linux-kernel - Re: [PATCH v2 06/10] vfio: ccw: Make FSM functions atomic

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Date:   Tue, 5 Jun 2018 15:35:39 +0200
From:   Cornelia Huck <cohuck@...hat.com>
To:     Pierre Morel <pmorel@...ux.ibm.com>
Cc:     Pierre Morel <pmorel@...ux.vnet.ibm.com>, pasic@...ux.vnet.ibm.com,
        bjsdjshi@...ux.vnet.ibm.com, linux-s390@...r.kernel.org,
        linux-kernel@...r.kernel.org, kvm@...r.kernel.org
Subject: Re: [PATCH v2 06/10] vfio: ccw: Make FSM functions atomic

On Tue, 5 Jun 2018 15:10:11 +0200
Pierre Morel <pmorel@...ux.ibm.com> wrote:

> On 05/06/2018 13:38, Cornelia Huck wrote:
> > On Fri, 25 May 2018 12:21:14 +0200
> > Pierre Morel <pmorel@...ux.vnet.ibm.com> wrote:
> >  
> >> We use mutex around the FSM function call to make the FSM
> >> event handling and state change atomic.  
> > I'm still not really clear as to what this mutex is supposed to
> > serialize:
> >
> > - Modification of the state?
> > - Any calls in the state machine?
> > - A combination? (That would imply that we only deal with the state in
> >    the state machine.)  
> 
> yes to all

But wouldn't that imply that you need to either take the mutex if you
do something dependent on the state, or fire an event in that case?

> 
> >  
> >> Signed-off-by: Pierre Morel <pmorel@...ux.vnet.ibm.com>
> >> ---
> >>   drivers/s390/cio/vfio_ccw_drv.c     | 3 +--
> >>   drivers/s390/cio/vfio_ccw_private.h | 3 +++
> >>   2 files changed, 4 insertions(+), 2 deletions(-)
> >>
> >> diff --git a/drivers/s390/cio/vfio_ccw_drv.c b/drivers/s390/cio/vfio_ccw_drv.c
> >> index 6b7112e..98951d5 100644
> >> --- a/drivers/s390/cio/vfio_ccw_drv.c
> >> +++ b/drivers/s390/cio/vfio_ccw_drv.c
> >> @@ -73,8 +73,6 @@ static void vfio_ccw_sch_io_todo(struct work_struct *work)
> >>   
> >>   	private = container_of(work, struct vfio_ccw_private, io_work);
> >>   	vfio_ccw_fsm_event(private, VFIO_CCW_EVENT_INTERRUPT);
> >> -	if (private->mdev)
> >> -		private->state = VFIO_CCW_STATE_IDLE;  
> > Looks like an unrelated change? If you want to do all state changes
> > under the mutex, that should rather be moved than deleted, shouldn't it?  
> 
> It is moved to fsm_irq() which is called under mutex.
> fsm_irq() returns VFIO_CCW_STATE_IDLE.

So, should that go into another patch?

> 
> >  
> >>   }
> >>   
> >>   static void vfio_ccw_sch_event_todo(struct work_struct *work)  
> 
>