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    Subject[PATCH RT 03/12 rc3] intel-iommu: Fix AB-BA lockdep report
    From: Roland Dreier <roland@purestorage.com>

    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>
    ---
    drivers/pci/intel-iommu.c | 4 ++--
    1 files changed, 2 insertions(+), 2 deletions(-)

    diff --git a/drivers/pci/intel-iommu.c b/drivers/pci/intel-iommu.c
    index 8c2564d..bc05a51 100644
    --- a/drivers/pci/intel-iommu.c
    +++ b/drivers/pci/intel-iommu.c
    @@ -3569,6 +3569,8 @@ static void domain_remove_one_dev_info(struct dmar_domain *domain,
    found = 1;
    }

    + spin_unlock_irqrestore(&device_domain_lock, flags);
    +
    if (found == 0) {
    unsigned long tmp_flags;
    spin_lock_irqsave(&domain->iommu_lock, tmp_flags);
    @@ -3585,8 +3587,6 @@ static void domain_remove_one_dev_info(struct dmar_domain *domain,
    spin_unlock_irqrestore(&iommu->lock, tmp_flags);
    }
    }
    -
    - spin_unlock_irqrestore(&device_domain_lock, flags);
    }

    static void vm_domain_remove_all_dev_info(struct dmar_domain *domain)
    --
    1.7.7.1

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