Bug 21162

	Kernel panics when a snapshot is deleted, and syncer is forced
	to sync.  However, I don't think this has anything to do with
	snapshots, just big files in general.

Fix: 

No idea, but here's the stack trace.  I have the vmcore and
	kernel still, so just let me know if you need any more info
	than what's supplied here.

(kgdb) bt -10
#10 0xc017c358 in panic (
    fmt=0xc02c7fe0 "handle_workitem_freeblocks: block count")
    at /usr/src/sys/kern/kern_shutdown.c:553
#11 0xc02437df in handle_workitem_freeblocks (freeblks=0xc09d1880)
    at /usr/src/sys/ufs/ffs/ffs_softdep.c:2020
#12 0xc02431b4 in softdep_setup_freeblocks (ip=0xc09c3600, length=0)
    at /usr/src/sys/ufs/ffs/ffs_softdep.c:1709
#13 0xc023e2da in ffs_truncate (vp=0xc6146dc0, length=0, flags=0, cred=0x0, 
    p=0xc5675f40) at /usr/src/sys/ufs/ffs/ffs_inode.c:199
#14 0xc024c08f in ufs_inactive (ap=0xc5c78efc)
    at /usr/src/sys/ufs/ufs/ufs_inode.c:88
#15 0xc0251195 in ufs_vnoperate (ap=0xc5c78efc)
    at /usr/src/sys/ufs/ufs/ufs_vnops.c:2263
#16 0xc01ac0ee in vput (vp=0xc6146dc0) at vnode_if.h:794
#17 0xc024454c in handle_workitem_remove (dirrem=0xc09d5fc0)
    at /usr/src/sys/ufs/ffs/ffs_softdep.c:2701
#18 0xc0241b50 in softdep_process_worklist (matchmnt=0x0)
    at /usr/src/sys/ufs/ffs/ffs_softdep.c:555
#19 0xc01ab97c in sched_sync () at /usr/src/sys/kern/vfs_subr.c:1020
(kgdb) sel 11
(kgdb) frame
#11 0xc02437df in handle_workitem_freeblocks (freeblks=0xc09d1880)
    at /usr/src/sys/ufs/ffs/ffs_softdep.c:2020
2020			panic("handle_workitem_freeblocks: block count");
(kgdb) list
2015			blocksreleased += btodb(bsize);
2016		}
2017	
2018	#ifdef DIAGNOSTIC
2019		if (freeblks->fb_chkcnt != blocksreleased)
2020			panic("handle_workitem_freeblocks: block count");
2021		if (allerror)
2022			softdep_error("handle_workitem_freeblks", allerror);
2023	#endif /* DIAGNOSTIC */
2024		WORKITEM_FREE(freeblks, D_FREEBLKS);
(kgdb) print freeblks->fb_chkcnt
$1 = 14048
(kgdb) print blocksreleased
$2 = 14032
(kgdb) print freeblks->fb_oldsize
$3 = 1536000000
(kgdb) quit
How-To-Repeat: 
	# mount -u -o snapshot /bigfilesystem/snap /bigfilesystem
	# rm /bigfilesystem/snap
	# sync
	# sync
	# sync
	# sync
	# sync
	[...sync like there's no tomorrow until it panics...]

	I couldn't reproduce this on a filesystem with not very many
	inodes (~4500), so I think you need a lot ( ~112000 in my
	case) to get it to panic.  Probably a race condition of some sort?