slimbus: qcom-ngd-ctrl: Avoid ABBA on tx_lock/ctrl->lock

In the Linux kernel, the following vulnerability has been resolved:

slimbus: qcom-ngd-ctrl: Avoid ABBA on tx_lock/ctrl->lock

During the SSR/PDR down notification the tx_lock is taken with the

intent to provide synchronization with active DMA transfers.

But during this period qcom_slim_ngd_down() is invoked, which ends up in

slim_report_absent(), which takes the slim_controller lock. In multiple

other codepaths these two locks are taken in the opposite order (i.e.

slim_controller then tx_lock).

The result is a lockdep splat, and a possible deadlock:

rprocctl/449 is trying to acquire lock:

ffff00009793e620 (&ctrl->lock){+.+.}-{4:4}, at: slim_report_absent (drivers/slimbus/core.c:322) slimbus

but task is already holding lock:

ffff00009793fb50 (&ctrl->tx_lock){+.+.}-{4:4}, at: qcom_slim_ngd_ssr_pdr_notify (drivers/slimbus/qcom-ngd-ctrl.c:1475) slim_qcom_ngd_ctrl

which lock already depends on the new lock.

Possible unsafe locking scenario:

CPU0 CPU1

---- ----

lock(&ctrl->tx_lock);

lock(&ctrl->lock);

lock(&ctrl->tx_lock);

lock(&ctrl->lock);

The assumption is that the comment refers to the desire to not call

qcom_slim_ngd_exit_dma() while we have an ongoing DMA TX transaction.

But any such transaction is initiated and completed within a single

qcom_slim_ngd_xfer_msg().

Prior to calling qcom_slim_ngd_exit_dma() the slim_controller is torn

down, all child devices are notified that the slimbus is gone and the

child devices are removed.

Stop taking the tx_lock in qcom_slim_ngd_ssr_pdr_notify() to avoid the

deadlock.