Re: [RFC PATCH v4 4/4] vfio-ccw: Reset FSM state to IDLE before io_mutex

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

On Wed, 2021-04-21 at 08:58 -0400, Eric Farman wrote:
> On Wed, 2021-04-21 at 12:25 +0200, Cornelia Huck wrote:
> > On Tue, 13 Apr 2021 20:24:10 +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()
> > > 					fsm_io_helper()
> > > 	vfio_ccw_sch_io_todo()
> > > 					fsm_irq()
> > > 					vfio_ccw_sch_io_todo()
> > > 
> > > Let's move the reset of the FSM state to the point where the
> > > channel_program struct is cleaned up, which is only done for
> > > solicited interrupts anyway.
> > > 
> > > Signed-off-by: Eric Farman <farman@linux.ibm.com>
> > > ---
> > >  drivers/s390/cio/vfio_ccw_drv.c | 7 +++----
> > >  1 file changed, 3 insertions(+), 4 deletions(-)
> > > 
> > > diff --git a/drivers/s390/cio/vfio_ccw_drv.c
> > > b/drivers/s390/cio/vfio_ccw_drv.c
> > > index 8c625b530035..e51318f23ca8 100644
> > > --- a/drivers/s390/cio/vfio_ccw_drv.c
> > > +++ b/drivers/s390/cio/vfio_ccw_drv.c
> > > @@ -94,16 +94,15 @@ 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);
> > > +			private->state = VFIO_CCW_STATE_IDLE;
> > > +		}
> > >  	}
> > >  	mutex_lock(&private->io_mutex);
> > >  	memcpy(private->io_region->irb_area, irb, sizeof(*irb));
> > >  	mutex_unlock(&private->io_mutex);
> > >  
> > > -	if (private->mdev && is_final)
> > > -		private->state = VFIO_CCW_STATE_IDLE;
> > 
> > Isn't that re-allowing new I/O requests a bit too early?
> 
> Hrm... I guess I don't see what work vfio-ccw has left to do that is
> presenting it from carrying on. The copying of the IRB data back into
> the io_region seems like a flimsy gate to me. But...
> 
> It seems you're (rightly) concerned with userspace doing SSCH + SSCH,
> whereas I'v been focused on the CSCH + SSCH sequence. So with this
> change, we're inviting the possibility of a second SSCH being able to
> be submitted/started before the IRB data for the first SSCH is copied
> (and presumably before userspace is tapped to read that data back).
> 
> Sigh... I guess that's not the greatest behavior either. Gotta
> ruminate
> on this.
> 
> >  Maybe remember
> > that we had a final I/O interrupt for an I/O request and only
> > change
> > the state in this case?
> 
> As a local flag within this routine? Hrm... I have entirely too many
> "Let's try this" branches that didn't work, but I don't see that one
> jumping out at me. Will give it a try.

Still going strong, so that bodes really well (knock wood). I need to
spend a little time with patch 2 before I send the next version, but
that shouldn't be too long.

Eric

> 
> > 
> > > -
> > >  	if (private->io_trigger)
> > >  		eventfd_signal(private->io_trigger, 1);
> > >  }