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

From: "Michael S. Tsirkin" <[email-masked]>
Date: Thu, 3 Mar 2022 16:01:06 -0500

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 <[email-masked]>
> > Signed-off-by: Lee Jones <[email-masked]>
> > ---
> >  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.
 */

> >  		if (dev->vqs[i]->error_ctx)
> >  			eventfd_ctx_put(dev->vqs[i]->error_ctx);
> > -- 
> > 2.35.1.574.g5d30c73bfb-goog
> >