[g_bio(9) PANIC] Duplicate free of item 0xc56ee8c4 from zone 0xc185d780(g_bio) ...

This panic occured whilst there was was a tonne of I/O (procmail+sendmail):

	kern.maxfiles limit exceeded by uid 2072, please see tuning(7).
	kern.maxfiles limit exceeded by uid 2072, please see tuning(7).
	Slab at 0xc56eef70, freei 17 = 0.
	panic: Duplicate free of item 0xc56ee8c4 from zone 0xc185d780(g_bio)
	
	cpuid = 1
	KDB: enter: panic
	[thread pid 18 tid 100012 ]
	Stopped at      kdb_enter+0x32: leave
	db> tr
	Tracing pid 18 tid 100012 td 0xc430c000
	kdb_enter(c09ecabf,1,c0a08052,e2d47bc4,c430c000,...) at kdb_enter+0x32
	panic(c0a08052,c56ee8c4,c185d780,c09e6906,c0a0794a,...) at panic+0x191
	uma_dbg_free(c185d780,0,c56ee8c4,8d9,c185f300,...) at uma_dbg_free+0x16c
	uma_zfree_arg(c185d780,c56ee8c4,0,e2d47c50,c06d8f3b,...) at uma_zfree_arg+0x6f
	g_destroy_bio(c56ee8c4,c56ee8c4,c09e5c68,c6,c6000180,...) at g_destroy_bio+0x22
	g_disk_done(c56ee8c4,e2d47c78,c07517fb,c6000218,e2d47ca0,...) at g_disk_done+0x8 9
	ad_done(c6000180,c09f058d,e2d47ca0,c0716a82,c437689c,...) at ad_done+0x2f
	ata_completed(c6000180,0,c09f058d,52,0,...) at ata_completed+0x1c7
	taskqueue_run(c4376880,0,c09e9cac,30e,c09ee59f,...) at taskqueue_run+0x137
	ithread_loop(c4412530,e2d47d38,c09e9a7a,328,c430bb40,...) at ithread_loop+0x211
	fork_exit(c0707ec4,c4412530,e2d47d38) at fork_exit+0xab
	fork_trampoline() at fork_trampoline+0x8
	--- trap 0x1, eip = 0, esp = 0xe2d47d6c, ebp = 0 ---
	db>
	
	shell>sudo kgdb kernel.debug.20070219 vmcore.0
	[GDB will not be able to debug user-mode threads: /usr/lib/libthread_db.so: Undefined symbol "ps_pglobal_lookup"]
	GNU gdb 6.1.1 [FreeBSD]
	Copyright 2004 Free Software Foundation, Inc.
	GDB is free software, covered by the GNU General Public License, and you are
	welcome to change it and/or distribute copies of it under certain conditions.
	Type "show copying" to see the conditions.
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	This GDB was configured as "i386-marcel-freebsd".
	
	Unread portion of the kernel message buffer:
	ad0: TIMEOUT - READ_DMA retrying (1 retry left) LBA=3351
	Slab at 0xc56eef70, freei 17 = 0.
	panic: Duplicate free of item 0xc56ee8c4 from zone 0xc185d780(g_bio)
	
	cpuid = 1
	KDB: enter: panic
	panic: from debugger
	cpuid = 1
	KDB: stack backtrace:
	Physical memory: 1007 MB
	Dumping 297 MB: 282 266 250 234 218 202 186 170 154 138 122 106 90 74 58 42 26 10
	
	#0  doadump () at pcpu.h:166
	166     pcpu.h: No such file or directory.
	        in pcpu.h
	(kgdb) where
	#0  doadump () at pcpu.h:166
	#1  0xc047e576 in db_fncall (dummy1=-1066121253, dummy2=0, dummy3=-1, dummy4=0xe2d479b4 "") at /usr/src/sys/ddb/db_command.c:486
	#2  0xc047e870 in db_command_loop () at /usr/src/sys/ddb/db_command.c:401
	#3  0xc04805fb in db_trap (type=3, code=0) at /usr/src/sys/ddb/db_main.c:222
	#4  0xc0744c19 in kdb_trap (type=0, code=0, tf=0xe2d47b48) at /usr/src/sys/kern/subr_kdb.c:502
	#5  0xc0960ea5 in trap (frame=0xe2d47b48) at /usr/src/sys/i386/i386/trap.c:621
	#6  0xc0948dbb in calltrap () at /usr/src/sys/i386/i386/exception.s:139
	#7  0x00000000 in ?? ()
	(kgdb)-


Please let me know if you need me to help dig deeper.

 -aW