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

[Cornelia Huck <cohuck@redhat.com>]

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).

> 
> > +		ret = -EFAULT;
> > +		goto out_unlock;
> > +	}
> >  
> >  	vfio_ccw_fsm_event(private, VFIO_CCW_EVENT_IO_REQ);
> > -	if (region->ret_code != 0) {
> > -		private->state = VFIO_CCW_STATE_IDLE;
> > -		return region->ret_code;
> > -	}
> > +	ret = (region->ret_code != 0) ? region->ret_code : count;
> >  
> > -	return count;
> > +out_unlock:
> > +	mutex_unlock(&private->io_mutex);
> > +	return ret;
> >  }
> >    
> [..]
>