PCI: Avoid race while enabling upstream bridges

When we enable a device, we first enable any upstream bridges.  If a bridge
has multiple downstream devices and we enable them simultaneously, the race
to enable the upstream bridge may cause problems.  Consider this hierarchy:

  bridge A --+-- device B
             +-- device C

If drivers for B and C call pci_enable_device() simultaneously, both will
attempt to enable A, which involves setting PCI_COMMAND_MASTER via
pci_set_master() and PCI_COMMAND_MEMORY via pci_enable_resources().

In the following sequence, B's update to set A's PCI_COMMAND_MEMORY is
lost, and neither B nor C will work correctly:

      B                                C
  pci_set_master(A)
    cmd = read(A, PCI_COMMAND)
    cmd |= PCI_COMMAND_MASTER
                                   pci_set_master(A)
                                     cmd = read(A, PCI_COMMAND)
                                     cmd |= PCI_COMMAND_MASTER
    write(A, PCI_COMMAND, cmd)
  pci_enable_device(A)
    pci_enable_resources(A)
      cmd = read(A, PCI_COMMAND)
      cmd |= PCI_COMMAND_MEMORY
      write(A, PCI_COMMAND, cmd)
                                     write(A, PCI_COMMAND, cmd)

Avoid this race by holding a new pci_bridge_mutex while enabling a bridge.
This ensures that both PCI_COMMAND_MASTER and PCI_COMMAND_MEMORY will be
updated before another thread can start enabling the bridge.

Note that although pci_enable_bridge() is recursive, it enables any
upstream bridges *before* acquiring the mutex.  When it acquires the mutex
and calls pci_set_master() and pci_enable_device(), any upstream bridges
have already been enabled so pci_enable_device() will not deadlock by
calling pci_enable_bridge() again.

Signed-off-by: Srinath Mannam <srinath.mannam@broadcom.com>
[bhelgaas: changelog, comment]
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>

drivers/pci/pci.c

@@ -52,6 +52,7 @@ static void pci_pme_list_scan(struct work_struct *work);
 static LIST_HEAD(pci_pme_list);
 static DEFINE_MUTEX(pci_pme_list_mutex);
 static DECLARE_DELAYED_WORK(pci_pme_work, pci_pme_list_scan);
+static DEFINE_MUTEX(pci_bridge_mutex);
 
 struct pci_pme_device {
 	struct list_head list;
@@ -1348,17 +1349,25 @@ static void pci_enable_bridge(struct pci_dev *dev)
 	if (bridge)
 		pci_enable_bridge(bridge);
 
+	/*
+	 * Hold pci_bridge_mutex to prevent a race when enabling two
+	 * devices below the bridge simultaneously.  The race may cause a
+	 * PCI_COMMAND_MEMORY update to be lost (see changelog).
+	 */
+	mutex_lock(&pci_bridge_mutex);
 	if (pci_is_enabled(dev)) {
 		if (!dev->is_busmaster)
 			pci_set_master(dev);
-		return;
+		goto end;
 	}
 
 	retval = pci_enable_device(dev);
 	if (retval)
 		dev_err(&dev->dev, "Error enabling bridge (%d), continuing\n",
 			retval);
 	pci_set_master(dev);
+end:
+	mutex_unlock(&pci_bridge_mutex);
 }
 
 static int pci_enable_device_flags(struct pci_dev *dev, unsigned long flags)
@@ -1383,7 +1392,7 @@ static int pci_enable_device_flags(struct pci_dev *dev, unsigned long flags)
 		return 0;		/* already enabled */
 
 	bridge = pci_upstream_bridge(dev);
-	if (bridge)
+	if (bridge && !pci_is_enabled(bridge))
 		pci_enable_bridge(bridge);
 
 	/* only skip sriov related */