Re: [RFC PATCH v5 3/3] vfio-ccw: Serialize FSM IDLE state with I/O completion

[Eric Farman <farman@linux.ibm.com>]

On Tue, 2021-05-11 at 13:31 +0200, Cornelia Huck wrote:
> On Mon, 10 May 2021 22:56:46 +0200
> Eric Farman <farman@linux.ibm.com> wrote:
> 
> > Today, the stacked call to vfio_ccw_sch_io_todo() does three
> > things:
> > 
> >   1) Update a solicited IRB with CP information, and release the CP
> >      if the interrupt was the end of a START operation.
> >   2) Copy the IRB data into the io_region, under the protection of
> >      the io_mutex
> >   3) Reset the vfio-ccw FSM state to IDLE to acknowledge that
> >      vfio-ccw can accept more work.
> > 
> > The trouble is that step 3 is (A) invoked for both solicited and
> > unsolicited interrupts, and (B) sitting after the mutex for step 2.
> > This second piece becomes a problem if it processes an interrupt
> > for a CLEAR SUBCHANNEL while another thread initiates a START,
> > thus allowing the CP and FSM states to get out of sync. That is:
> > 
> >     CPU 1                           CPU 2
> >     fsm_do_clear()
> >     fsm_irq()
> >                                     fsm_io_request()
> >     vfio_ccw_sch_io_todo()
> >                                     fsm_io_helper()
> > 
> > Since the FSM state and CP should be kept in sync, let's make a
> > note when the CP is released, and rely on that as an indication
> > that the FSM should also be reset at the end of this routine and
> > open up the device for more work.
> > 
> > Signed-off-by: Eric Farman <farman@linux.ibm.com>
> > ---
> >  drivers/s390/cio/vfio_ccw_drv.c | 8 +++++---
> >  1 file changed, 5 insertions(+), 3 deletions(-)
> > 
> > diff --git a/drivers/s390/cio/vfio_ccw_drv.c
> > b/drivers/s390/cio/vfio_ccw_drv.c
> > index 8c625b530035..ef39182edab5 100644
> > --- a/drivers/s390/cio/vfio_ccw_drv.c
> > +++ b/drivers/s390/cio/vfio_ccw_drv.c
> > @@ -85,7 +85,7 @@ static void vfio_ccw_sch_io_todo(struct
> > work_struct *work)
> >  {
> >  	struct vfio_ccw_private *private;
> >  	struct irb *irb;
> > -	bool is_final;
> > +	bool is_final, is_finished = false;
> 
> <bikeshed>
> "is_finished" does not really say what is finished; maybe call it
> "cp_is_finished"?
> </bikeshed>

Sure, that's a bit clearer.

> 
> >  
> >  	private = container_of(work, struct vfio_ccw_private, io_work);
> >  	irb = &private->irb;
> > @@ -94,14 +94,16 @@ static void vfio_ccw_sch_io_todo(struct
> > work_struct *work)
> >  		     (SCSW_ACTL_DEVACT | SCSW_ACTL_SCHACT));
> >  	if (scsw_is_solicited(&irb->scsw)) {
> >  		cp_update_scsw(&private->cp, &irb->scsw);
> > -		if (is_final && private->state ==
> > VFIO_CCW_STATE_CP_PENDING)
> > +		if (is_final && private->state ==
> > VFIO_CCW_STATE_CP_PENDING) {
> >  			cp_free(&private->cp);
> > +			is_finished = true;
> > +		}
> >  	}
> >  	mutex_lock(&private->io_mutex);
> >  	memcpy(private->io_region->irb_area, irb, sizeof(*irb));
> >  	mutex_unlock(&private->io_mutex);
> >  
> > -	if (private->mdev && is_final)
> > +	if (private->mdev && is_finished)
> 
> Maybe add a comment?
> 
> /*
>  * Reset to idle if processing of a channel program
>  * has finished; but do not overwrite a possible
>  * processing state if we got a final interrupt for hsch
>  * or csch.
>  */
> 
> Otherwise, I see us scratching our heads again in a few months :)

Almost certainly. :)

> 
> >  		private->state = VFIO_CCW_STATE_IDLE;
> >  
> >  	if (private->io_trigger)
> 
> Patch looks good to me.
> 

Thanks. Will make the above improvements and send as non-RFC.