I have just had 3 of these kernel warnings with backtraces while using my USB 
serial adapter.
I'm using the current kernel released with Lucid, on 32-bit x86.  My serial 
adapter is an Arkmicro Technologies Inc. ARK3116 Serial.

It's not just innocuous warnings.  A script talking to the device at the
other and of the serial cable is getting garbage back from the serial
port.  I've seen the garbage repeatedly during an hour long session (in
fact, can't use the script - the data coming back is too unreliable).
Specifically, it contains repeated portions of text sent earlier, over
and over.  strace confirms the kernel serial driver is returning these
repeated bits of text, not a bug in the script.  It's using Perl and
libexpect-perl.  Strangely, when I use minicom to talk to the device
manually I don't get these repeated selections of old text back.

I had suspected the device at the other end, until I started getting
apport crash reports towards the end of my hour long session.  Now I
suspect a bug in the usb-serial driver or the tty layer - both were
changed near this kernel version.

I will keep a copy of my script, to see if this can be reproduced
another time.

Rather than a one-off, this looks quite likely to duplicated as at least
some of these other reports:

      #533982 WARNING: at 
/build/buildd/linux-2.6.32/drivers/usb/serial/usb-serial.c:440 
serial_unthrottle 0x7b/0x80 [usbserial]()
      #433604 WARNING: at 
/build/buildd/linux-2.6.31/drivers/usb/serial/usb-serial.c:448 
serial_unthrottle+0x76/0x80 [usbserial]()
      #471157 WARNING: at 
/build/buildd/linux-2.6.31/drivers/usb/serial/usb-serial.c:446 
serial_unthrottle+0x76/0x80 [usbserial]()
      #470007 WARNING: at 
/build/buildd/linux-2.6.31/drivers/usb/serial/usb-serial.c:446 
serial_unthrottle+0x76/0x80 [usbserial]()
      #407939 System freeze after WARNING: at 
/build/buildd/linux-2.6.31/drivers/usb/serial/usb-serial.c:374 
serial_write+0xb9/0xc0 [usbserial]()
      #286703 WARNING: at 
/build/buildd/linux-2.6.27/drivers/usb/serial/usb-serial.c:311 
serial_write+0xa4/0xb0 [usbserial]()
      #438311 WARNING: at 
/build/buildd/linux-2.6.31/drivers/usb/serial/usb-serial.c:401 
serial_write+0xbb/0xc0 [usbserial]()

Notice a common theme?  They are all USB serial.

These are my variations on the warnings:

