bug of feature?

[Toxa]

On 6.0-CURRENT I have this:

1. Plug in usb flash memory drive
2. 'mount_msdosfs /dev/da0s1 /mnt/flash'
3. Plug  OUT the drive
4. Try to umount removed device (umount /mnt/flash), get an error
"/dev/da0s1: no such file or directory"
4.1. 'mount' shows us /mnt/flash is still mounted..  
5. 'cd /mnt/flash ; ls'
5. System HANGS....

Ofcourse, this is abnormal situation, because usually we're to smart not
to plug out the drive unil umount, but just in case...

P.S: usb-related part of dmesg attached
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ohci0: <NEC uPD 9210 USB controller> mem 0xe8200000-0xe8200fff irq 9 at device 7.0 on pci2
ohci0: [GIANT-LOCKED]
usb0: OHCI version 1.0, legacy support
usb0: <NEC uPD 9210 USB controller> on ohci0
usb0: USB revision 1.0
uhub0: NEC OHCI root hub, class 9/0, rev 1.00/1.00, addr 1
uhub0: 3 ports with 3 removable, self powered
ohci1: <NEC uPD 9210 USB controller> mem 0xe8201000-0xe8201fff irq 9 at device 7.1 on pci2
ohci1: [GIANT-LOCKED]
usb1: OHCI version 1.0, legacy support
usb1: <NEC uPD 9210 USB controller> on ohci1
usb1: USB revision 1.0
uhub1: NEC OHCI root hub, class 9/0, rev 1.00/1.00, addr 1
uhub1: 2 ports with 2 removable, self powered
umass0: Sony USB Memory Stick Slot, rev 1.10/1.90, addr 2
ehci0: <NEC uPD 720100 USB 2.0 controller> mem 0xe8203800-0xe82038ff irq 9 at device 7.2 on pci2
ehci0: [GIANT-LOCKED]
ehci_pci_attach: companion usb0
ehci_pci_attach: companion usb1
usb2: EHCI version 1.0
usb2: companion controllers, 3 ports each: usb0 usb1
usb2: <NEC uPD 720100 USB 2.0 controller> on ehci0
usb2: USB revision 2.0
uhub2: NEC EHCI root hub, class 9/0, rev 2.00/1.00, addr 1
uhub2: 5 ports with 5 removable, self powered
[snip...]
umass0: at uhub1 port 2 (addr 2) disconnected
umass0: detached
umass0: Sony USB Memory Stick Slot, rev 1.10/1.90, addr 2
(probe0:umass-sim0:0:0:0): INQUIRY. CDB: 12 0 0 0 24 0 
(probe0:umass-sim0:0:0:0): CAM Status: SCSI Status Error
(probe0:umass-sim0:0:0:0): SCSI Status: Check Condition
(probe0:umass-sim0:0:0:0): UNIT ATTENTION asc:29,0
(probe0:umass-sim0:0:0:0): Power on, reset, or bus device reset occurred
(probe0:umass-sim0:0:0:0): Retrying Command (per Sense Data)
(probe0:umass-sim0:0:0:0): INQUIRY. CDB: 12 0 0 0 24 0 
(probe0:umass-sim0:0:0:0): CAM Status: SCSI Status Error
(probe0:umass-sim0:0:0:0): SCSI Status: Check Condition
(probe0:umass-sim0:0:0:0): NOT READY asc:3a,0
(probe0:umass-sim0:0:0:0): Medium not present
(probe0:umass-sim0:0:0:0): Unretryable error
[snip...]
(probe2:umass-sim0:0:0:0): INQUIRY. CDB: 12 0 0 0 24 0 
(probe2:umass-sim0:0:0:0): CAM Status: SCSI Status Error
(probe2:umass-sim0:0:0:0): SCSI Status: Check Condition
(probe2:umass-sim0:0:0:0): NOT READY asc:3a,0
(probe2:umass-sim0:0:0:0): Medium not present
(probe2:umass-sim0:0:0:0): Unretryable error