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    SubjectRE: [PATCH 8/8] vfio/mdev: Improve the create/remove sequence
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    > -----Original Message-----
    > From: Maxim Levitsky <mlevitsk@redhat.com>
    > Sent: Monday, March 25, 2019 8:24 AM
    > To: Parav Pandit <parav@mellanox.com>; kvm@vger.kernel.org; linux-
    > kernel@vger.kernel.org; kwankhede@nvidia.com;
    > alex.williamson@redhat.com
    > Subject: Re: [PATCH 8/8] vfio/mdev: Improve the create/remove sequence
    >
    > On Fri, 2019-03-22 at 18:20 -0500, Parav Pandit 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 */
    >
    > Agree.
    > You probably mean mdev_device_create here.
    >
    > >
    > > 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.
    > Agreee here too.
    >
    >
    >
    > >
    > > 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()
    > Looks like an issue to me too.
    >
    > >
    > > 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().
    > >
    > > 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.
    > > to avoid explicit groups creation and removal.
    > > 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.
    > All these fixes seem reasonable and correct to me
    >
    >
    > >
    > > Fixes: 7b96953bc640 ("vfio: Mediated device Core driver")
    > > Signed-off-by: Parav Pandit <parav@mellanox.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)
    >
    > Tiny nitpic: maybe a better name? or a comment for this function that state
    > that it tries removes the device even if in use
    >
    > > {
    > > - 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);
    > >
    > > + /* 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;
    > > -}
    > > -
    > > 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.
    > > + */
    > > + 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);
    > > if (ret)
    > > - goto create_fail;
    > > + goto dev_fail;
    > >
    > > ret = mdev_create_sysfs_files(&mdev->dev, type);
    > > - if (ret) {
    > > - mdev_device_remove_ops(mdev, true);
    > > - goto create_fail;
    > > - }
    > > + if (ret)
    > > + goto sysfs_fail;
    > >
    > > mdev->type_kobj = kobj;
    > > mdev->active = true;
    > > dev_dbg(&mdev->dev, "MDEV: created\n");
    > > + srcu_read_unlock(&parent->unreg_srcu, srcu_idx);
    > >
    > > return 0;
    > >
    > > +sysfs_fail:
    > > + device_del(&mdev->dev);
    > > +dev_fail:
    > > + type->parent->ops->remove(mdev);
    > > create_fail:
    > > - device_unregister(&mdev->dev);
    > > + put_device(&mdev->dev);
    > > mdev_fail:
    > > + srcu_read_unlock(&parent->unreg_srcu, srcu_idx);
    > > mdev_put_parent(parent);
    > > return ret;
    > > }
    > >
    > > -int mdev_device_remove(struct device *dev, bool force_remove)
    > > +int mdev_device_remove(struct device *dev)
    > > {
    > > + struct mdev_parent *valid_parent;
    > > struct mdev_device *mdev;
    > > struct mdev_parent *parent;
    > > - struct mdev_type *type;
    > > + int srcu_idx;
    > > int ret;
    > >
    > > mdev = to_mdev_device(dev);
    > > + parent = mdev->parent;
    > > + srcu_idx = srcu_read_lock(&parent->unreg_srcu);
    > > + valid_parent = srcu_dereference(parent->self, &parent->unreg_srcu);
    > > + if (!valid_parent) {
    > > + srcu_read_unlock(&parent->unreg_srcu, srcu_idx);
    > > + /* parent is undergoing unregistration */
    > > + return -ENODEV;
    > > + }
    > > +
    > > + mutex_lock(&mdev_list_lock);
    > > if (!mdev->active) {
    > > mutex_unlock(&mdev_list_lock);
    > > - return -EAGAIN;
    > > + srcu_read_unlock(&parent->unreg_srcu, srcu_idx);
