Re: [PATCH V2 5/8] mdev: introduce device specific ops

[Alex Williamson <alex.williamson@redhat.com>]

On Wed, 25 Sep 2019 10:11:00 -0400
Rob Miller <rob.miller@broadcom.com> wrote:
> > > On Tue, 24 Sep 2019 21:53:29 +0800
> > > Jason Wang <jasowang@redhat.com> wrote:
> > > > diff --git a/drivers/vfio/mdev/vfio_mdev.c  
> > > b/drivers/vfio/mdev/vfio_mdev.c  
> > > > index 891cf83a2d9a..95efa054442f 100644
> > > > --- a/drivers/vfio/mdev/vfio_mdev.c
> > > > +++ b/drivers/vfio/mdev/vfio_mdev.c
> > > > @@ -14,6 +14,7 @@
> > > >  #include <linux/slab.h>
> > > >  #include <linux/vfio.h>
> > > >  #include <linux/mdev.h>
> > > > +#include <linux/vfio_mdev.h>
> > > >
> > > >  #include "mdev_private.h"
> > > >
> > > > @@ -24,16 +25,16 @@
> > > >  static int vfio_mdev_open(void *device_data)
> > > >  {
> > > >     struct mdev_device *mdev = device_data;
> > > > -   struct mdev_parent *parent = mdev->parent;
> > > > +   const struct vfio_mdev_device_ops *ops =  
> > > mdev_get_dev_ops(mdev);  
> > > >     int ret;
> > > >
> > > > -   if (unlikely(!parent->ops->open))
> > > > +   if (unlikely(!ops->open))
> > > >             return -EINVAL;
> > > >
> > > >     if (!try_module_get(THIS_MODULE))
> > > >             return -ENODEV;  
> >  
> 
> RJM>] My understanding lately is that this call to  
> try_module_get(THIS_MODULE) is no longer needed as is considered as a
> latent bug.
> Quote from
> https://stackoverflow.com/questions/1741415/linux-kernel-modules-when-to-use-try-module-get-module-put
>  :
> There are a number of uses of try_module_get(THIS_MODULE) in the kernel
> source but most if not all of them are latent bugs that should be cleaned
> up.

This use seems to fall exactly into the case where it is necessary, the
open here is not a direct VFS call, it's an internal interface between
modules.  The user is interacting with filesystem objects from the vfio
module and the module reference we're trying to acquire here is to the
vfio-mdev module.  Thanks,

Alex
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