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

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Message-Id: <6c39016f-8125-c5cc-3b9b-0bc517e5f642@linux.ibm.com>
Date:   Tue, 5 Jun 2018 16:21:03 +0200
From:   Pierre Morel <pmorel@...ux.ibm.com>
To:     Cornelia Huck <cohuck@...hat.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 05/06/2018 15:35, Cornelia Huck wrote:
> 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?

Yes, if it is not I forgot something important (like I did in patch 10)
vfio_ccw_fsm_event(private, event) takes the mutex on firering an event.

I have some cases where I do not respect this.
This is false and I must handle this with a new private variable,
this is where I test the state after having fired an event.
I will need to change this, in quiesce, reset, probe and open (others?).


>
>>>   
>>>> 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?

I will see if I can put it inside the patch 01/10 moving state change 
out of IRQ context.


-- 
Pierre Morel
Linux/KVM/QEMU in Böblingen - Germany