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    PCI: Avoid race while enabling upstream bridges · 40f11adc
    Srinath Mannam 提交于
    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: NSrinath Mannam <srinath.mannam@broadcom.com>
    [bhelgaas: changelog, comment]
    Signed-off-by: NBjorn Helgaas <bhelgaas@google.com>
    40f11adc
pci.c 142.0 KB