On Mon, 10 May 2021 22:56:46 +0200
Eric Farman <farman@xxxxxxxxxxxxx> 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@xxxxxxxxxxxxx>
> ---
> 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>
>
> 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 :)
> private->state = VFIO_CCW_STATE_IDLE;
>
> if (private->io_trigger)
Patch looks good to me.
