Virtualization - Mar 2022 - [PATCH 1/1] vhost: Provide a kernel warning if mutex is held whilst clean-up in progress

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 at redhat.com>
> Signed-off-by: Lee Jones <lee.jones at linaro.org>
> ---
>  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
>  		if (dev->vqs[i]->error_ctx)
>  			eventfd_ctx_put(dev->vqs[i]->error_ctx);
> -- 
> 2.35.1.574.g5d30c73bfb-goog
>

On Thu, 03 Mar 2022, 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 at redhat.com>
> > Signed-off-by: Lee Jones <lee.jones at linaro.org>
> > ---
> >  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.
The suggestion for this patch came from the maintainer.

Please see the conversation here:

https://lore.kernel.org/all/20220302082021-mutt-send-email-mst at kernel.org/

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

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 at redhat.com>
> > Signed-off-by: Lee Jones <lee.jones at linaro.org>
> > ---
> >  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
> >