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Date: Mon, 25 Mar 2019 23:34:28 +0000
From: Parav Pandit <parav@...lanox.com>
To: Alex Williamson <alex.williamson@...hat.com>
CC: "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
"kwankhede@...dia.com" <kwankhede@...dia.com>
Subject: RE: [PATCH 8/8] vfio/mdev: Improve the create/remove sequence
> -----Original Message-----
> From: Alex Williamson <alex.williamson@...hat.com>
> Sent: Monday, March 25, 2019 6:19 PM
> To: Parav Pandit <parav@...lanox.com>
> Cc: kvm@...r.kernel.org; linux-kernel@...r.kernel.org;
> kwankhede@...dia.com
> Subject: Re: [PATCH 8/8] vfio/mdev: Improve the create/remove sequence
>
> On Fri, 22 Mar 2019 18:20:35 -0500
> Parav Pandit <parav@...lanox.com> wrote:
>
> > There are five problems with current code structure.
> > 1. mdev device is placed on the mdev bus before it is created in the
> > vendor driver. Once a device is placed on the mdev bus without
> > creating its supporting underlying vendor device, an open() can get
> > triggered by userspace on partially initialized device.
> > Below ladder diagram highlight it.
> >
> > cpu-0 cpu-1
> > ----- -----
> > create_store()
> > mdev_create_device()
> > device_register()
> > ...
> > vfio_mdev_probe()
> > ...creates char device
> > vfio_mdev_open()
> > parent->ops->open(mdev)
> > vfio_ap_mdev_open()
> > matrix_mdev = NULL
> > [...]
> > parent->ops->create()
> > vfio_ap_mdev_create()
> > mdev_set_drvdata(mdev, matrix_mdev);
> > /* Valid pointer set above */
> >
> > 2. Current creation sequence is,
> > parent->ops_create()
> > groups_register()
> >
> > Remove sequence is,
> > parent->ops->remove()
> > groups_unregister()
> > However, remove sequence should be exact mirror of creation sequence.
> > Once this is achieved, all users of the mdev will be terminated first
> > before removing underlying vendor device.
> > (Follow standard linux driver model).
> > At that point vendor's remove() ops shouldn't failed because device is
> > taken off the bus that should terminate the users.
> >
> > 3. Additionally any new mdev driver that wants to work on mdev device
> > during probe() routine registered using mdev_register_driver() needs
> > to get stable mdev structure.
> >
> > 4. In following sequence, 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_register()
> > parent_remove_sysfs_files()
> > /* BUG: device added by cpu-0
> > * whose parent is getting removed.
> > */
> >
> > issue-2:
> > --------
> > cpu-0 cpu-1
> > ----- -----
> > create_store()
> > mdev_device_create() [...]
> > device_register()
> >
> > [...] mdev_unregister_device()
> > device_for_each_child()
> > mdev_device_remove_cb()
> > mdev_device_remove()
> >
> > mdev_create_sysfs_files()
> > /* BUG: create is adding
> > * sysfs files for a device
> > * which is undergoing removal.
> > */
> > parent_remove_sysfs_files()
>
> In both cases above, it looks like the device will hold a reference to the
> parent, so while there is a race, the parent object isn't released.
Yes, parent object is not released but parent fields are not stable.
>
> >
> > 5. 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()
> > remove type
> > [...]
> > mdev_remove_ops() crashes.
> >
> > This is similar race like create() racing with mdev_unregister_device().
>
> Not sure I catch this, the device should have a reference to the parent, and
> we don't specifically clear parent->ops, so what's getting removed that
> causes this oops? Is .remove pointing at bad text regardless?
>
I guess the mdev_attr_groups being stale now.
> > mtty mtty: MDEV: Registered
> > iommu: Adding device 83b8f4f2-509f-382f-3c1e-e6bfe0fa1001 to group 57
> > vfio_mdev 83b8f4f2-509f-382f-3c1e-e6bfe0fa1001: MDEV: group_id = 57
> > mdev_device_remove sleep started mtty mtty: MDEV: Unregistering
> > mtty_dev: Unloaded!
> > BUG: unable to handle kernel paging request at ffffffffc027d668 PGD
> > af9818067 P4D af9818067 PUD af981a067 PMD 8583c3067 PTE 0
> > Oops: 0000 [#1] SMP PTI
> > CPU: 15 PID: 3517 Comm: bash Kdump: loaded Not tainted
> > 5.0.0-rc7-vdevbus+ #2 Hardware name: Supermicro
> > SYS-6028U-TR4+/X10DRU-i+, BIOS 2.0b 08/09/2016
> > RIP: 0010:mdev_device_remove_ops+0x1a/0x50 [mdev] Call Trace:
> > mdev_device_remove+0xef/0x130 [mdev]
> > remove_store+0x77/0xa0 [mdev]
> > kernfs_fop_write+0x113/0x1a0
> > __vfs_write+0x33/0x1b0
> > ? rcu_read_lock_sched_held+0x64/0x70
> > ? rcu_sync_lockdep_assert+0x2a/0x50
> > ? __sb_start_write+0x121/0x1b0
> > ? vfs_write+0x17c/0x1b0
> > vfs_write+0xad/0x1b0
> > ? trace_hardirqs_on_thunk+0x1a/0x1c
> > ksys_write+0x55/0xc0
> > do_syscall_64+0x5a/0x210
> >
> > Therefore, mdev core is improved in following ways to overcome above
> > issues.
