Re: [PATCH 3/9] driver core: warn should device_move try to move a need_parent_lock device

[tarunsahu@google.com]

David Jeffery <djeffery@redhat.com> writes:

> Currently, no device has need_parent_lock set and is moved by
> device_move. need_parent_lock is only set by the usb bus and very
> few device types ever use device_move.
>
> Add a warning to device_move to catch should it ever be used on a
> device with need_parent_lock set. The combination would break
> the immutable relationship needed between parent and child for
> need_parent_lock when locking and unlocking both.
>
> Signed-off-by: David Jeffery <djeffery@redhat.com>
> Tested-by: Laurence Oberman <loberman@redhat.com>
> ---
>  drivers/base/core.c | 8 ++++++++
>  1 file changed, 8 insertions(+)
>
> diff --git a/drivers/base/core.c b/drivers/base/core.c
> index bd9c2921e326..75f5931165a8 100644
> --- a/drivers/base/core.c
> +++ b/drivers/base/core.c
> @@ -4712,6 +4712,14 @@ int device_move(struct device *dev, struct device *new_parent,
>  	if (!dev)
>  		return -EINVAL;
>  
> +	/*
> +	 * device_move() should not be used on devices with need_parent_lock
> +	 * set. Concurrent reparenting will violate the immutable
> +	 * relationship needed while locking and unlocking both parent and
> +	 * child.
> +	 */
> +	WARN_ON(dev->bus && dev->bus->need_parent_lock);
> +
>  	device_pm_lock();
>  	new_parent = get_device(new_parent);
>  	new_parent_kobj = get_device_parent(dev, new_parent);
> -- 

Thankyou for adding this, Please feel free to add
Suggested-by: Tarun Sahu <tarunsahu@google.com>

Also, I have seen sashiko complaining about another problem with
device_move which is impractical/impossible. So I mentioning it below
for discussion/information.

Sashiko Claim: There is potential deadlock if a device is being
registered with DL_FLAG_STATELESS which skips the reordering of
the list so device_kset->list will have device dependent devices out of
order. Also same problem can be create by device_move.

=> This is impractical because a device being registered with
DL_FLAG_STATELESS must have its supplier already registered first
(Documentation/driver-api/device_link.rst) which inherently puts
them in the order.

Similarily for device_move() affecting topological order is very
impractical. Unless someone mis-use the API
device_move(..., DPM_ORDER_NONE). Which, as well, not favourable
on upstream.

Once topological order is messed up, neither async shutdown nor serial
(sync) shutdown can work. As it is precondition for it. except
DL_FLAG_SYNC_STATE_ONLY, which is handled in both implementation.


~Tarun

> 2.55.0