Re: [PATCH net-next 11/19] vfio/mdev: Improvise mdev life cycle and parent removal scheme

Cornelia Huck
2019-11-08T13:01:23Z

On Thu, 7 Nov 2019 10:08:26 -0600
Parav Pandit <parav@mellanox.com> wrote:

I guess that should be s/Improvise/improve/ in $SUBJECT, no?

> mdev creation and removal sequence synchronization with parent device
> removal is improved in [1].
>
> However such improvement using semaphore either limiting or leads to
> complex locking scheme when used across multiple subsystem such as mdev
> and devlink.
>
> When mdev devices are used with devlink eswitch device, following
> deadlock sequence can be witnessed.
>
> mlx5_core 0000:06:00.0: E-Switch: Disable: mode(OFFLOADS), nvfs(4), active vports(5)
> mlx5_core 0000:06:00.0: MDEV: Unregistering
>
> WARNING: possible circular locking dependency detected
> ------------------------------------------------------
> devlink/42094 is trying to acquire lock:
> 00000000eb6fb4c7 (&parent->unreg_sem){++++}, at: mdev_unregister_device+0xf1/0x160 [mdev]
> 012but task is already holding lock:
> 00000000efcd208e (devlink_mutex){+.+.}, at: devlink_nl_pre_doit+0x1d/0x170
> 012which lock already depends on the new lock.
> 012the existing dependency chain (in reverse order) is:
> 012-> #1 (devlink_mutex){+.+.}:
> lock_acquire+0xbd/0x1a0
> __mutex_lock+0x84/0x8b0
> devlink_unregister+0x17/0x60
> mlx5_sf_unload+0x21/0x60 [mlx5_core]
> mdev_remove+0x1e/0x40 [mdev]
> device_release_driver_internal+0xdc/0x1a0
> bus_remove_device+0xef/0x160
> device_del+0x163/0x360
> mdev_device_remove_common+0x1e/0xa0 [mdev]
> mdev_device_remove+0x8d/0xd0 [mdev]
> remove_store+0x71/0x90 [mdev]
> kernfs_fop_write+0x113/0x1a0
> vfs_write+0xad/0x1b0
> ksys_write+0x5c/0xd0
> do_syscall_64+0x5a/0x270
> entry_SYSCALL_64_after_hwframe+0x49/0xbe
> 012-> #0 (&parent->unreg_sem){++++}:
> check_prev_add+0xb0/0x810
> __lock_acquire+0xd4b/0x1090
> lock_acquire+0xbd/0x1a0
> down_write+0x33/0x70
> mdev_unregister_device+0xf1/0x160 [mdev]
> esw_offloads_disable+0xe/0x70 [mlx5_core]
> mlx5_eswitch_disable+0x149/0x190 [mlx5_core]
> mlx5_devlink_eswitch_mode_set+0xd0/0x180 [mlx5_core]
> devlink_nl_cmd_eswitch_set_doit+0x3e/0xb0
> genl_family_rcv_msg+0x3a2/0x420
> genl_rcv_msg+0x47/0x90
> netlink_rcv_skb+0xc9/0x100
> genl_rcv+0x24/0x40
> netlink_unicast+0x179/0x220
> netlink_sendmsg+0x2f6/0x3f0
> sock_sendmsg+0x30/0x40
> __sys_sendto+0xdc/0x160
> __x64_sys_sendto+0x24/0x30
> do_syscall_64+0x5a/0x270
> entry_SYSCALL_64_after_hwframe+0x49/0xbe
> Possible unsafe locking scenario:
> CPU0 CPU1
> ---- ----
> lock(devlink_mutex);
> lock(&parent->unreg_sem);
> lock(devlink_mutex);
> lock(&parent->unreg_sem);
> 012 *** DEADLOCK ***
> 3 locks held by devlink/42094:
> 0: 0000000097a0c4aa (cb_lock){++++}, at: genl_rcv+0x15/0x40
> 1: 00000000baf61ad2 (genl_mutex){+.+.}, at: genl_rcv_msg+0x66/0x90
> 2: 00000000efcd208e (devlink_mutex){+.+.}, at: devlink_nl_pre_doit+0x1d/0x170
>
> To summarize,
> mdev_remove()
> read locks -> unreg_sem [ lock-A ]
> [..]
> devlink_unregister();
> mutex lock devlink_mutex [ lock-B ]
>
> devlink eswitch->switchdev-legacy mode change.
> devlink_nl_cmd_eswitch_set_doit()
> mutex lock devlink_mutex [ lock-B ]
> mdev_unregister_device()
> write locks -> unreg_sem [ lock-A]

So, this problem starts to pop up once you hook up that devlink stuff
with the mdev stuff, and previous users of mdev just did not have a
locking scheme similar to devlink?

>
> Hence, instead of using semaphore, such synchronization is achieved
> using srcu which is more flexible that eliminates nested locking.
>
> SRCU based solution is already proposed before at [2].
>
> [1] commit 5715c4dd66a3 ("vfio/mdev: Synchronize device create/remove with parent removal")
> [2] https://lore.kernel.org/patchwork/patch/1055254/

I don't quite recall the discussion there... is this a rework of a
patch you proposed before? Confused.

>
> Signed-off-by: Parav Pandit <parav@mellanox.com>
> ---
> drivers/vfio/mdev/mdev_core.c | 56 +++++++++++++++++++++++---------
> drivers/vfio/mdev/mdev_private.h | 3 +-
> 2 files changed, 43 insertions(+), 16 deletions(-)

(...)

> @@ -207,6 +207,7 @@ int mdev_register_device(struct device *dev, const struct mdev_parent_ops *ops)
> dev_warn(dev, "Failed to create compatibility class link\n");
>
> list_add(&parent->next, &parent_list);
> + rcu_assign_pointer(parent->self, parent);
> mutex_unlock(&parent_list_lock);
>
> dev_info(dev, "MDEV: Registered\n");
> @@ -250,14 +251,29 @@ void mdev_unregister_device(struct device *dev)
> list_del(&parent->next);
> mutex_unlock(&parent_list_lock);
>
> - down_write(&parent->unreg_sem);
> + /*
> + * 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.
> + */

So, you're putting an srcu-handled self reference there and use that as
an indication whether the parent is unregistering?

>
> 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);
> - up_write(&parent->unreg_sem);
>
> mdev_put_parent(parent);
>