Date:   Mon, 25 Mar 2019 22:07:36 +0000
From:   Parav Pandit <parav@...lanox.com>
To:     Alex Williamson <alex.williamson@...hat.com>
CC:     Kirti Wankhede <kwankhede@...dia.com>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: RE: [PATCH 7/8] vfio/mdev: Fix aborting mdev child device removal if
 one fails

Hi Alex,

> -----Original Message-----
> From: Alex Williamson <alex.williamson@...hat.com>
> Sent: Monday, March 25, 2019 4:52 PM
> To: Parav Pandit <parav@...lanox.com>
> Cc: Kirti Wankhede <kwankhede@...dia.com>; kvm@...r.kernel.org; linux-
> kernel@...r.kernel.org
> Subject: Re: [PATCH 7/8] vfio/mdev: Fix aborting mdev child device removal if
> one fails
> 
> On Mon, 25 Mar 2019 21:36:42 +0000
> Parav Pandit <parav@...lanox.com> wrote:
> 
> > > -----Original Message-----
> > > From: Alex Williamson <alex.williamson@...hat.com>
> > > Sent: Monday, March 25, 2019 3:50 PM
> > > To: Kirti Wankhede <kwankhede@...dia.com>
> > > Cc: Parav Pandit <parav@...lanox.com>; kvm@...r.kernel.org; linux-
> > > kernel@...r.kernel.org
> > > Subject: Re: [PATCH 7/8] vfio/mdev: Fix aborting mdev child device
> > > removal if one fails
> > >
> > > On Tue, 26 Mar 2019 01:05:34 +0530
> > > Kirti Wankhede <kwankhede@...dia.com> wrote:
> > >
> > > > On 3/23/2019 4:50 AM, Parav Pandit wrote:
> > > > > device_for_each_child() stops executing callback function for
> > > > > remaining child devices, if callback hits an error.
> > > > > Each child mdev device is independent of each other.
> > > > > While unregistering parent device, mdev core must remove all
> > > > > child mdev devices.
> > > > > Therefore, mdev_device_remove_cb() always returns success so
> > > > > that device_for_each_child doesn't abort if one child removal hits
> error.
> > > > >
> > > >
> > > > When unregistering parent device, force_remove is set to true amd
> > > > mdev_device_remove_ops() always returns success.
> > >
> > > Can we know that?  mdev_device_remove() doesn't guarantee to return
> > > zero.
> > >
> > > > > While at it, improve remove and unregister functions for below
> > > simplicity.
> > > > >
> > > > > There isn't need to pass forced flag pointer during mdev parent
> > > > > removal which invokes mdev_device_remove().
> > > >
> > > > There is a need to pass the flag, pasting here the comment above
> > > > mdev_device_remove_ops() which explains why the flag is needed:
> > > >
> > > > /*
> > > >  * mdev_device_remove_ops gets called from sysfs's 'remove' and
> > > > when parent
> > > >  * device is being unregistered from mdev device framework.
> > > >  * - 'force_remove' is set to 'false' when called from sysfs's 'remove'
> > > > which
> > > >  *   indicates that if the mdev device is active, used by VMM or
> userspace
> > > >  *   application, vendor driver could return error then don't remove the
> > > > device.
> > > >  * - 'force_remove' is set to 'true' when called from
> > > > mdev_unregister_device()
> > > >  *   which indicate that parent device is being removed from mdev
> device
> > > >  *   framework so remove mdev device forcefully.
> > > >  */
> > >
> > > I don't see that this changes the force behavior, it's simply noting
> > > that in order to continue the device_for_each_child() iterator, we
> > > need to return zero, regardless of what mdev_device_remove()
> > > returns, and the parent remove path is the only caller of
> > > mdev_device_remove_cb(), so we can assume force = true when calling
> > > mdev_device_remove().  Aside from maybe a WARN_ON if
> > > mdev_device_remove() returns non-zero, that much looks reasonable to
> me.
> > >
> > > >  So simplify the flow.
> > > > >
> > > > > mdev_device_remove() is called from two paths.
> > > > > 1. mdev_unregister_driver()
> > > > >      mdev_device_remove_cb()
> > > > >        mdev_device_remove()
> > > > > 2. remove_store()
> > > > >      mdev_device_remove()
> > > > >
> > > > > When device is removed by user using remote_store(), device
> > > > > under removal is mdev device.
> > > > > When device is removed during parent device removal using
> > > > > generic child iterator, mdev check is already done using
> dev_is_mdev().
> > > > >
> > > > > Hence, remove the unnecessary loop in mdev_device_remove().
> > >
> > > I don't think knowing the device type is the only reason for this loop
> though.
> > > Both paths you mention above can race with each other, so we need to
