Re: [PATCH RESEND] nvme-pci: Fix EEH failure on ppc after subsystem reset

[Keith Busch <kbusch@kernel.org>]

On Sat, Mar 09, 2024 at 07:59:11PM +0530, Nilay Shroff wrote:
> On 3/8/24 21:11, Keith Busch wrote:
> > On Fri, Feb 09, 2024 at 10:32:16AM +0530, Nilay Shroff wrote:
> >> @@ -2776,6 +2776,14 @@ static void nvme_reset_work(struct work_struct *work)
> >>   out_unlock:
> >>  	mutex_unlock(&dev->shutdown_lock);
> >>   out:
> >> +	/*
> >> +	 * If PCI recovery is ongoing then let it finish first
> >> +	 */
> >> +	if (pci_channel_offline(to_pci_dev(dev->dev))) {
> >> +		dev_warn(dev->ctrl.device, "PCI recovery is ongoing so let it finish\n");
> >> +		return;
> >> +	}
> >> +
> >>  	/*
> >>  	 * Set state to deleting now to avoid blocking nvme_wait_reset(), which
> >>  	 * may be holding this pci_dev's device lock.
> >> @@ -3295,9 +3303,11 @@ static pci_ers_result_t nvme_error_detected(struct pci_dev *pdev,
> >>  	case pci_channel_io_frozen:
> >>  		dev_warn(dev->ctrl.device,
> >>  			"frozen state error detected, reset controller\n");
> >> -		if (!nvme_change_ctrl_state(&dev->ctrl, NVME_CTRL_RESETTING)) {
> >> -			nvme_dev_disable(dev, true);
> >> -			return PCI_ERS_RESULT_DISCONNECT;
> >> +		if (nvme_ctrl_state(&dev->ctrl) != NVME_CTRL_RESETTING) {
> >> +			if (!nvme_change_ctrl_state(&dev->ctrl, NVME_CTRL_RESETTING)) {
> >> +				nvme_dev_disable(dev, true);
> >> +				return PCI_ERS_RESULT_DISCONNECT;
> >> +			}
> >>  		}
> >>  		nvme_dev_disable(dev, false);
> >>  		return PCI_ERS_RESULT_NEED_RESET;
> > 
> > I get what you're trying to do, but it looks racy. The reset_work may
> > finish before pci sets channel offline, or the error handling work
> > happens to see RESETTING state, but then transitions to CONNECTING state
> > after and deadlocks on the '.resume()' side. You are counting on a very
> > specific sequence tied to the PCIe error handling module, and maybe you
> > are able to count on that sequence for your platform in this unique
> > scenario, but these link errors could happen anytime.
> > 
> I am not sure about the deadlock in '.resume()' side you mentioned above.
> Did you mean that deadlock occur due to someone holding this pci_dev's device lock?
> Or deadlock occur due to the flush_work() from nvme_error_resume() would never 
> return?

Your patch may observe a ctrl in "RESETTING" state from
error_detected(), then disable the controller, which quiesces the admin
queue. Meanwhile, reset_work may proceed to CONNECTING state and try
nvme_submit_sync_cmd(), which blocks forever because no one is going to
unquiesce that admin queue.