Panic in g_up (bg_fsck/cdparanoia interaction issues?)

After a crash and reboot,  I start ripping the latter half of the CD,
trying to reproduce the panic:

    % cdparanoia -Z -B "7-14[3:21.14]"

It got 3/4 of the way through track 13 before a panic.  Note that bg_fsck
has finished processing ad1s1d (/home), and is now working on ad1s1e
(/usr).  I am writing to ad1s1d:

    Fatal trap 12 = page fault while in kernel mode
    cpuid = 0; apic id = 00

    fault virtual address    = 0xa00000a
    fault code               = supervisor read, page not present
    instruction pointer      = 0x8:0xc067b104
    stack pointer            = 0x10:0xd4293b98
    frame pointer            = 0x10:0xd4293ba8
    code segment             = base 0xa0, limit 0xfffff, type 0x1b
                             = DPL 0, pres 1, dev32 1, gran 1
    processor eflags         = interrupt enabled, resume, IOPL = 0
    current process          = 3 (g_up)
    kernel: type 12 trap, code=0
    Stopped at      handle_allocindir_partdone+0x40:        testb   $0x2,0xa(%ecx)
    db> panic
    panic: from debugger
    cpuid = 0;
    Debugger("panic")


    Fatal trap 3: breakpoint instruction fault while in kernel mode
    cpuid = 0; apic id = 00

    instruction pointer      = 0x8:0xc06f8a8e
    stack pointer            = 0x10:0xd4293944
    frame pointer            = 0x10:0xd4293950
    code segment             = base 0x0, limit 0xfffff, type 0x1b
                             = DPL 0, pres 1, def32 1, gran 1
    prcoessor eflags         = IOPL = 0
    current process          = 3 (g_up)
    Stopped at      handle_allocindir_partdone+0x40:        testb   $0x2,0xa(%ecx)
    db> trace
    handle_allocindir_partdone(c2125440,0,0,0,cbf3af78) at handle_allocindir_partdone+0x40
    softdep_disk_write_complete(cbf3af78,47c89c0,0,246,cbf3af78) at softdep_disk_write_complete+0x14d
    vfs_backgroundwritedone(cbf3af78,d4293c04,c055bd14,d4293c28,c055c8fb) at vfs_backgroundwritedone+0x121
    bufdone(cbf3af78,0,c101e500,cbf3af78,d4293cbc) at bufdone + 0x12e
    bufdonebio(cbf3af78,c0755ba5,c1a9ebb0,cbf3af78,0) at biodone+0xb1
    g_dev_done(c1a9ebb0,0,c1aa5990,4,c1a9ebb0) at g_dev_done+0x88
    biodone(c1a9ebb0,c07b6ce8,24c,c0755227,a) at biodone+0xb1
    g_io_schedule_up(c1984420,c19cabe0,d4293d34,c0520776,0) at g_io_schedule_up+0x117
    g_up_procbody(0xd4293d48,0,0,0) at g_up_procbody+0x1e
    fork_exit(c04feb24,0,d4293d48) at fork_exit+0x7a
    fork_trampoline() at fork_trampoline+0x8
    --- trap 0x1, eip = 0, esp = 0xd4293d7c, ebp = 0 ---
    db> reset

  - Damian