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vm_object_terminate() on a device-backed object at the same time that another processor, call it Pa, is performing dev_pager_alloc() on the same device. The problem is that vm_pager_object_lookup() should not be allowed to return a doomed object, i.e., an object with OBJ_DEAD set, but it does. In detail, the unfortunate sequence of events is: Pt in vm_object_terminate() holds the doomed object's lock and sets OBJ_DEAD on the object. Pa in dev_pager_alloc() holds dev_pager_sx and calls vm_pager_object_lookup(), which returns the doomed object. Next, Pa calls vm_object_reference(), which requires the doomed object's lock, so Pa waits for Pt to release the doomed object's lock. Pt proceeds to the point in vm_object_terminate() where it releases the doomed object's lock. Pa is now able to complete vm_object_reference() because it can now complete the acquisition of the doomed object's lock. So, now the doomed object has a reference count of one! Pa releases dev_pager_sx and returns the doomed object from dev_pager_alloc(). Pt now acquires dev_pager_mtx, removes the doomed object from dev_pager_object_list, releases dev_pager_mtx, and finally calls uma_zfree with the doomed object. However, the doomed object is still in use by Pa. Repeating my key point, vm_pager_object_lookup() must not return a doomed object. Moreover, the test for the object's state, i.e., doomed or not, and the increment of the object's reference count should be carried out atomically. Reviewed by: kib Approved by: re (kensmith) MFC after: 3 weeks
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