> >
> > 1. Before placing mdev devices on the bus, perform vendor drivers
> > creation which supports the mdev creation.
> > This ensures that mdev specific all necessary fields are initialized
> > before a given mdev can be accessed by bus driver.
> >
> > 2. During remove flow, first remove the device from the bus. This
> > ensures that any bus specific devices and data is cleared.
> > Once device is taken of the mdev bus, perform remove() of mdev from
> > the vendor driver.
> >
> > 3. Linux core device model provides way to register and auto
> > unregister the device sysfs attribute groups at dev->groups.
> > Make use of this groups to let core create the groups and simplify
> > code to avoid explicit groups creation and removal.
> >
> > 4. Wait for any ongoing mdev create() and remove() to finish before
> > unregistering parent device using srcu. This continues to allow
> > multiple create and remove to progress in parallel. At the same time
> > guard parent removal while parent is being access by create() and remove
> callbacks.
>
> So there should be 4-5 separate patches here? Wishful thinking?
>
create, remove racing with unregister is handled using srcu.
Change-3 cannot be done without fixing the sequence so it should be in patch that fixes it.
Change described changes 1-2-3 are just one change. It is just the patch description to bring clarity.
Change-4 can be possibly done as split to different patch.
> > Fixes: 7b96953bc640 ("vfio: Mediated device Core driver")
> > Signed-off-by: Parav Pandit <parav@...lanox.com>
> > ---
> > drivers/vfio/mdev/mdev_core.c | 142 +++++++++++++++++++++--------------
> ----
> > drivers/vfio/mdev/mdev_private.h | 7 +-
> > drivers/vfio/mdev/mdev_sysfs.c | 6 +-
> > 3 files changed, 84 insertions(+), 71 deletions(-)
> >
> > diff --git a/drivers/vfio/mdev/mdev_core.c
> > b/drivers/vfio/mdev/mdev_core.c index 944a058..8fe0ed1 100644
> > --- a/drivers/vfio/mdev/mdev_core.c
> > +++ b/drivers/vfio/mdev/mdev_core.c
> > @@ -84,6 +84,7 @@ static void mdev_release_parent(struct kref *kref)
> > ref);
> > struct device *dev = parent->dev;
> >
> > + cleanup_srcu_struct(&parent->unreg_srcu);
> > kfree(parent);
> > put_device(dev);
> > }
> > @@ -103,56 +104,30 @@ static inline void mdev_put_parent(struct
> mdev_parent *parent)
> > kref_put(&parent->ref, mdev_release_parent); }
> >
> > -static int mdev_device_create_ops(struct kobject *kobj,
> > - struct mdev_device *mdev)
> > +static int mdev_device_must_remove(struct mdev_device *mdev)
>
> Naming is off here, mdev_device_remove_common()?
>
Yes, sounds better.
> > {
> > - struct mdev_parent *parent = mdev->parent;
> > + struct mdev_parent *parent;
> > + struct mdev_type *type;
> > int ret;
> >
> > - ret = parent->ops->create(kobj, mdev);
> > - if (ret)
> > - return ret;
> > + type = to_mdev_type(mdev->type_kobj);
> >
> > - ret = sysfs_create_groups(&mdev->dev.kobj,
> > - parent->ops->mdev_attr_groups);
> > + mdev_remove_sysfs_files(&mdev->dev, type);
> > + device_del(&mdev->dev);
> > + parent = mdev->parent;
> > + ret = parent->ops->remove(mdev);
> > if (ret)
> > - parent->ops->remove(mdev);
> > + dev_err(&mdev->dev, "Remove failed: err=%d\n", ret);
>
> Let the caller decide whether to be verbose with the error, parent removal
> might want to warn, sysfs remove might just return an error.
>
I didn't follow. Caller meaning mdev_device_remove_common() or vendor driver?
> >
> > + /* Balances with device_initialize() */
> > + put_device(&mdev->dev);
> > return ret;
> > }
> >
> > -/*
> > - * 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.
> > - */
> > -static int mdev_device_remove_ops(struct mdev_device *mdev, bool
> > force_remove) -{
> > - struct mdev_parent *parent = mdev->parent;
> > - int ret;
> > -
> > - /*
> > - * Vendor driver can return error if VMM or userspace application is
> > - * using this mdev device.
