On Tue, Oct 20, 2020 at 12:41 AM Michael S. Tsirkin <mst@redhat.com> wrote:

> On Mon, Oct 19, 2020 at 11:56:35PM +0800, 谢永吉 wrote:
> >
> >
> >
> > On Mon, Oct 19, 2020 at 11:47 PM Michael S. Tsirkin <mst@redhat.com>
> wrote:
> >
> >     On Mon, Oct 19, 2020 at 11:44:36PM +0800, 谢永吉 wrote:
> >     >
> >     >
> >     > On Mon, Oct 19, 2020 at 11:05 PM Michael S. Tsirkin <
> mst@redhat.com>
> >     wrote:
> >     >
> >     >     On Mon, Oct 19, 2020 at 10:56:22PM +0800, Xie Yongji wrote:
> >     >     > The module should not be unloaded if any vduse device exists.
> >     >     > So increase the module's reference count when creating vduse
> >     >     > device. And the reference count is kept until the device is
> >     >     > destroyed.
> >     >     >
> >     >     > Signed-off-by: Xie Yongji <xieyongji@bytedance.com>
> >     >     > ---
> >     >     >  drivers/vdpa/vdpa_user/vduse_dev.c | 2 ++
> >     >     >  1 file changed, 2 insertions(+)
> >     >     >
> >     >     > diff --git a/drivers/vdpa/vdpa_user/vduse_dev.c
> b/drivers/vdpa/
> >     vdpa_user/
> >     >     vduse_dev.c
> >     >     > index 6787ba66725c..f04aa02de8c1 100644
> >     >     > --- a/drivers/vdpa/vdpa_user/vduse_dev.c
> >     >     > +++ b/drivers/vdpa/vdpa_user/vduse_dev.c
> >     >     > @@ -887,6 +887,7 @@ static int vduse_destroy_dev(u32 id)
> >     >     >       kfree(dev->vqs);
> >     >     >       vduse_iova_domain_destroy(dev->domain);
> >     >     >       vduse_dev_destroy(dev);
> >     >     > +     module_put(THIS_MODULE);
> >     >     >
> >     >     >       return 0;
> >     >     >  }
> >     >     > @@ -931,6 +932,7 @@ static int vduse_create_dev(struct
> >     vduse_dev_config
> >     >     *config)
> >     >     >
> >     >     >       dev->connected = true;
> >     >     >       list_add(&dev->list, &vduse_devs);
> >     >     > +     __module_get(THIS_MODULE);
> >     >     >
> >     >     >       return fd;
> >     >     >  err_fd:
> >     >
> >     >     This kind of thing is usually an indicator of a bug. E.g.
> >     >     if the refcount drops to 0 on module_put(THIS_MODULE) it
> >     >     will be unloaded and the following return will not run.
> >     >
> >     >
> >     >
> >     > Should this happen?  The refcount should be only decreased to 0
> after the
> >     > misc_device is closed?
> >     >
> >     > Thanks,
> >     > Yongji
> >     >
> >
> >     OTOH if it never drops to 0 anyway then why do you need to increase
> it?
> >
> >
> >
> > To prevent unloading the module in the case that the device is created,
> but no
> > user process using it (e.g. the user process crashed).
> >
> > Thanks,
> > Yongji
>
> Looks like it can drop to 0 if that is the case then?
>
> Could you give me an example?  In my understanding, vduse_create_dev()
should be called only after we open the chardev which will grab the
module's reference.

Thanks,
Yongji