vm_prefault: Warning, backing object race averted lobject

[YONETANI Tomokazu]

Hello.
As I wrote in issue2118, after the fix (1f8fc82a) I started seeing
warning messages on the console under load:

  vm_prefault: Warning, backing object race averted lobject 0xffffffe0749d3210

The system used to panic(backtrace at the end of this message) within
a day or two, but with the kernel built from more recent source(fa7cb8ce),
it no longer panics (well, at least for 4 days) running the same load
(pkgsrc bulk build with MAKE_JOBS=5 defined), just the warning messages
shown above all over the console.

    #4  0xffffffff8039c50f in panic (
	fmt=0xffffffff805edf90 "Bad link elm %p prev->next != elm")
	at /usr/src/sys/kern/kern_shutdown.c:833
    #5  0xffffffff80532b35 in vm_object_deallocate_locked (
	object=0xffffffe0690b2d10) at /usr/src/sys/vm/vm_object.c:601
    #6  0xffffffff80532c0f in vm_object_deallocate (object=0xffffffe06b9eed10)
	at /usr/src/sys/vm/vm_object.c:451
    #7  0xffffffff8052d5bd in vm_map_entry_delete (countp=<optimized out>,
	entry=<optimized out>, map=<optimized out>)
	at /usr/src/sys/vm/vm_map.c:2608
    #8  vm_map_delete (map=0xffffffe05b0d9a00, start=<optimized out>,
	end=140737488355328, countp=<optimized out>)
	at /usr/src/sys/vm/vm_map.c:2753
    #9  0xffffffff8052d661 in vm_map_remove (map=0xffffffe05b0d9a00, start=0,
	end=<unavailable>) at /usr/src/sys/vm/vm_map.c:2774
    #10 0xffffffff8052d6e9 in vmspace_terminate (vm=0xffffffe05b0d9a00)
	at /usr/src/sys/vm/vm_map.c:310
    #11 0xffffffff80370579 in _sysref_put (sr=0xffffffe05b0d9bd0)
	at /usr/src/sys/kern/kern_sysref.c:325
    #12 0xffffffff80388079 in sysref_put (sr=<optimized out>)
	at /usr/src/sys/sys/sysref2.h:83
    #13 exit1 (rv=<optimized out>) at /usr/src/sys/kern/kern_exit.c:417
    #14 0xffffffff803884a4 in sys_exit (uap=<optimized out>)
	at /usr/src/sys/kern/kern_exit.c:121
    #15 0xffffffff805b0308 in syscall2 (frame=0xffffffe076d33c08)
	at /usr/src/sys/platform/pc64/x86_64/trap.c:1188
    #16 0xffffffff805a88df in Xfast_syscall ()
	at /usr/src/sys/platform/pc64/x86_64/exception.S:320
    #17 0x000000000000002b in ?? ()


Best Regards,
YONETANI Tomokazu