RE: [PATCHv2 10/10] vfio/mdev: Synchronize device create/remove with parent removal

[Parav Pandit <parav@mellanox.com>]

> -----Original Message-----
> From: Alex Williamson <alex.williamson@redhat.com>
> Sent: Wednesday, May 8, 2019 9:46 PM
> To: Parav Pandit <parav@mellanox.com>
> Cc: kvm@vger.kernel.org; linux-kernel@vger.kernel.org;
> kwankhede@nvidia.com; cjia@nvidia.com
> Subject: Re: [PATCHv2 10/10] vfio/mdev: Synchronize device create/remove
> with parent removal
> 
> On Tue, 30 Apr 2019 17:49:37 -0500
> Parav Pandit <parav@mellanox.com> wrote:
> 
> > In following sequences, child devices created while removing mdev
> > parent device can be left out, or it may lead to race of removing half
> > initialized child mdev devices.
> >
> > issue-1:
> > --------
> >        cpu-0                         cpu-1
> >        -----                         -----
> >                                   mdev_unregister_device()
> >                                     device_for_each_child()
> >                                       mdev_device_remove_cb()
> >                                         mdev_device_remove()
> > create_store()
> >   mdev_device_create()                   [...]
> >     device_add()
> >                                   parent_remove_sysfs_files()
> >
> > /* BUG: device added by cpu-0
> >  * whose parent is getting removed
> >  * and it won't process this mdev.
> >  */
> >
> > issue-2:
> > --------
> > Below crash is observed when user initiated remove is in progress and
> > mdev_unregister_driver() completes parent unregistration.
> >
> >        cpu-0                         cpu-1
> >        -----                         -----
> > remove_store()
> >    mdev_device_remove()
> >    active = false;
> >                                   mdev_unregister_device()
> >                                   parent device removed.
> >    [...]
> >    parents->ops->remove()
> >  /*
> >   * BUG: Accessing invalid parent.
> >   */
> >
> > This is similar race like create() racing with mdev_unregister_device().
> >
> > BUG: unable to handle kernel paging request at ffffffffc0585668 PGD
> > e8f618067 P4D e8f618067 PUD e8f61a067 PMD 85adca067 PTE 0
> > Oops: 0000 [#1] SMP PTI
> > CPU: 41 PID: 37403 Comm: bash Kdump: loaded Not tainted
> > 5.1.0-rc6-vdevbus+ #6 Hardware name: Supermicro
> > SYS-6028U-TR4+/X10DRU-i+, BIOS 2.0b 08/09/2016
> > RIP: 0010:mdev_device_remove+0xfa/0x140 [mdev] Call Trace:
> >  remove_store+0x71/0x90 [mdev]
> >  kernfs_fop_write+0x113/0x1a0
> >  vfs_write+0xad/0x1b0
> >  ksys_write+0x5a/0xe0
> >  do_syscall_64+0x5a/0x210
> >  entry_SYSCALL_64_after_hwframe+0x49/0xbe
> >
> > Therefore, mdev core is improved as below to overcome above issues.
> >
> > Wait for any ongoing mdev create() and remove() to finish before
> > unregistering parent device using refcount and completion.
> > This continues to allow multiple create and remove to progress in
> > parallel for different mdev devices as most common case.
> > At the same time guard parent removal while parent is being access by
> > create() and remove callbacks.
> >
> > Code is simplified from kref to use refcount as unregister_device()
> > has to wait anyway for all create/remove to finish.
> >
> > While removing mdev devices during parent unregistration, there isn't
> > need to acquire refcount of parent device, hence code is restructured
> > using mdev_device_remove_common() to avoid it.
> 
> Did you consider calling parent_remove_sysfs_files() earlier in
> mdev_unregister_device() and adding srcu support to know there are no in-
> flight callers of the create path?  I think that would address issue-1.
> 
parent_remove_sysfs_files() cannot be done until create is completed because child mdev are under the parent's supported_types/../devices.
So once parent directory is removed, it removes the child link too.
And doing mdev_device_remove_common() on such child (for whom create was ongoing), results in warning.

Secondly, I dropped the srcu approach because srcu are slow, and call_rcu() is not helpful,
because once mdev_unregister_device() is completed, we want to be sure that there are no references to this parent device.

