linux-kernel - [PATCH] PCI: Fix "try" semantics of bus and slot reset

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Date:   Fri, 15 Feb 2019 15:22:12 -0700
From:   Alex Williamson <alex.williamson@...hat.com>
To:     linux-pci@...r.kernel.org
Cc:     linux-kernel@...r.kernel.org, bhelgaas@...gle.com, hch@....de,
        okaya@...nel.org
Subject: [PATCH] PCI: Fix "try" semantics of bus and slot reset

The commit referenced below introduced device locking around save and
restore of state for each device during a PCI bus "try" reset, making
it decidely non-"try" and prone to deadlock in the event that a device
is already locked.  Restore __pci_reset_bus() and __pci_reset_slot()
to their advertised locking semantics by pushing the save and restore
functions into the branch where the entire tree is already locked.
Extend the helper function names with "_locked" and update the comment
to reflect this calling requirement.

Fixes: b014e96d1abb ("PCI: Protect pci_error_handlers->reset_notify() usage with device_lock()")
Signed-off-by: Alex Williamson <alex.williamson@...hat.com>
---
 drivers/pci/pci.c |   53 ++++++++++++++++++++++++++---------------------------
 1 file changed, 26 insertions(+), 27 deletions(-)

diff --git a/drivers/pci/pci.c b/drivers/pci/pci.c
index c25acace7d91..870834cb6672 100644
--- a/drivers/pci/pci.c
+++ b/drivers/pci/pci.c
@@ -5058,39 +5058,42 @@ static int pci_slot_trylock(struct pci_slot *slot)
 	return 0;
 }
 
-/* Save and disable devices from the top of the tree down */
-static void pci_bus_save_and_disable(struct pci_bus *bus)
+/*
+ * Save and disable devices from the top of the tree down while holding
+ * the @dev mutex lock for the entire tree.
+ */
+static void pci_bus_save_and_disable_locked(struct pci_bus *bus)
 {
 	struct pci_dev *dev;
 
 	list_for_each_entry(dev, &bus->devices, bus_list) {
-		pci_dev_lock(dev);
 		pci_dev_save_and_disable(dev);
-		pci_dev_unlock(dev);
 		if (dev->subordinate)
-			pci_bus_save_and_disable(dev->subordinate);
+			pci_bus_save_and_disable_locked(dev->subordinate);
 	}
 }
 
 /*
- * Restore devices from top of the tree down - parent bridges need to be
- * restored before we can get to subordinate devices.
+ * Restore devices from top of the tree down while holding @dev mutex lock
+ * for the entire tree.  Parent bridges need to be restored before we can
+ * get to subordinate devices.
  */
-static void pci_bus_restore(struct pci_bus *bus)
+static void pci_bus_restore_locked(struct pci_bus *bus)
 {
 	struct pci_dev *dev;
 
 	list_for_each_entry(dev, &bus->devices, bus_list) {
-		pci_dev_lock(dev);
 		pci_dev_restore(dev);
-		pci_dev_unlock(dev);
 		if (dev->subordinate)
-			pci_bus_restore(dev->subordinate);
+			pci_bus_restore_locked(dev->subordinate);
 	}
 }
 
-/* Save and disable devices from the top of the tree down */
-static void pci_slot_save_and_disable(struct pci_slot *slot)
+/*
+ * Save and disable devices from the top of the tree down while holding
+ * the @dev mutex lock for the entire tree.
+ */
+static void pci_slot_save_and_disable_locked(struct pci_slot *slot)
 {
 	struct pci_dev *dev;
 
@@ -5099,26 +5102,25 @@ static void pci_slot_save_and_disable(struct pci_slot *slot)
 			continue;
 		pci_dev_save_and_disable(dev);
 		if (dev->subordinate)
-			pci_bus_save_and_disable(dev->subordinate);
+			pci_bus_save_and_disable_locked(dev->subordinate);
 	}
 }
 
 /*
- * Restore devices from top of the tree down - parent bridges need to be
- * restored before we can get to subordinate devices.
+ * Restore devices from top of the tree down while holding @dev mutex lock
+ * for the entire tree.  Parent bridges need to be restored before we can
+ * get to subordinate devices.
  */
-static void pci_slot_restore(struct pci_slot *slot)
+static void pci_slot_restore_locked(struct pci_slot *slot)
 {
 	struct pci_dev *dev;
 
 	list_for_each_entry(dev, &slot->bus->devices, bus_list) {
 		if (!dev->slot || dev->slot != slot)
 			continue;
-		pci_dev_lock(dev);
 		pci_dev_restore(dev);
-		pci_dev_unlock(dev);
 		if (dev->subordinate)
-			pci_bus_restore(dev->subordinate);
+			pci_bus_restore_locked(dev->subordinate);
 	}
 }
 
@@ -5177,16 +5179,15 @@ static int __pci_reset_slot(struct pci_slot *slot)
 	if (rc)
 		return rc;
 
-	pci_slot_save_and_disable(slot);
-
 	if (pci_slot_trylock(slot)) {
+		pci_slot_save_and_disable_locked(slot);
 		might_sleep();
 		rc = pci_reset_hotplug_slot(slot->hotplug, 0);
+		pci_slot_restore_locked(slot);
 		pci_slot_unlock(slot);
 	} else
 		rc = -EAGAIN;
 
-	pci_slot_restore(slot);
 
 	return rc;
 }
@@ -5273,17 +5274,15 @@ static int __pci_reset_bus(struct pci_bus *bus)
 	if (rc)
 		return rc;
 
-	pci_bus_save_and_disable(bus);
-
 	if (pci_bus_trylock(bus)) {
+		pci_bus_save_and_disable_locked(bus);
 		might_sleep();
 		rc = pci_bridge_secondary_bus_reset(bus->self);
+		pci_bus_restore_locked(bus);
 		pci_bus_unlock(bus);
 	} else
 		rc = -EAGAIN;
 
-	pci_bus_restore(bus);
-
 	return rc;
 }