RE: [PATCHv1 7/7] vfio/mdev: Fix race conditions with mdev device life cycle APIs

Parav Pandit
2019-04-04T00:02:34Z

> -----Original Message-----
> From: Alex Williamson <alex.williamson@redhat.com>
> Sent: Wednesday, April 3, 2019 4:27 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: [PATCHv1 7/7] vfio/mdev: Fix race conditions with mdev device
> life cycle APIs
>
> On Tue, 26 Mar 2019 22:45:45 -0500
> Parav Pandit <parav@mellanox.com> wrote:
>
> > Below race condition and call trace exist with current device life
> > cycle sequence.
> >
> > 1. 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()
> >
> > 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()
> > 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
> > 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 to overcome above issues.
> >
> > 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.
> >
> > mdev_device_remove() is refactored to not block on srcu when device is
> > removed as part of parent removal.
> >
> > Fixes: 7b96953bc640 ("vfio: Mediated device Core driver")
> > Signed-off-by: Parav Pandit <parav@mellanox.com>
> > ---
> > drivers/vfio/mdev/mdev_core.c | 83
> ++++++++++++++++++++++++++++++++++------
> > drivers/vfio/mdev/mdev_private.h | 6 +++
> > 2 files changed, 77 insertions(+), 12 deletions(-)
> >
> > diff --git a/drivers/vfio/mdev/mdev_core.c
> > b/drivers/vfio/mdev/mdev_core.c index aefcf34..fa233c8 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);
> > }
> > @@ -147,10 +148,30 @@ static int mdev_device_remove_ops(struct
> mdev_device *mdev, bool force_remove)
> > return 0;
> > }
> >
> > +static int mdev_device_remove_common(struct mdev_device *mdev,
> > + bool force_remove)
> > +{
> > + struct mdev_type *type;
> > + int ret;
> > +
> > + type = to_mdev_type(mdev->type_kobj);
>
> I know you're just moving this into the common function, but I think we're
> just caching this for aesthetics, the mdev object is still valid after the remove
> ops and I don't see anything touching this field. If so, maybe we should
> remove 'type' or at least set it right before it's used so it doesn't appear that
> we're preserving it before the remove op.
>
Sure, yes.
Type assignment should be done just before calling mdev_remove_sysfs_files().
Will send v2.

> > +
> > + ret = mdev_device_remove_ops(mdev, force_remove);
> > + if (ret && !force_remove) {
> > + mutex_lock(&mdev_list_lock);
> > + mdev->active = true;
> > + mutex_unlock(&mdev_list_lock);
>
> The mutex around this is a change from the previous code and I'm not sure
> it adds anything. If there's a thread testing for active racing with this thread
> setting active to true, there's no meaningful difference in the result by
> acquiring the mutex. 'active' may change from false->true during the critical
> section of the other thread, but I don't think there are any strange out of
> order things that give the wrong result, the other thread either sees true or
> false and continues or exits, regardless of this mutex.
>
Yes, I can drop the mutex.
In future remove sequence fix, this will anyway vanish.

Shall we finish this series with these 7 patches?
Once you ack it will send v2 for these 7 patches and follow on to that we cleanup the sequencing?

> > + return ret;
> > + }
> > + mdev_remove_sysfs_files(&mdev->dev, type);
> > + device_unregister(&mdev->dev);
> > + return ret;
> > +}
> > +
> > static int mdev_device_remove_cb(struct device *dev, void *data) {
> > if (dev_is_mdev(dev))
> > - mdev_device_remove(dev, true);
> > + mdev_device_remove_common(to_mdev_device(dev), true);
> >
> > return 0;
> > }
> > @@ -193,6 +214,7 @@ 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;
> > @@ -213,6 +235,7 @@ 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);
> >
> > @@ -251,13 +274,31 @@ void mdev_unregister_device(struct device *dev)
> > dev_info(dev, "MDEV: Unregistering\n");
> >
> > list_del(&parent->next);
> > + mutex_unlock(&parent_list_lock);
> > +
> > + /*
> > + * 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.
> > + */
>
> Thanks for the good documentation here.
>
> Alex
>
> > 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 +319,24 @@ int mdev_device_create(struct kobject *kobj,
> > struct device *dev, const guid_t *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 */
> > @@ -334,44 +385,52 @@ int mdev_device_create(struct kobject *kobj,
> > mdev->type_kobj = kobj;
> > mdev->active = true;
> > dev_dbg(&mdev->dev, "MDEV: created\n");
> > + srcu_read_unlock(&parent->unreg_srcu, srcu_idx);
> >
> > return 0;
> >
> > create_fail:
> > device_unregister(&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) {
> > + 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);
> > + srcu_read_unlock(&parent->unreg_srcu, srcu_idx);
> > return -EAGAIN;
> > }
> >
> > 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;
> > + ret = mdev_device_remove_common(mdev, force_remove);
> > + srcu_read_unlock(&parent->unreg_srcu, srcu_idx);
> > + if (ret)
> > return ret;
> > - }
> >
> > - mdev_remove_sysfs_files(dev, type);
> > - device_unregister(dev);
> > mdev_put_parent(parent);
> >
> > return 0;
> > diff --git a/drivers/vfio/mdev/mdev_private.h
> > b/drivers/vfio/mdev/mdev_private.h
> > index ddcf9c7..b799978 100644
> > --- a/drivers/vfio/mdev/mdev_private.h
> > +++ b/drivers/vfio/mdev/mdev_private.h
> > @@ -23,6 +23,12 @@ 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 {