mutex_lock
> > here suggests to me that workers can still run here.
> >
> > Thanks
>
>
> "Ideally" here is misleading, we need a bigger detailed comment
> along the lines of:
>
> /*
> * By design, no workers can run here. But if there's a bug and the
> * driver did not flush all work properly then they might, and we
> * encountered such bugs in the past.  With no proper flush guest won't
> * work correctly but avoiding host memory corruption in this case
> * sounds like a good idea.
> */

Can we use vhost_vq_get_backend() to check this situation?

IIUC all the vhost devices clear the backend to stop the workers.
This is not racy (if we do after the mutex_lock) and should cover all 
cases.

Thanks,
Stefano