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From: Cornelia Huck <cohuck@redhat.com>
To: Eric Farman <farman@linux.ibm.com>
Cc: Matthew Rosato <mjrosato@linux.ibm.com>,
	Halil Pasic <pasic@linux.ibm.com>,
	Jared Rossi <jrossi@linux.ibm.com>,
	linux-s390@vger.kernel.org, kvm@vger.kernel.org
Subject: Re: [RFC PATCH v5 3/3] vfio-ccw: Serialize FSM IDLE state with I/O completion
Date: Tue, 11 May 2021 13:31:54 +0200	[thread overview]
Message-ID: <20210511133154.66440087.cohuck@redhat.com> (raw)
In-Reply-To: <20210510205646.1845844-4-farman@linux.ibm.com>

On Mon, 10 May 2021 22:56:46 +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 | 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.


  reply	other threads:[~2021-05-11 11:32 UTC|newest]

Thread overview: 6+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-05-10 20:56 [RFC PATCH v5 0/3] vfio-ccw: Fix interrupt handling for HALT/CLEAR Eric Farman
2021-05-10 20:56 ` [RFC PATCH v5 1/3] vfio-ccw: Check initialized flag in cp_init() Eric Farman
2021-05-10 20:56 ` [RFC PATCH v5 2/3] vfio-ccw: Reset FSM state to IDLE inside FSM Eric Farman
2021-05-10 20:56 ` [RFC PATCH v5 3/3] vfio-ccw: Serialize FSM IDLE state with I/O completion Eric Farman
2021-05-11 11:31   ` Cornelia Huck [this message]
2021-05-11 18:02     ` Eric Farman

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