intel-iommu: Fix AB-BA lockdep report

When unbinding a device so that I could pass it through to a KVM VM, I
got the lockdep report below.  It looks like a legitimate lock
ordering problem:

 - domain_context_mapping_one() takes iommu->lock and calls
   iommu_support_dev_iotlb(), which takes device_domain_lock (inside
   iommu->lock).

 - domain_remove_one_dev_info() starts by taking device_domain_lock
   then takes iommu->lock inside it (near the end of the function).

So this is the classic AB-BA deadlock.  It looks like a safe fix is to
simply release device_domain_lock a bit earlier, since as far as I can
tell, it doesn't protect any of the stuff accessed at the end of
domain_remove_one_dev_info() anyway.

BTW, the use of device_domain_lock looks a bit unsafe to me... it's
at least not obvious to me why we aren't vulnerable to the race below:

  iommu_support_dev_iotlb()
                                          domain_remove_dev_info()

  lock device_domain_lock
    find info
  unlock device_domain_lock

                                          lock device_domain_lock
                                            find same info
                                          unlock device_domain_lock

                                          free_devinfo_mem(info)

  do stuff with info after it's free

However I don't understand the locking here well enough to know if
this is a real problem, let alone what the best fix is.

Anyway here's the full lockdep output that prompted all of this:

     =======================================================
     [ INFO: possible circular locking dependency detected ]
     2.6.39.1+ #1
     -------------------------------------------------------
     bash/13954 is trying to acquire lock:
      (&(&iommu->lock)->rlock){......}, at: [<ffffffff812f6421>] domain_remove_one_dev_info+0x121/0x230

     but task is already holding lock:
      (device_domain_lock){-.-...}, at: [<ffffffff812f6508>] domain_remove_one_dev_info+0x208/0x230

     which lock already depends on the new lock.

     the existing dependency chain (in reverse order) is:

     -> #1 (device_domain_lock){-.-...}:
            [<ffffffff8109ca9d>] lock_acquire+0x9d/0x130
            [<ffffffff81571475>] _raw_spin_lock_irqsave+0x55/0xa0
            [<ffffffff812f8350>] domain_context_mapping_one+0x600/0x750
            [<ffffffff812f84df>] domain_context_mapping+0x3f/0x120
            [<ffffffff812f9175>] iommu_prepare_identity_map+0x1c5/0x1e0
            [<ffffffff81ccf1ca>] intel_iommu_init+0x88e/0xb5e
            [<ffffffff81cab204>] pci_iommu_init+0x16/0x41
            [<ffffffff81002165>] do_one_initcall+0x45/0x190
            [<ffffffff81ca3d3f>] kernel_init+0xe3/0x168
            [<ffffffff8157ac24>] kernel_thread_helper+0x4/0x10

     -> #0 (&(&iommu->lock)->rlock){......}:
            [<ffffffff8109bf3e>] __lock_acquire+0x195e/0x1e10
            [<ffffffff8109ca9d>] lock_acquire+0x9d/0x130
            [<ffffffff81571475>] _raw_spin_lock_irqsave+0x55/0xa0
            [<ffffffff812f6421>] domain_remove_one_dev_info+0x121/0x230
            [<ffffffff812f8b42>] device_notifier+0x72/0x90
            [<ffffffff8157555c>] notifier_call_chain+0x8c/0xc0
            [<ffffffff81089768>] __blocking_notifier_call_chain+0x78/0xb0
            [<ffffffff810897b6>] blocking_notifier_call_chain+0x16/0x20
            [<ffffffff81373a5c>] __device_release_driver+0xbc/0xe0
            [<ffffffff81373ccf>] device_release_driver+0x2f/0x50
            [<ffffffff81372ee3>] driver_unbind+0xa3/0xc0
            [<ffffffff813724ac>] drv_attr_store+0x2c/0x30
            [<ffffffff811e4506>] sysfs_write_file+0xe6/0x170
            [<ffffffff8117569e>] vfs_write+0xce/0x190
            [<ffffffff811759e4>] sys_write+0x54/0xa0
            [<ffffffff81579a82>] system_call_fastpath+0x16/0x1b

     other info that might help us debug this:

     6 locks held by bash/13954:
      #0:  (&buffer->mutex){+.+.+.}, at: [<ffffffff811e4464>] sysfs_write_file+0x44/0x170
      #1:  (s_active#3){++++.+}, at: [<ffffffff811e44ed>] sysfs_write_file+0xcd/0x170
      #2:  (&__lockdep_no_validate__){+.+.+.}, at: [<ffffffff81372edb>] driver_unbind+0x9b/0xc0
      #3:  (&__lockdep_no_validate__){+.+.+.}, at: [<ffffffff81373cc7>] device_release_driver+0x27/0x50
      #4:  (&(&priv->bus_notifier)->rwsem){.+.+.+}, at: [<ffffffff8108974f>] __blocking_notifier_call_chain+0x5f/0xb0
      #5:  (device_domain_lock){-.-...}, at: [<ffffffff812f6508>] domain_remove_one_dev_info+0x208/0x230

     stack backtrace:
     Pid: 13954, comm: bash Not tainted 2.6.39.1+ #1
     Call Trace:
      [<ffffffff810993a7>] print_circular_bug+0xf7/0x100
      [<ffffffff8109bf3e>] __lock_acquire+0x195e/0x1e10
      [<ffffffff810972bd>] ? trace_hardirqs_off+0xd/0x10
      [<ffffffff8109d57d>] ? trace_hardirqs_on_caller+0x13d/0x180
      [<ffffffff8109ca9d>] lock_acquire+0x9d/0x130
      [<ffffffff812f6421>] ? domain_remove_one_dev_info+0x121/0x230
      [<ffffffff81571475>] _raw_spin_lock_irqsave+0x55/0xa0
      [<ffffffff812f6421>] ? domain_remove_one_dev_info+0x121/0x230
      [<ffffffff810972bd>] ? trace_hardirqs_off+0xd/0x10
      [<ffffffff812f6421>] domain_remove_one_dev_info+0x121/0x230
      [<ffffffff812f8b42>] device_notifier+0x72/0x90
      [<ffffffff8157555c>] notifier_call_chain+0x8c/0xc0
      [<ffffffff81089768>] __blocking_notifier_call_chain+0x78/0xb0
      [<ffffffff810897b6>] blocking_notifier_call_chain+0x16/0x20
      [<ffffffff81373a5c>] __device_release_driver+0xbc/0xe0
      [<ffffffff81373ccf>] device_release_driver+0x2f/0x50
      [<ffffffff81372ee3>] driver_unbind+0xa3/0xc0
      [<ffffffff813724ac>] drv_attr_store+0x2c/0x30
      [<ffffffff811e4506>] sysfs_write_file+0xe6/0x170
      [<ffffffff8117569e>] vfs_write+0xce/0x190
      [<ffffffff811759e4>] sys_write+0x54/0xa0
      [<ffffffff81579a82>] system_call_fastpath+0x16/0x1b

Signed-off-by: Roland Dreier <roland@purestorage.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>