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

[Mukesh Ojha]

On Mon, Mar 09, 2026 at 11:09:08PM -0500, Bjorn Andersson wrote:
> 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@xxxxxxxxxxxxxxx
> Signed-off-by: Bjorn Andersson <bjorn.andersson@xxxxxxxxxxxxxxxx>
> ---
>  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);


is it not much more safer, to put this tx_lock around qcom_slim_ngd_exit_dma() ?


>  		break;
>  	case QCOM_SSR_AFTER_POWERUP:
>  	case SERVREG_SERVICE_STATE_UP:
> 
> -- 
> 2.51.0
> 

-- 
-Mukesh Ojha