Date: Sun, 8 Feb 2009 20:00:33 +0000 (UTC) From: Konstantin Belousov <kib@FreeBSD.org> To: src-committers@freebsd.org, svn-src-all@freebsd.org, svn-src-head@freebsd.org Subject: svn commit: r188325 - head/sys/vm Message-ID: <200902082000.n18K0XQl032762@svn.freebsd.org>
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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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