Commit 40f11adc authored Aug 18, 2017 by Srinath Mannam Committed by Bjorn Helgaas Aug 18, 2017

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>

parent 62ce94a7

drivers/pci/pci.c

+11 −2

Original line number	Diff line number	Diff line
		@@ -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;
		@@ -1348,10 +1349,16 @@ 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);
		@@ -1359,6 +1366,8 @@ static void pci_enable_bridge(struct pci_dev *dev)
		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 */

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