From 3e7abe2556b583e87dabda3e0e6178a67b20d06f Mon Sep 17 00:00:00 2001 From: Roland Dreier Date: Wed, 20 Jul 2011 06:22:21 -0700 Subject: [PATCH] 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: [] domain_remove_one_dev_info+0x121/0x230 but task is already holding lock: (device_domain_lock){-.-...}, at: [] 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){-.-...}: [] lock_acquire+0x9d/0x130 [] _raw_spin_lock_irqsave+0x55/0xa0 [] domain_context_mapping_one+0x600/0x750 [] domain_context_mapping+0x3f/0x120 [] iommu_prepare_identity_map+0x1c5/0x1e0 [] intel_iommu_init+0x88e/0xb5e [] pci_iommu_init+0x16/0x41 [] do_one_initcall+0x45/0x190 [] kernel_init+0xe3/0x168 [] kernel_thread_helper+0x4/0x10 -> #0 (&(&iommu->lock)->rlock){......}: [] __lock_acquire+0x195e/0x1e10 [] lock_acquire+0x9d/0x130 [] _raw_spin_lock_irqsave+0x55/0xa0 [] domain_remove_one_dev_info+0x121/0x230 [] device_notifier+0x72/0x90 [] notifier_call_chain+0x8c/0xc0 [] __blocking_notifier_call_chain+0x78/0xb0 [] blocking_notifier_call_chain+0x16/0x20 [] __device_release_driver+0xbc/0xe0 [] device_release_driver+0x2f/0x50 [] driver_unbind+0xa3/0xc0 [] drv_attr_store+0x2c/0x30 [] sysfs_write_file+0xe6/0x170 [] vfs_write+0xce/0x190 [] sys_write+0x54/0xa0 [] system_call_fastpath+0x16/0x1b other info that might help us debug this: 6 locks held by bash/13954: #0: (&buffer->mutex){+.+.+.}, at: [] sysfs_write_file+0x44/0x170 #1: (s_active#3){++++.+}, at: [] sysfs_write_file+0xcd/0x170 #2: (&__lockdep_no_validate__){+.+.+.}, at: [] driver_unbind+0x9b/0xc0 #3: (&__lockdep_no_validate__){+.+.+.}, at: [] device_release_driver+0x27/0x50 #4: (&(&priv->bus_notifier)->rwsem){.+.+.+}, at: [] __blocking_notifier_call_chain+0x5f/0xb0 #5: (device_domain_lock){-.-...}, at: [] domain_remove_one_dev_info+0x208/0x230 stack backtrace: Pid: 13954, comm: bash Not tainted 2.6.39.1+ #1 Call Trace: [] print_circular_bug+0xf7/0x100 [] __lock_acquire+0x195e/0x1e10 [] ? trace_hardirqs_off+0xd/0x10 [] ? trace_hardirqs_on_caller+0x13d/0x180 [] lock_acquire+0x9d/0x130 [] ? domain_remove_one_dev_info+0x121/0x230 [] _raw_spin_lock_irqsave+0x55/0xa0 [] ? domain_remove_one_dev_info+0x121/0x230 [] ? trace_hardirqs_off+0xd/0x10 [] domain_remove_one_dev_info+0x121/0x230 [] device_notifier+0x72/0x90 [] notifier_call_chain+0x8c/0xc0 [] __blocking_notifier_call_chain+0x78/0xb0 [] blocking_notifier_call_chain+0x16/0x20 [] __device_release_driver+0xbc/0xe0 [] device_release_driver+0x2f/0x50 [] driver_unbind+0xa3/0xc0 [] drv_attr_store+0x2c/0x30 [] sysfs_write_file+0xe6/0x170 [] vfs_write+0xce/0x190 [] sys_write+0x54/0xa0 [] system_call_fastpath+0x16/0x1b Signed-off-by: Roland Dreier Signed-off-by: David Woodhouse --- drivers/iommu/intel-iommu.c | 4 ++-- 1 file changed, 2 insertions(+), 2 deletions(-) diff --git a/drivers/iommu/intel-iommu.c b/drivers/iommu/intel-iommu.c index c621c98c99da..d9514c46177f 100644 --- a/drivers/iommu/intel-iommu.c +++ b/drivers/iommu/intel-iommu.c @@ -3568,6 +3568,8 @@ 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); @@ -3584,8 +3586,6 @@ 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) -- 2.39.2