Author: kib
Date: Sun Feb 8 20:00:33 2009
New Revision: 188325
URL: http://svn.freebsd.org/changeset/base/188325
Log:
Add the comments to vm_map_simplify_entry() and vmspace_fork(),
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
Modified:
head/sys/vm/vm_map.c
Modified: head/sys/vm/vm_map.c
==============================================================================
--- head/sys/vm/vm_map.c Sun Feb 8 19:55:13 2009 (r188324)
+++ head/sys/vm/vm_map.c Sun Feb 8 20:00:33 2009 (r188325)
@@ -1350,6 +1350,16 @@ vm_map_simplify_entry(vm_map_t map, vm_m
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_m
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;
}
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