    > > + return -ENODEV;
    > > }
    > > -
    > > mdev->active = false;
    > > mutex_unlock(&mdev_list_lock);
    > >
    > > - type = to_mdev_type(mdev->type_kobj);
    > > - parent = mdev->parent;
    > > -
    > > - ret = mdev_device_remove_ops(mdev, force_remove);
    > > - if (ret) {
    > > - mdev->active = true;
    > > - return ret;
    > > - }
    > > + ret = mdev_device_must_remove(mdev);
    > > + srcu_read_unlock(&parent->unreg_srcu, srcu_idx);
    > >
    > > - mdev_remove_sysfs_files(dev, type);
    > > - device_unregister(dev);
    > > mdev_put_parent(parent);
    > > -
    > > - return 0;
    > > + return ret;
    > > }
    > >
    > > static int __init mdev_init(void)
    > > diff --git a/drivers/vfio/mdev/mdev_private.h
    > > b/drivers/vfio/mdev/mdev_private.h
    > > index 84b2b6c..3d17db9 100644
    > > --- a/drivers/vfio/mdev/mdev_private.h
    > > +++ b/drivers/vfio/mdev/mdev_private.h
    > > @@ -23,6 +23,11 @@ struct mdev_parent {
    > > struct list_head next;
    > > struct kset *mdev_types_kset;
    > > struct list_head type_list;
    > > + /* Protects unregistration to wait until create/remove
    > > + * are completed.
    > > + */
    > > + struct srcu_struct unreg_srcu;
    > > + struct mdev_parent __rcu *self;
    > > };
    > >
    > > struct mdev_device {
    > > @@ -58,6 +63,6 @@ struct mdev_type {
    > > void mdev_remove_sysfs_files(struct device *dev, struct mdev_type
    > > *type);
    > >
    > > int mdev_device_create(struct kobject *kobj, struct device *dev,
    > > uuid_le uuid); -int mdev_device_remove(struct device *dev, bool
    > > force_remove);
    > > +int mdev_device_remove(struct device *dev);
    > >
    > > #endif /* MDEV_PRIVATE_H */
    > > diff --git a/drivers/vfio/mdev/mdev_sysfs.c
    > > b/drivers/vfio/mdev/mdev_sysfs.c index c782fa9..68a8191 100644
    > > --- a/drivers/vfio/mdev/mdev_sysfs.c
    > > +++ b/drivers/vfio/mdev/mdev_sysfs.c
    > > @@ -236,11 +236,9 @@ static ssize_t remove_store(struct device *dev,
    > > struct device_attribute *attr,
    > > if (val && device_remove_file_self(dev, attr)) {
    > > int ret;
    > >
    > > - ret = mdev_device_remove(dev, false);
    > > - if (ret) {
    > > - device_create_file(dev, attr);
    > > + ret = mdev_device_remove(dev);
    > > + if (ret)
    > > return ret;
    > > - }
    > > }
    > >
    > > return count;
    >
    > The patch looks OK to me, especially looking at the code after the changes
    > were apllied. I might have missed something though due to amount of
    > changes done.
    >
    > I lightly tested the whole patch series with my mdev driver, and it seems to
    > survive, but my testing doesn't test much of the error paths so there that.
    >
    > I'll keep this applied so if I notice any errors I'll let you know.
    >
    > If you could split this into few patches, this would be even better, but
    > anyway thanks a lot for this work!
    >
    > Reviewed-by: Maxim Levitsky <mlevitsk@redhat.com>
    >
    Thanks a lot Maxim for the review.
    This particular patch cannot be cut into more patches because it touches serialization between multiple functions.
    So all of those have to be touched.
    I tried to cut into two where previous patch does the mdev cleanup, but I guess for correctness I got to merge here.
    Only the commit message is probably big, but I had to explain all 4-5 cases for this refactor.

    I will send v1 with below changes.
    1. drop patch-1 in the series.
    2. simplify patch 7 to drop the bool part...
    3. move the loop_removal code in mdev_remove() from patch-7 to 8 for correctness.
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