[PATCH 7/7] slimbus: qcom-ngd-ctrl: Avoid ABBA on tx_lock/ctrl->lock

[Bjorn Andersson <bjorn.andersson@oss.qualcomm.com>]

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.

Fixes: a899d324863a ("slimbus: qcom-ngd-ctrl: add Sub System Restart support")
Cc: stable@vger.kernel.org
Signed-off-by: Bjorn Andersson <bjorn.andersson@oss.qualcomm.com>
---
 drivers/slimbus/qcom-ngd-ctrl.c | 3 ---
 1 file changed, 3 deletions(-)

diff --git a/drivers/slimbus/qcom-ngd-ctrl.c b/drivers/slimbus/qcom-ngd-ctrl.c
index 54a4c6ee1e71fe55794f09575979826d9aa5be9f..75d70de0909a8d17e2410d30f7811f32d5eebea3 100644
--- a/drivers/slimbus/qcom-ngd-ctrl.c
+++ b/drivers/slimbus/qcom-ngd-ctrl.c
@@ -1471,15 +1471,12 @@ static int qcom_slim_ngd_ssr_pdr_notify(struct qcom_slim_ngd_ctrl *ctrl,
 	switch (action) {
 	case QCOM_SSR_BEFORE_SHUTDOWN:
 	case SERVREG_SERVICE_STATE_DOWN:
-		/* Make sure the last dma xfer is finished */
-		mutex_lock(&ctrl->tx_lock);
 		if (ctrl->state != QCOM_SLIM_NGD_CTRL_DOWN) {
 			pm_runtime_get_noresume(ctrl->ctrl.dev);
 			ctrl->state = QCOM_SLIM_NGD_CTRL_DOWN;
 			qcom_slim_ngd_down(ctrl);
 			qcom_slim_ngd_exit_dma(ctrl);
 		}
-		mutex_unlock(&ctrl->tx_lock);
 		break;
 	case QCOM_SSR_AFTER_POWERUP:
 	case SERVREG_SERVICE_STATE_UP:

-- 
2.51.0