> > - */
> > - ret = parent->ops->remove(mdev);
> > - if (ret && !force_remove)
> > - return ret;
> > -
> > - sysfs_remove_groups(&mdev->dev.kobj, parent->ops-
> >mdev_attr_groups);
> > - return 0;
> > -}
>
> Seems like there's easily a separate patch in pushing the create/remove ops
> into the calling function and separating for the iterator callback, that would
> make this easier to review.
>
> > -
> > static int mdev_device_remove_cb(struct device *dev, void *data) {
> > if (dev_is_mdev(dev))
> > - mdev_device_remove(dev, true);
> > -
> > + mdev_device_must_remove(to_mdev_device(dev));
> > return 0;
> > }
> >
> > @@ -194,6 +169,7 @@ int mdev_register_device(struct device *dev, const
> struct mdev_parent_ops *ops)
> > }
> >
> > kref_init(&parent->ref);
> > + init_srcu_struct(&parent->unreg_srcu);
> >
> > parent->dev = dev;
> > parent->ops = ops;
> > @@ -214,6 +190,7 @@ int mdev_register_device(struct device *dev, const
> struct mdev_parent_ops *ops)
> > if (ret)
> > dev_warn(dev, "Failed to create compatibility class link\n");
> >
> > + rcu_assign_pointer(parent->self, parent);
> > list_add(&parent->next, &parent_list);
> > mutex_unlock(&parent_list_lock);
> >
> > @@ -244,21 +221,36 @@ void mdev_unregister_device(struct device *dev)
> >
> > mutex_lock(&parent_list_lock);
> > parent = __find_parent_device(dev);
> > -
> > if (!parent) {
> > mutex_unlock(&parent_list_lock);
> > return;
> > }
> > + list_del(&parent->next);
> > + mutex_unlock(&parent_list_lock);
> > +
> > dev_info(dev, "MDEV: Unregistering\n");
> >
> > - list_del(&parent->next);
> > + /* Publish that this mdev parent is unregistering. So any new
> > + * create/remove cannot start on this parent anymore by user.
> > + */
>
> Comment style, we're not in netdev.
Yep. Will fix it.
>
> > + rcu_assign_pointer(parent->self, NULL);
> > +
> > + /*
> > + * Wait for any active create() or remove() mdev ops on the parent
> > + * to complete.
> > + */
> > + synchronize_srcu(&parent->unreg_srcu);
> > +
> > + /* At this point it is confirmed that any pending user initiated
> > + * create or remove callbacks accessing the parent are completed.
> > + * It is safe to remove the parent now.
> > + */
> > 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);
> > }
> > EXPORT_SYMBOL(mdev_unregister_device);
> > @@ -278,14 +270,24 @@ static void mdev_device_release(struct device
> > *dev) int mdev_device_create(struct kobject *kobj, struct device
> > *dev, uuid_le uuid) {
> > int ret;
> > + struct mdev_parent *valid_parent;
> > struct mdev_device *mdev, *tmp;
> > struct mdev_parent *parent;
> > struct mdev_type *type = to_mdev_type(kobj);
> > + int srcu_idx;
> >
> > parent = mdev_get_parent(type->parent);
> > if (!parent)
> > return -EINVAL;
> >
> > + srcu_idx = srcu_read_lock(&parent->unreg_srcu);
> > + valid_parent = srcu_dereference(parent->self, &parent->unreg_srcu);
> > + if (!valid_parent) {
> > + /* parent is undergoing unregistration */
> > + ret = -ENODEV;
> > + goto mdev_fail;
> > + }
> > +
> > mutex_lock(&mdev_list_lock);
> >
> > /* Check for duplicate */
> > @@ -310,68 +312,76 @@ int mdev_device_create(struct kobject *kobj,
> > struct device *dev, uuid_le uuid)
> >
> > mdev->parent = parent;
> >
> > + device_initialize(&mdev->dev);
> > mdev->dev.parent = dev;
> > mdev->dev.bus = &mdev_bus_type;
> > mdev->dev.release = mdev_device_release;
> > + mdev->dev.groups = type->parent->ops->mdev_attr_groups;
> > dev_set_name(&mdev->dev, "%pUl", uuid.b);
> >
> > - ret = device_register(&mdev->dev);
> > + ret = type->parent->ops->create(kobj, mdev);
> > if (ret)
> > - goto mdev_fail;
> > + goto create_fail;
> >
> > - ret = mdev_device_create_ops(kobj, mdev);
> > + ret = device_add(&mdev->dev);
>
> Separating device_initialize() and device_add() also looks like a separate
> patch, then the srcu could be added at the end. Thanks,
>
> Alex
I saw little more core generated that way, but I think its fine.
Basically, create/remove callback sequencing that does the device_inititailze/add etc in one patch and
User side race handling using srcu in another patch.
Sounds good?
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