> Issue-2 suggests a bug in our handling of the parent device krefs, the parent
> object should exist until all child devices which have a kref reference to the
> parent are removed, but clearly
> mdev_unregister_device() is not blocking for that to occur allowing the
> parent driver .remove callback to finish.  This seems similar to
> vfio_del_group_dev() where we need to block a vfio bus driver from
> removing a device until it becomes unused, could a similar solution with a
> wait_queue and wait_woken be used here?
> 
mdev_unregister_device(), removes all the child mdevs by first by detaching from the drivers, followed by calling remove() of the vendor driver.
once those remove() call completes, there shouldn't be any active reference to parent.

Once mdevs are created or removed, they drop the reference to the parent.
If we keep around the reference to parent, than parent cannot publish that it is going away.
An additional refcount or another sync mechanism is needed to achieve that.
I don't' see what would that buy us.

> I'm not immediately sold on the idea that removing a kref to solve this
> problem is a good thing, it seems odd to me that mdevs don't hold a
> reference to the parent throughout their life with this change, and the
> remove_store path branch to exit if we find we're racing the parent remove
> path is rather ugly.  
Mdev's are anchored to its parent. When parent is going away, its removing the child mdevs after ensuring that no removes() are progressing.
so remove_store and create are not very different in what they do.

> BTW, why is the sanitization loop in
> mdev_device_remove() still here, wasn't that fixed by the previous two
> patches?  Thanks,
> 
The loop still exist because remove_store() does't hold any reference to the struct device getting removed.
So it is very much possible that remove_store() didn't yet invoke the mdev_device_remove(), but such device is already removed.
This sanitization loop for now helps there.
I want to do more sane things for remove by uuid (similar to create file) and not by self-file, but at later point...