> > > serialize them and pick a winner.  The mdev_list_lock allows us to do
> that.
> > > Additionally...
> > >
> > > > >
> > > > > Fixes: 7b96953bc640 ("vfio: Mediated device Core driver")
> > > > > Signed-off-by: Parav Pandit <parav@...lanox.com>
> > > > > ---
> > > > >  drivers/vfio/mdev/mdev_core.c | 24 +++++-------------------
> > > > >  1 file changed, 5 insertions(+), 19 deletions(-)
> > > > >
> > > > > diff --git a/drivers/vfio/mdev/mdev_core.c
> > > > > b/drivers/vfio/mdev/mdev_core.c index ab05464..944a058 100644
> > > > > --- a/drivers/vfio/mdev/mdev_core.c
> > > > > +++ b/drivers/vfio/mdev/mdev_core.c
> > > > > @@ -150,10 +150,10 @@ static int mdev_device_remove_ops(struct
> > > > > mdev_device *mdev, bool force_remove)
> > > > >
> > > > >  static int mdev_device_remove_cb(struct device *dev, void *data)  {
> > > > > -	if (!dev_is_mdev(dev))
> > > > > -		return 0;
> > > > > +	if (dev_is_mdev(dev))
> > > > > +		mdev_device_remove(dev, true);
> > > > >
> > > > > -	return mdev_device_remove(dev, data ? *(bool *)data :
> true);
> > > > > +	return 0;
> > > > >  }
> > > > >
> > > > >  /*
> > > > > @@ -241,7 +241,6 @@ int mdev_register_device(struct device *dev,
> > > > > const struct mdev_parent_ops *ops)  void
> > > > > mdev_unregister_device(struct device *dev)  {
> > > > >  	struct mdev_parent *parent;
> > > > > -	bool force_remove = true;
> > > > >
> > > > >  	mutex_lock(&parent_list_lock);
> > > > >  	parent = __find_parent_device(dev); @@ -255,8 +254,7 @@
> void
> > > > > mdev_unregister_device(struct device *dev)
> > > > >  	list_del(&parent->next);
> > > > >  	class_compat_remove_link(mdev_bus_compat_class, dev,
> NULL);
> > > > >
> > > > > -	device_for_each_child(dev, (void *)&force_remove,
> > > > > -			      mdev_device_remove_cb);
> > > > > +	device_for_each_child(dev, NULL, mdev_device_remove_cb);
> > > > >
> > > > >  	parent_remove_sysfs_files(parent);
> > > > >
> > > > > @@ -346,24 +344,12 @@ int mdev_device_create(struct kobject
> > > > > *kobj, struct device *dev, uuid_le uuid)
> > > > >
> > > > >  int mdev_device_remove(struct device *dev, bool force_remove)  {
> > > > > -	struct mdev_device *mdev, *tmp;
> > > > > +	struct mdev_device *mdev;
> > > > >  	struct mdev_parent *parent;
> > > > >  	struct mdev_type *type;
> > > > >  	int ret;
> > > > >
> > > > >  	mdev = to_mdev_device(dev);
> > > > > -
> > > > > -	mutex_lock(&mdev_list_lock);
> > >
> > > Acquiring the lock is removed, but...
> > >
> > Crap. Missed the lower part.
> >
> > > > > -	list_for_each_entry(tmp, &mdev_list, next) {
> > > > > -		if (tmp == mdev)
> > > > > -			break;
> > > > > -	}
> > > > > -
> > > > > -	if (tmp != mdev) {
> > > > > -		mutex_unlock(&mdev_list_lock);
> > > > > -		return -ENODEV;
> > > > > -	}
> > > > > -
> > > > >  	if (!mdev->active) {
> > > > >  		mutex_unlock(&mdev_list_lock);
> > > > >  		return -EAGAIN;
> > > > >
> > >
> > > We still release it in this path and the code below here.  If we
> > > don't find the device on the list under lock, then we're working
> > > with a stale device and playing with the 'active' flag of that
> > > device outside of any sort of mutual exclusion is racy.  Thanks,
> > Subsequent patch makes the order sane.
> > I think I should merge this change with patch-8 in the series.
> 
> Please don't incorporate more fixes into patch 8, it has too many already.  I'd
> really prefer to see patch 8 split into issues you've identified as much as
> possible.  Thanks,
> 
I tried to split into two patches.
one for user initiated race conditions, second for driver side race conditions.
But its generating more code churn as synchronization is inter-related. So dropped it.

This patch is just fine, only thing I messed up is accidental mutex lock removal.
Below is the fixup patch for patch-7 that I want to roll in v2.
Rest all stays same in patch-7 and 8.

diff --git a/drivers/vfio/mdev/mdev_core.c b/drivers/vfio/mdev/mdev_core.c
index 5bd8d22..e09b94f 100644
--- a/drivers/vfio/mdev/mdev_core.c
+++ b/drivers/vfio/mdev/mdev_core.c
@@ -349,6 +349,7 @@ int mdev_device_remove(struct device *dev, bool force_remove)
        struct mdev_type *type;
        int ret;

+       mutex_lock(&mdev_list_lock);
        mdev = to_mdev_device(dev);
        if (!mdev->active) {
                mutex_unlock(&mdev_list_lock);


> Alex

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