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Subject[BUG] mysterious tty deadlock
One such stuck process had the following backtrace:

#0 schedule_timeout (timeout=-150765944) at timer.c:864
#1 0xc01a28a3 in uart_wait_until_sent (tty=0xf7669000, timeout=2147483647)
at core.c:1320
#2 0xc01afca8 in tty_wait_until_sent (tty=0xf7669000, timeout=0)
at tty_ioctl.c:66
#3 0xc01b0049 in set_termios (tty=0xf7669000, arg=3221221720, opt=2)
at tty_ioctl.c:164
#4 0xc01b03dc in n_tty_ioctl (tty=0xf7669000, file=0xf716b8a0, cmd=21507,
arg=3221221720) at tty_ioctl.c:409
#5 0xc01ac3b0 in tty_ioctl (inode=0xf72b9cf4, file=0xf716b8a0, cmd=21507,
arg=3221221720) at tty_io.c:1798
#6 0xc0152cf6 in sys_ioctl (fd=0, cmd=21507, arg=3221221720) at ioctl.c:128
#7 0xc01078df in syscall_call () at process.c:982

This doesn't appear to be serial-specific. Another stuck process is:

(gdb) bt
#0 schedule_timeout (timeout=2147483647) at timer.c:836
#1 0xc01af23d in read_chan (tty=0xf73da000, file=0xf781f6a0,
buf=0xbffffd93 "", nr=1) at n_tty.c:1043
#2 0xc01aa4b6 in tty_read (file=0xf781f6a0, buf=0xbffffd93 "", count=1,
ppos=0xf781f6c0) at tty_io.c:677
#3 0xc0142e2c in vfs_read (file=0xf781f6a0, buf=0xbffffd93 "", count=1,
pos=0xf781f6c0) at read_write.c:193
#4 0xc0142ffe in sys_read (fd=0, buf=0xbffffd93 "", count=1)
at read_write.c:232
#5 0xc01078df in syscall_call () at process.c:982

It's actually possible to kick these by sending them signal-generating
characters, though the forward progress one can make this way is limited.

(1) type "ls &"
(2) This will not echo.
(3) type ^Z (^C doesn't work for some reason)
(4) "ls &" echoes
(5) no prompt appears
(6) type ^Z again
(7) the prompt doesn't appear
(8) type ^Z again
(9) the prompt appears

... this is a little oversimplified. Some pounding on the return keys is
usually also required. serial and non-serial behave identically here.

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