Re: [qemu-s390x] [Qemu-devel] [PATCH v2 2/5] vfio-ccw: concurrent I/O handling

[Cornelia Huck]

On Mon, 21 Jan 2019 21:20:18 +0100
Halil Pasic <address@hidden> wrote:

> On Mon, 21 Jan 2019 12:03:51 +0100
> Cornelia Huck <address@hidden> 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 <address@hidden>
> > ---  
> 
> [..]
> 
> >  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;
> >  }
> >    
> [..]
>