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SubjectRe: [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.

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?

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. BTW, why is the sanitization loop in
mdev_device_remove() still here, wasn't that fixed by the previous two
patches? Thanks,

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;

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