Re: [PATCH v2 3/3] PCI: Add missing bridge lock to pci_bus_lock()

[Keith Busch <kbusch@xxxxxxxxxx>]

On Thu, May 30, 2024 at 06:04:35PM -0700, Dan Williams wrote:
> @@ -5444,6 +5444,7 @@ static void pci_bus_lock(struct pci_bus *bus)
>  {
>  	struct pci_dev *dev;
>  
> +	pci_dev_lock(bus->self);
>  	list_for_each_entry(dev, &bus->devices, bus_list) {
>  		pci_dev_lock(dev);
>  		if (dev->subordinate)

The very next line is:

	pci_bus_lock(dev->subordinate);

The code had just locked "dev". The recursive call will then lock
dev->subordinate->self, which is "dev", the same that was just locked a
moment ago: deadlocked.

Here's an idea:

---
diff --git a/drivers/pci/pci.c b/drivers/pci/pci.c
index 46beaf1815fab..4f7cf693857f4 100644
--- a/drivers/pci/pci.c
+++ b/drivers/pci/pci.c
@@ -5487,8 +5487,9 @@ static void pci_bus_lock(struct pci_bus *bus)
 
 	pci_dev_lock(bus->self);
 	list_for_each_entry(dev, &bus->devices, bus_list) {
-		pci_dev_lock(dev);
-		if (dev->subordinate)
+		if (!dev->subordinate)
+			pci_dev_lock(dev);
+		else
 			pci_bus_lock(dev->subordinate);
 	}
 }
@@ -5501,7 +5502,8 @@ static void pci_bus_unlock(struct pci_bus *bus)
 	list_for_each_entry(dev, &bus->devices, bus_list) {
 		if (dev->subordinate)
 			pci_bus_unlock(dev->subordinate);
-		pci_dev_unlock(dev);
+		else
+			pci_dev_unlock(dev);
 	}
 	pci_dev_unlock(bus->self);
 }
--