kern/189355: zfs panic on 10-stable

[Radim Kolar]

crash stack from patched kernel

#7  0xc092336c in calltrap () at /usr/src/sys/i386/i386/exception.s:170
#8  0xc069345d in kdb_enter (why=0xc09a7ef8 "panic",
    msg=<value optimized out>) at cpufunc.h:71
#9  0xc065710f in panic (fmt=<value optimized out>)
    at /usr/src/sys/kern/kern_shutdown.c:823
#10 0xc0939acb in dblfault_handler () at /usr/src/sys/i386/i386/trap.c:1072
#11 0xc0cb8187 in vdev_queue_io_to_issue (vq=0xc46ab300)
    at /usr/src/sys/modules/zfs/../../cddl/contrib/opensolaris/uts/common/fs/zfs/vdev_queue.c:489
#12 0xc0cb8119 in vdev_queue_io (zio=0xc486c5b8)
    at /usr/src/sys/modules/zfs/../../cddl/contrib/opensolaris/uts/common/fs/zfs/vdev_queue.c:768
#13 0xc0cd8704 in zio_vdev_io_start (ziop=<value optimized out>)
    at /usr/src/sys/modules/zfs/../../cddl/contrib/opensolaris/uts/common/fs/zfs/zio.c:2617
#14 0xc0cd4e28 in zio_execute (zio=0xc486c5b8)
    at /usr/src/sys/modules/zfs/../../cddl/contrib/opensolaris/uts/common/fs/zfs/zio.c:1357
#15 0xc0cb7604 in vdev_mirror_io_start (zio=0xc476f000)
    at /usr/src/sys/modules/zfs/../../cddl/contrib/opensolaris/uts/common/fs/zfs/vdev_mirror.c:284
#16 0xc0cd854f in zio_vdev_io_start (ziop=<value optimized out>)
    at /usr/src/sys/modules/zfs/../../cddl/contrib/opensolaris/uts/common/fs/zfs/zio.c:2537
#17 0xc0cd4e28 in zio_execute (zio=0xc476f000)
    at /usr/src/sys/modules/zfs/../../cddl/contrib/opensolaris/uts/common/fs/zfs/zio.c:1357
#18 0xc0ca0ddc in spa_load_verify_cb (zilog=0x0, bp=<value optimized out>,
    dnp=0xc4b0fa00, arg=<value optimized out>)
    at /usr/src/sys/modules/zfs/../../cddl/contrib/opensolaris/uts/common/fs/zfs/spa.c:1887
#19 0xc0c5eb79 in traverse_visitbp (td=0xd93f1120, dnp=0xc46ab300,
    bp=0xc4b18100, zb=0xd93f06b0)
    at /usr/src/sys/modules/zfs/../../cddl/contrib/opensolaris/uts/common/fs/zfs/dmu_traverse.c:272
#20 0xc0c5edb0 in traverse_visitbp (td=0xd93f1120, dnp=0xc4b0fa00, bp=0x80,
    zb=0xd93f07a8)
    at /usr/src/sys/modules/zfs/../../cddl/contrib/opensolaris/uts/common/fs/zfs/dmu_traverse.c:306

failing line 489 is:

        if (avl_numnodes(&vq->vq_active_tree) >= zfs_vdev_max_active)
                return (ZIO_PRIORITY_NUM_QUEUEABLE);