KVM: Fix possible circular locking in kvm_vm_ioctl_assign_device()

One possible order is:

KVM_CREATE_IRQCHIP ioctl(took kvm->lock) -> kvm_iobus_register_dev() ->
down_write(kvm->slots_lock).

The other one is in kvm_vm_ioctl_assign_device(), which take kvm->slots_lock
first, then kvm->lock.

Update the comment of lock order as well.

Observe it due to kernel locking debug warnings.

Cc: stable@kernel.org
Signed-off-by: Sheng Yang <sheng@linux.intel.com>
Signed-off-by: Avi Kivity <avi@redhat.com>

2 files changed, 4 insertions, 4 deletions

diff --git a/virt/kvm/assigned-dev.c b/virt/kvm/assigned-dev.c
index fd9c097b760a..f73de631e3ee 100644
--- a/virt/kvm/assigned-dev.c
+++ b/virt/kvm/assigned-dev.c
@@ -508,8 +508,8 @@ static int kvm_vm_ioctl_assign_device(struct kvm *kvm,
         struct kvm_assigned_dev_kernel *match;
         struct pci_dev *dev;
 
-        down_read(&kvm->slots_lock);
         mutex_lock(&kvm->lock);
+        down_read(&kvm->slots_lock);
 
         match = kvm_find_assigned_dev(&kvm->arch.assigned_dev_head,
                                       assigned_dev->assigned_dev_id);
@@ -573,8 +573,8 @@ static int kvm_vm_ioctl_assign_device(struct kvm *kvm,
         }
 
 out:
-        mutex_unlock(&kvm->lock);
         up_read(&kvm->slots_lock);
+        mutex_unlock(&kvm->lock);
         return r;
 out_list_del:
         list_del(&match->list);
@@ -585,8 +585,8 @@ out_put:
         pci_dev_put(dev);
 out_free:
         kfree(match);
-        mutex_unlock(&kvm->lock);
         up_read(&kvm->slots_lock);
+        mutex_unlock(&kvm->lock);
         return r;
 }
 
diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
index b5af88167613..7db071ba466c 100644
--- a/virt/kvm/kvm_main.c
+++ b/virt/kvm/kvm_main.c
@@ -64,7 +64,7 @@ MODULE_LICENSE("GPL");
 /*
  * Ordering of locks:
  *
- *              kvm->slots_lock --> kvm->lock --> kvm->irq_lock
+ *              kvm->lock --> kvm->slots_lock --> kvm->irq_lock
  */
 
 DEFINE_SPINLOCK(kvm_lock);