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

Cornelia Huck
2021-04-21T10:26:38Z

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? Maybe remember
that we had a final I/O interrupt for an I/O request and only change
the state in this case?

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