Re: [PATCH v6 3/3] vfio-ccw: Serialize FSM IDLE state with I/O completion

[Cornelia Huck <cohuck@redhat.com>]

On Tue, 11 May 2021 21:56:31 +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 | 12 ++++++++++--
>  1 file changed, 10 insertions(+), 2 deletions(-)

Looking good :)

Reviewed-by: Cornelia Huck <cohuck@redhat.com>