Mar 19 10:16:17 localhost kernel: [33130.516205] usb 3-2: new full speed USB 
device using uhci_hcd and address 2
Mar 19 10:16:17 localhost kernel: [33130.672508] usb 3-2: configuration #1 
chosen from 1 choice
Mar 19 10:16:18 localhost kernel: [33130.698253] usbcore: registered new 
interface driver usbserial
Mar 19 10:16:18 localhost kernel: [33130.698283] USB Serial support registered 
for generic
Mar 19 10:16:18 localhost kernel: [33130.698343] usbcore: registered new 
interface driver usbserial_generic
Mar 19 10:16:18 localhost kernel: [33130.698348] usbserial: USB Serial Driver 
core
Mar 19 10:16:18 localhost kernel: [33130.700253] USB Serial support registered 
for ark3116
Mar 19 10:16:18 localhost kernel: [33130.700275] ark3116 3-2:1.0: ark3116 
converter detected
Mar 19 10:16:18 localhost kernel: [33130.727487] usb 3-2: ark3116 converter now 
attached to ttyUSB0
Mar 19 10:16:18 localhost kernel: [33130.727527] usbcore: registered new 
interface driver ark3116
Mar 19 12:34:53 localhost kernel: [41446.080220] ------------[ cut here 
]------------
Mar 19 12:34:53 localhost kernel: [41446.080240] WARNING: at 
/build/buildd/linux-2.6.32/drivers/usb/serial/usb-serial.c:406 
serial_write_room+0x75/0x80 [usbserial]()
Mar 19 12:34:53 localhost kernel: [41446.080247] Hardware name: Amilo Si 1520
Mar 19 12:34:53 localhost kernel: [41446.080251] Modules linked in: ark3116 
usbserial binfmt_misc ppdev kqemu kvm_intel kvm sha256_generic aes_i586 
aes_generic dm_crypt joydev snd_hda_codec_conexant snd_hda_intel snd_hda_codec 
snd_hwdep snd_pcm_oss snd_mixer_oss snd_pcm snd_seq_dummy arc4 snd_seq_oss 
snd_seq_midi snd_rawmidi snd_seq_midi_event snd_seq snd_timer snd_seq_device 
sbp2 iwl3945 snd iwlcore soundcore mac80211 ricoh_mmc sdhci_pci sdhci led_class 
psmouse serio_raw snd_page_alloc cfg80211 lp parport fbcon tileblit font 
bitblit softcursor vga16fb vgastate ohci1394 i915 drm_kms_helper drm 
i2c_algo_bit e100 mii ieee1394 video intel_agp output agpgart
Mar 19 12:34:53 localhost kernel: [41446.080367] Pid: 9, comm: events/0 Not 
tainted 2.6.32-16-generic #25-Ubuntu
Mar 19 12:34:53 localhost kernel: [41446.080372] Call Trace:
Mar 19 12:34:53 localhost kernel: [41446.080386]  [<c014a9d2>] 
warn_slowpath_common+0x72/0xa0
Mar 19 12:34:53 localhost kernel: [41446.080396]  [<fe486735>] ? 
serial_write_room+0x75/0x80 [usbserial]
Mar 19 12:34:53 localhost kernel: [41446.080406]  [<fe486735>] ? 
serial_write_room+0x75/0x80 [usbserial]
Mar 19 12:34:53 localhost kernel: [41446.080413]  [<c014aa1a>] 
warn_slowpath_null+0x1a/0x20
Mar 19 12:34:53 localhost kernel: [41446.080422]  [<fe486735>] 
serial_write_room+0x75/0x80 [usbserial]
Mar 19 12:34:53 localhost kernel: [41446.080432]  [<c03ad8d3>] ? 
tty_put_char+0x33/0x40
Mar 19 12:34:53 localhost kernel: [41446.080440]  [<c03b3ecd>] 
tty_write_room+0x1d/0x20
Mar 19 12:34:53 localhost kernel: [41446.080447]  [<c03b191d>] 
process_echoes+0x4d/0x2d0
Mar 19 12:34:53 localhost kernel: [41446.080453]  [<c03b148d>] ? 
echo_char+0x4d/0xa0
Mar 19 12:34:53 localhost kernel: [41446.080461]  [<c03b35a1>] 
n_tty_receive_char+0x3a1/0x780
Mar 19 12:34:53 localhost kernel: [41446.080468]  [<c03b3b4e>] 
n_tty_receive_buf+0x1ce/0x410
Mar 19 12:34:53 localhost kernel: [41446.080478]  [<c012a2f8>] ? 
default_spin_lock_flags+0x8/0x10
Mar 19 12:34:53 localhost kernel: [41446.080488]  [<c059dcbf>] ? 
_spin_lock_irqsave+0x2f/0x50
Mar 19 12:34:53 localhost kernel: [41446.080496]  [<c012a2f8>] ? 
default_spin_lock_flags+0x8/0x10
Mar 19 12:34:53 localhost kernel: [41446.080504]  [<c03b62a1>] 
flush_to_ldisc+0x151/0x180
Mar 19 12:34:53 localhost kernel: [41446.080513]  [<c0161c6e>] 
run_workqueue+0x8e/0x150
Mar 19 12:34:53 localhost kernel: [41446.080520]  [<c03b6150>] ? 
flush_to_ldisc+0x0/0x180
Mar 19 12:34:53 localhost kernel: [41446.080528]  [<c0161db4>] 
worker_thread+0x84/0xe0
Mar 19 12:34:53 localhost kernel: [41446.080536]  [<c0165d30>] ? 
autoremove_wake_function+0x0/0x50