> Alex
> 
> > Fixes: 7b96953bc640 ("vfio: Mediated device Core driver")
> > Signed-off-by: Parav Pandit <parav@mellanox.com>
> > ---
> >  drivers/vfio/mdev/mdev_core.c    | 86 ++++++++++++++++++++------------
> >  drivers/vfio/mdev/mdev_private.h |  6 ++-
> >  2 files changed, 60 insertions(+), 32 deletions(-)
> >
> > diff --git a/drivers/vfio/mdev/mdev_core.c
> > b/drivers/vfio/mdev/mdev_core.c index 2b98da2ee361..a5da24d662f4
> > 100644
> > --- a/drivers/vfio/mdev/mdev_core.c
> > +++ b/drivers/vfio/mdev/mdev_core.c
> > @@ -78,34 +78,41 @@ static struct mdev_parent
> *__find_parent_device(struct device *dev)
> >  	return NULL;
> >  }
> >
> > -static void mdev_release_parent(struct kref *kref)
> > +static bool mdev_try_get_parent(struct mdev_parent *parent)
> >  {
> > -	struct mdev_parent *parent = container_of(kref, struct
> mdev_parent,
> > -						  ref);
> > -	struct device *dev = parent->dev;
> > -
> > -	kfree(parent);
> > -	put_device(dev);
> > +	if (parent)
> > +		return refcount_inc_not_zero(&parent->refcount);
> > +	return false;
> >  }
> >
> > -static struct mdev_parent *mdev_get_parent(struct mdev_parent
> > *parent)
> > +static void mdev_put_parent(struct mdev_parent *parent)
> >  {
> > -	if (parent)
> > -		kref_get(&parent->ref);
> > -
> > -	return parent;
> > +	if (parent && refcount_dec_and_test(&parent->refcount))
> > +		complete(&parent->unreg_completion);
> >  }
> >
> > -static void mdev_put_parent(struct mdev_parent *parent)
> > +static void mdev_device_remove_common(struct mdev_device *mdev)
> >  {
> > -	if (parent)
> > -		kref_put(&parent->ref, mdev_release_parent);
> > +	struct mdev_parent *parent;
> > +	struct mdev_type *type;
> > +	int ret;
> > +
> > +	type = to_mdev_type(mdev->type_kobj);
> > +	mdev_remove_sysfs_files(&mdev->dev, type);
> > +	device_del(&mdev->dev);
> > +	parent = mdev->parent;
> > +	ret = parent->ops->remove(mdev);
> > +	if (ret)
> > +		dev_err(&mdev->dev, "Remove failed: err=%d\n", ret);
> > +
> > +	/* Balances with device_initialize() */
> > +	put_device(&mdev->dev);
> >  }
> >
> >  static int mdev_device_remove_cb(struct device *dev, void *data)  {
> >  	if (dev_is_mdev(dev))
> > -		mdev_device_remove(dev);
> > +		mdev_device_remove_common(to_mdev_device(dev));
> >
> >  	return 0;
> >  }
> > @@ -147,7 +154,8 @@ int mdev_register_device(struct device *dev, const
> struct mdev_parent_ops *ops)
> >  		goto add_dev_err;
> >  	}
> >
> > -	kref_init(&parent->ref);
> > +	refcount_set(&parent->refcount, 1);
> > +	init_completion(&parent->unreg_completion);
> >
> >  	parent->dev = dev;
> >  	parent->ops = ops;
> > @@ -206,14 +214,27 @@ void mdev_unregister_device(struct device *dev)
> >  	dev_info(dev, "MDEV: Unregistering\n");
> >
> >  	list_del(&parent->next);
> > +	mutex_unlock(&parent_list_lock);
> > +
> > +	/* Release the initial reference so that new create cannot start */
> > +	mdev_put_parent(parent);
> > +
> > +	/*
> > +	 * Wait for all the create and remove references to drop.
> > +	 */
> > +	wait_for_completion(&parent->unreg_completion);
> > +
> > +	/*
> > +	 * New references cannot be taken and all users are done
> > +	 * using the parent. So it is safe to unregister parent.
> > +	 */
> >  	class_compat_remove_link(mdev_bus_compat_class, dev, NULL);
> >
> >  	device_for_each_child(dev, NULL, mdev_device_remove_cb);
> >
> >  	parent_remove_sysfs_files(parent);
> > -
> > -	mutex_unlock(&parent_list_lock);
> > -	mdev_put_parent(parent);
> > +	kfree(parent);
> > +	put_device(dev);
> >  }
> >  EXPORT_SYMBOL(mdev_unregister_device);
> >
> > @@ -237,10 +258,11 @@ int mdev_device_create(struct kobject *kobj,
> >  	struct mdev_parent *parent;
> >  	struct mdev_type *type = to_mdev_type(kobj);
> >
> > -	parent = mdev_get_parent(type->parent);
> > -	if (!parent)
> > +	if (!mdev_try_get_parent(type->parent))
> >  		return -EINVAL;
> >
> > +	parent = type->parent;
> > +
> >  	mutex_lock(&mdev_list_lock);
> >
> >  	/* Check for duplicate */
> > @@ -287,6 +309,7 @@ int mdev_device_create(struct kobject *kobj,
> >
> >  	mdev->active = true;
> >  	dev_dbg(&mdev->dev, "MDEV: created\n");
> > +	mdev_put_parent(parent);
> >
> >  	return 0;
> >
> > @@ -306,7 +329,6 @@ int mdev_device_remove(struct device *dev)
> >  	struct mdev_device *mdev, *tmp;
> >  	struct mdev_parent *parent;
> >  	struct mdev_type *type;
> > -	int ret;
> >
> >  	mdev = to_mdev_device(dev);
> >
> > @@ -330,15 +352,17 @@ int mdev_device_remove(struct device *dev)
> >  	mutex_unlock(&mdev_list_lock);
> >
> >  	type = to_mdev_type(mdev->type_kobj);
> > -	mdev_remove_sysfs_files(dev, type);
> > -	device_del(&mdev->dev);
> > -	parent = mdev->parent;
> > -	ret = parent->ops->remove(mdev);
> > -	if (ret)
> > -		dev_err(&mdev->dev, "Remove failed: err=%d\n", ret);
> > +	if (!mdev_try_get_parent(type->parent)) {
> > +		/*
> > +		 * Parent unregistration have started.
> > +		 * No need to remove here.
> > +		 */
> > +		mutex_unlock(&mdev_list_lock);
> > +		return -ENODEV;
> > +	}
> >
> > -	/* Balances with device_initialize() */
> > -	put_device(&mdev->dev);
> > +	parent = mdev->parent;
> > +	mdev_device_remove_common(mdev);
> >  	mdev_put_parent(parent);
> >
> >  	return 0;
> > diff --git a/drivers/vfio/mdev/mdev_private.h
> > b/drivers/vfio/mdev/mdev_private.h
> > index 067dc5d8c5de..781f111d66d2 100644
> > --- a/drivers/vfio/mdev/mdev_private.h
> > +++ b/drivers/vfio/mdev/mdev_private.h
> > @@ -19,7 +19,11 @@ void mdev_bus_unregister(void);  struct
> mdev_parent
> > {
> >  	struct device *dev;
> >  	const struct mdev_parent_ops *ops;
> > -	struct kref ref;
> > +	/* Protects unregistration to wait until create/remove
> > +	 * are completed.
> > +	 */
> > +	refcount_t refcount;
> > +	struct completion unreg_completion;
> >  	struct list_head next;
> >  	struct kset *mdev_types_kset;
> >  	struct list_head type_list;