Re: [Qemu-devel] [PATCH v2 2/5] vfio-ccw: concurrent I/O handling

[Halil Pasic <pasic@linux.ibm.com>]

On Tue, 22 Jan 2019 11:29:26 +0100
Cornelia Huck <cohuck@redhat.com> wrote:

> On Mon, 21 Jan 2019 21:20:18 +0100
> Halil Pasic <pasic@linux.ibm.com> wrote:
> 
> > On Mon, 21 Jan 2019 12:03:51 +0100
> > Cornelia Huck <cohuck@redhat.com> wrote:
> > 
> > > Rework handling of multiple I/O requests to return -EAGAIN if
> > > we are already processing an I/O request. Introduce a mutex
> > > to disallow concurrent writes to the I/O region.
> > > 
> > > The expectation is that userspace simply retries the operation
> > > if it gets -EAGAIN.
> > > 
> > > We currently don't allow multiple ssch requests at the same
> > > time, as we don't have support for keeping channel programs
> > > around for more than one request.
> > > 
> > > Signed-off-by: Cornelia Huck <cohuck@redhat.com>
> > > ---  
> > 
> > [..]
> > 
> > >  static ssize_t vfio_ccw_mdev_write(struct mdev_device *mdev,
> > > @@ -188,25 +192,30 @@ static ssize_t vfio_ccw_mdev_write(struct mdev_device *mdev,
> > >  {
> > >  	struct vfio_ccw_private *private;
> > >  	struct ccw_io_region *region;
> > > +	int ret;
> > >  
> > >  	if (*ppos + count > sizeof(*region))
> > >  		return -EINVAL;
> > >  
> > >  	private = dev_get_drvdata(mdev_parent_dev(mdev));
> > > -	if (private->state != VFIO_CCW_STATE_IDLE)
> > > +	if (private->state == VFIO_CCW_STATE_NOT_OPER ||
> > > +	    private->state == VFIO_CCW_STATE_STANDBY)
> > >  		return -EACCES;
> > > +	if (!mutex_trylock(&private->io_mutex))
> > > +		return -EAGAIN;
> > >  
> > >  	region = private->io_region;
> > > -	if (copy_from_user((void *)region + *ppos, buf, count))
> > > -		return -EFAULT;
> > > +	if (copy_from_user((void *)region + *ppos, buf, count)) {  
> > 
> > This might race with vfio_ccw_sch_io_todo() on
> > private->io_region->irb_area, or?
> 
> Ah yes, this should also take the mutex (should work because we're on a
> workqueue).
> 

I'm not sure that will do the trick (assumed I understood the
intention correctly). Let's say the things happen in this order:
1) vfio_ccw_sch_io_todo() goes first, I guess updates
private->io_region->irb_area and releases the mutex.
2) Then vfio_ccw_mdev_write() destroys the irb_area by zeriong it out,
and finally,
3) userspace reads the destroyed irb_area using vfio_ccw_mdev_read().

Or am I misunderstanding something? 

Regards,
Halil