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

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

On Thu, 24 Jan 2019 21:37:44 -0500
Eric Farman <farman@linux.ibm.com> wrote:

> >> @@ -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;  
> > 
> > Ah, I see Halil's difficulty here.
> > 
> > It is true there is a race condition today, and that this doesn't 
> > address it.  That's fine, add it to the todo list.  But even with that, 
> > I don't see what the mutex is enforcing?  Two simultaneous SSCHs will be 
> > serialized (one will get kicked out with a failed trylock() call), while 
> > still leaving the window open between cc=0 on the SSCH and the 
> > subsequent interrupt.  In the latter case, a second SSCH will come 
> > through here, do the copy_from_user below, and then jump to fsm_io_busy 
> > to return EAGAIN.  Do we really want to stomp on io_region in that case? 
> >   Why can't we simply return EAGAIN if state==BUSY?  
> 
> (Answering my own questions as I skim patch 5...)
> 
> Because of course this series is for async handling, while I was looking 
> specifically at the synchronous code that exists today.  I guess then my 
> question just remains on how the mutex is adding protection in the sync 
> case, because that's still not apparent to me.  (Perhaps I missed it in 
> a reply to Halil; if so I apologize, there were a lot when I returned.)

Careful, at the end we have vfio_ccw_mdev_write_io_region() and the
write callback for the capchain regions. We could return EAGAIN if
state==BUSY in the vfio_ccw_mdev_write_io_region() (but I would prefer a
different error code -- see my other response).

I answered your mutex question as well. Just a small addendum the mutex
is not only for the cases the userspace acts sane (but also when it acts
insane;).

Halil