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Add the comments to vm_map_simplify_entry() and vmspace_fork(),

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describing why several calls to vm_deallocate_object() with locked map
do not result in the acquisition of the vnode lock after map lock.

Suggested and reviewed by:	tegge
This commit is contained in:
Konstantin Belousov 2009-02-08 20:00:33 +00:00
parent 1fac7d7f35
commit 7fd10fb3c7
1 changed files with 20 additions and 0 deletions
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20
sys/vm/vm_map.c
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@ -1350,6 +1350,16 @@ vm_map_simplify_entry(vm_map_t map, vm_map_entry_t entry)
entry->offset = prev->offset;
if (entry->prev != &map->header)
vm_map_entry_resize_free(map, entry->prev);
/*
* If the backing object is the vnode object,
* vm_object_deallocate() results in a call to
* vrele(). Because the reference to the
* object is not last, vrele() does not lock
* the vnode, and map lock can be kept without
* causing vnode lock to be taken after the
* map lock.
*/
if (prev->object.vm_object)
vm_object_deallocate(prev->object.vm_object);
vm_map_entry_dispose(map, prev);
@ -1371,6 +1381,10 @@ vm_map_simplify_entry(vm_map_t map, vm_map_entry_t entry)
vm_map_entry_unlink(map, next);
entry->end = next->end;
vm_map_entry_resize_free(map, entry);
/*
* See comment above.
*/
if (next->object.vm_object)
vm_object_deallocate(next->object.vm_object);
vm_map_entry_dispose(map, next);
@ -2729,6 +2743,12 @@ vmspace_fork(struct vmspace *vm1)
/* Transfer the second reference too. */
vm_object_reference(
old_entry->object.vm_object);
/*
* As in vm_map_simplify_entry(), the
* vnode lock may not be acquired in
* this call to vm_object_deallocate().
*/
vm_object_deallocate(object);
object = old_entry->object.vm_object;
}