On Thu, 24 Jan 2019 21:37:44 -0500
Eric Farman <farman@xxxxxxxxxxxxx> wrote:
> On 01/24/2019 09:25 PM, Eric Farman wrote:
> >
> >
> > On 01/21/2019 06:03 AM, Cornelia Huck wrote:
> > [1] I think these changes are cool. We end up going into (and staying
> > in) state=BUSY if we get cc=0 on the SSCH, rather than in/out as we
> > bumble along.
> >
> > But why can't these be separated out from this patch? It does change
> > the behavior of the state machine, and seem distinct from the addition
> > of the mutex you otherwise add here? At the very least, this behavior
> > change should be documented in the commit since it's otherwise lost in
> > the mutex/EAGAIN stuff.
That's a very good idea. I'll factor them out into a separate patch.
> >
> >> trace_vfio_ccw_io_fctl(scsw->cmd.fctl, get_schid(private),
> >> io_region->ret_code, errstr);
> >> }
> >> diff --git a/drivers/s390/cio/vfio_ccw_ops.c
> >> b/drivers/s390/cio/vfio_ccw_ops.c
> >> index f673e106c041..3fa9fc570400 100644
> >> --- a/drivers/s390/cio/vfio_ccw_ops.c
> >> +++ b/drivers/s390/cio/vfio_ccw_ops.c
> >> @@ -169,16 +169,20 @@ static ssize_t vfio_ccw_mdev_read(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));
> >> + mutex_lock(&private->io_mutex);
> >> region = private->io_region;
> >> if (copy_to_user(buf, (void *)region + *ppos, count))
> >> - return -EFAULT;
> >> -
> >> - return count;
> >> + ret = -EFAULT;
> >> + else
> >> + ret = count;
> >> + mutex_unlock(&private->io_mutex);
> >> + return ret;
> >> }
> >> 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;
> >
> > 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.)
My idea behind the mutex was to make sure that we get consistent data
when reading/writing (e.g. if one user space thread is reading the I/O
region while another is writing to it).