Mar 19 12:34:53 localhost kernel: [41446.080544]  [<c0161d30>] ? 
worker_thread+0x0/0xe0
Mar 19 12:34:53 localhost kernel: [41446.080550]  [<c0165aa4>] kthread+0x74/0x80
Mar 19 12:34:53 localhost kernel: [41446.080557]  [<c0165a30>] ? 
kthread+0x0/0x80
Mar 19 12:34:53 localhost kernel: [41446.080565]  [<c0104087>] 
kernel_thread_helper+0x7/0x10
Mar 19 12:34:53 localhost kernel: [41446.080570] ---[ end trace 
5d571f4d930148a4 ]---
Mar 19 12:36:14 localhost kernel: [41526.840310] ------------[ cut here 
]------------
Mar 19 12:36:14 localhost kernel: [41526.840329] WARNING: at 
/build/buildd/linux-2.6.32/drivers/usb/serial/usb-serial.c:440 
serial_unthrottle+0x7b/0x80 [usbserial]()
Mar 19 12:36:14 localhost kernel: [41526.840335] Hardware name: Amilo Si 1520
Mar 19 12:36:14 localhost kernel: [41526.840339] Modules linked in: ark3116 
usbserial binfmt_misc ppdev kqemu kvm_intel kvm sha256_generic aes_i586 
aes_generic dm_crypt joydev snd_hda_codec_conexant snd_hda_intel snd_hda_codec 
snd_hwdep snd_pcm_oss snd_mixer_oss snd_pcm snd_seq_dummy arc4 snd_seq_oss 
snd_seq_midi snd_rawmidi snd_seq_midi_event snd_seq snd_timer snd_seq_device 
sbp2 iwl3945 snd iwlcore soundcore mac80211 ricoh_mmc sdhci_pci sdhci led_class 
psmouse serio_raw snd_page_alloc cfg80211 lp parport fbcon tileblit font 
bitblit softcursor vga16fb vgastate ohci1394 i915 drm_kms_helper drm 
i2c_algo_bit e100 mii ieee1394 video intel_agp output agpgart
Mar 19 12:36:14 localhost kernel: [41526.840454] Pid: 8076, comm: 
catch_bootloade Tainted: G        W  2.6.32-16-generic #25-Ubuntu
Mar 19 12:36:14 localhost kernel: [41526.840460] Call Trace:
Mar 19 12:36:14 localhost kernel: [41526.840473]  [<c014a9d2>] 
warn_slowpath_common+0x72/0xa0
Mar 19 12:36:14 localhost kernel: [41526.840484]  [<fe48658b>] ? 
serial_unthrottle+0x7b/0x80 [usbserial]
Mar 19 12:36:14 localhost kernel: [41526.840493]  [<fe48658b>] ? 
serial_unthrottle+0x7b/0x80 [usbserial]
Mar 19 12:36:14 localhost kernel: [41526.840501]  [<c014aa1a>] 
warn_slowpath_null+0x1a/0x20
Mar 19 12:36:14 localhost kernel: [41526.840509]  [<fe48658b>] 
serial_unthrottle+0x7b/0x80 [usbserial]
Mar 19 12:36:14 localhost kernel: [41526.840519]  [<c059cb49>] ? 
mutex_lock+0x19/0x40
Mar 19 12:36:14 localhost kernel: [41526.840527]  [<c03b448b>] 
tty_unthrottle+0x3b/0x50
Mar 19 12:36:14 localhost kernel: [41526.840534]  [<c03b253e>] 
reset_buffer_flags+0xbe/0xf0
Mar 19 12:36:14 localhost kernel: [41526.840541]  [<c03b258b>] 
n_tty_flush_buffer+0x1b/0x80
Mar 19 12:36:14 localhost kernel: [41526.840548]  [<c03b598c>] 
tty_ldisc_flush+0x2c/0x50
Mar 19 12:36:14 localhost kernel: [41526.840555]  [<c03b68a0>] 
tty_port_close_end+0x20/0xe0
Mar 19 12:36:14 localhost kernel: [41526.840564]  [<fe486b6a>] 
serial_close+0x6a/0xb0 [usbserial]
Mar 19 12:36:14 localhost kernel: [41526.840573]  [<c03adc43>] ? 
tty_fasync+0x43/0x140
Mar 19 12:36:14 localhost kernel: [41526.840581]  [<c03afef0>] 
tty_release_dev+0x120/0x500
Mar 19 12:36:14 localhost kernel: [41526.840590]  [<c015d0cf>] ? 
ptrace_stop+0xbf/0x160
Mar 19 12:36:14 localhost kernel: [41526.840597]  [<c015d209>] ? 
ptrace_notify+0x99/0xc0
Mar 19 12:36:14 localhost kernel: [41526.840606]  [<c03b02e7>] 
tty_release+0x17/0x30
Mar 19 12:36:14 localhost kernel: [41526.840614]  [<c0206c3f>] __fput+0xdf/0x1f0
Mar 19 12:36:14 localhost kernel: [41526.840622]  [<c0206d6d>] fput+0x1d/0x30
Mar 19 12:36:14 localhost kernel: [41526.840629]  [<c02032ac>] 
filp_close+0x4c/0x80
Mar 19 12:36:14 localhost kernel: [41526.840636]  [<c0203358>] 
sys_close+0x78/0xc0
Mar 19 12:36:14 localhost kernel: [41526.840643]  [<c01033ec>] 
syscall_call+0x7/0xb
Mar 19 12:36:14 localhost kernel: [41526.840649] ---[ end trace 
5d571f4d930148a5 ]---

-- 
WARNING: at /build/buildd/linux-2.6.32/drivers/usb/serial/usb-serial.c:440 
serial_unthrottle 0x7b/0x80 [usbserial]()
https://bugs.launchpad.net/bugs/533982
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