Subject: Re: [PATCH] vhost-vdpa: log warning message if vhost_vdpa_remove gets blocked

From: "Michael S. Tsirkin" <[email-masked]>
Date: Sat, 3 Jul 2021 04:11:45 -0400

On Tue, Jun 15, 2021 at 10:33:22PM +0800, Jason Wang wrote:
> 
> 在 2021/6/6 下午9:29, Gautam Dawar 写道:
> > From: Gautam Dawar <[email-masked]>
> > 
> > If some module invokes vdpa_device_unregister (usually in the module
> > unload function) when the userspace app (eg. QEMU) which had opened
> > the vhost-vdpa character device is still running, vhost_vdpa_remove()
> > function will block indefinitely in call to wait_for_completion().
> > 
> > This causes the vdpa_device_unregister caller to hang and with a
> > usual side-effect of rmmod command not returning when this call
> > is in the module_exit function.
> > 
> > This patch converts the wait_for_completion call to its timeout based
> > counterpart (wait_for_completion_timeout) and also adds a warning
> > message to alert the user/administrator about this hang situation.
> > 
> > To eventually fix this problem, a mechanism will be required to let
> > vhost-vdpa module inform the userspace of this situation and
> > userspace will close the descriptor of vhost-vdpa char device.
> > This will enable vhost-vdpa to continue with graceful clean-up.
> > 
> > Signed-off-by: Gautam Dawar <[email-masked]>
> > ---
> >   drivers/vhost/vdpa.c | 6 +++++-
> >   1 file changed, 5 insertions(+), 1 deletion(-)
> > 
> > diff --git a/drivers/vhost/vdpa.c b/drivers/vhost/vdpa.c
> > index bfa4c6ef554e..572b64d09b06 100644
> > --- a/drivers/vhost/vdpa.c
> > +++ b/drivers/vhost/vdpa.c
> > @@ -1091,7 +1091,11 @@ static void vhost_vdpa_remove(struct vdpa_device *vdpa)
> >   		opened = atomic_cmpxchg(&v->opened, 0, 1);
> >   		if (!opened)
> >   			break;
> > -		wait_for_completion(&v->completion);
> > +		wait_for_completion_timeout(&v->completion,
> > +					    msecs_to_jiffies(1000));
> > +		dev_warn_ratelimited(&v->dev,
> > +				     "%s waiting for /dev/%s to be closed\n",
> > +				     __func__, dev_name(&v->dev));

Can fill up the kernel log in this case ... dev_warn_once seems more
appropriate.

> >   	} while (1);
> >   	put_device(&v->dev);
> 
> 
> Acked-by: Jason Wang <[email-masked]>
>