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

From: Lee Jones <[email-masked]>
Date: Fri, 4 Mar 2022 08:11:59 +0000

On Fri, 04 Mar 2022, Leon Romanovsky wrote:

> 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 <[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);

                HERE ---^

> > > 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.
> >  */
> 
> This description looks better, but the check is inherently racy.
> Why don't you add a comment and mutex_lock()?

We do, look up.  ^

> The WARN_ON here is more distraction than actual help.

The WARN() is just an indication that something else has gone wrong.

Stefano patched one problem in:

  vhost: Protect the virtqueue from being cleared whilst still in use

... but others may crop up and the WARN() is how we'll be informed.

-- 
Lee Jones [李琼斯]
Principal Technical Lead - Developer Services
Linaro.org │ Open source software for Arm SoCs
Follow Linaro: Facebook | Twitter | Blog