linux-kernel - Re: [PATCH v4 0/2] vfio/mdev: Device namespace protection

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Date:   Tue, 22 May 2018 19:17:07 +0200
From:   Halil Pasic <pasic@...ux.ibm.com>
To:     Alex Williamson <alex.williamson@...hat.com>, kwankhede@...dia.com,
        Dong Jia <bjsdjshi@...ux.vnet.ibm.com>
Cc:     kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        cohuck@...hat.com
Subject: Re: [PATCH v4 0/2] vfio/mdev: Device namespace protection

 From vfio-ccw perspective I join Connie's assessment: vfio-ccw should
be fine with these changes. I'm however not too deeply involved with
the mdev framework, thus I don't feel comfortable r-b-ing. That results
in
Acked-by: Halil Pasic <pasic@...ux.ibm.com>
for both patches.

While at it I have would like to ask about the semantics and intended
use of the mdev interfaces.

static int vfio_ccw_sch_probe(struct subchannel *sch)
{

/* HALIL: 8< Not so interesting stuff happens here. >8 */
         ret = vfio_ccw_mdev_reg(sch);
         if (ret)
                 goto out_disable;
/*
  * HALIL:  
  * This might be racy. Somewhere in vfio_ccw_mdev_reg() the create attribute
  * is made available (it calls mdev_register_device()). For instance create will
  * attempt to decrement private->avail which is initialized below. I fail to
  * understand how is  this well synchronized.
  */
         INIT_WORK(&private->io_work, vfio_ccw_sch_io_todo);
         atomic_set(&private->avail, 1);
         private->state = VFIO_CCW_STATE_STANDBY;

         return 0;

out_disable:
         cio_disable_subchannel(sch);
out_free:
         dev_set_drvdata(&sch->dev, NULL);
         kfree(private);
         return ret;
}

Should not initialization  of go before mdev_register_device(), and then rolled
back if necessary if mdev_register_device() fails?

In practice it does not seem very likely that userspace can trigger
mdev_device_create() before vfio_ccw_sch_probe() finishes so it should
not be a practical problem. But I would like to understand how synchronization
is supposed to work.

[Added Dong Jia, maybe he is also able to answer my question.]

Regards,
Halil

On 05/18/2018 09:10 PM, Alex Williamson wrote:
> v4: Fix the 'create' racing 'remove' gap noted by Kirti by moving
>      removal from mdev_list to mdev_device_release().  Fix missing
>      mdev_put_parent() cases in mdev_device_create(), also noted
>      by Kirti.  Added documention update regarding serialization as
>      noted by Cornelia.  Added additional commit log comment about
>      -EAGAIN vs -ENODEV for 'remove' racing 'create'.  Added second
>      patch to re-order sysfs attributes, with this my targeted
>      scripts can no longer hit the gap where -EAGAIN is regurned.
>      BTW, the gap where the current code returns -ENODEV in this
>      race condition is about 50% easier to hit than it exists in
>      this series with patch 1 alone.
> 
> Thanks,
> Alex
> 
> ---
> 
> Alex Williamson (2):
>        vfio/mdev: Check globally for duplicate devices
>        vfio/mdev: Re-order sysfs attribute creation
> 
> 
>   Documentation/vfio-mediated-device.txt |    5 ++
>   drivers/vfio/mdev/mdev_core.c          |  102 +++++++++++---------------------
>   drivers/vfio/mdev/mdev_private.h       |    2 -
>   drivers/vfio/mdev/mdev_sysfs.c         |   14 ++--
>   4 files changed, 49 insertions(+), 74 deletions(-)
>