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

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On Tue, 13 Apr 2021 20:24:10 +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()
> 					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@xxxxxxxxxxxxx>
> ---
>  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);
>  }




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