On 01/27/2016 02:26 AM, Richard Genoud wrote: > 2016-01-27 10:14 GMT+01:00 Richard Genoud <[email protected]>: >> 2016-01-26 18:13 GMT+01:00 Peter Hurley <[email protected]>: >>> Hi Richard, >>> >>> On 01/26/2016 08:19 AM, Richard Genoud wrote: >>>> [ sorry for the noise, I forgot to Cc the lkml ] >>>> >>>> Hi, >>>> I've found a case were calling >>>> stty -F /dev/ttyS1 clocal >>>> blocks forever. >>>> And I don't know if it's a very old bug or if it's meant to be like that. >>>> >>>> Here is how to reproduce the lock : >>>> NB: there's NO modem on ttyS1 >>>> stty -F /dev/ttyS1 clocal cread crtscts >>>> cat < /dev/ttyS1 >>>> >>>> #on another terminal : >>>> echo "dummy" > /dev/ttyS1 # This call doesn't block >>>> >>>> stty -F /dev/ttyS1 -crtscts # this blocks forever on ioctl(TCSETSW ) >>>> >>>> >>>> looking at tty_port_close_start(), it's pretty clear that nothing is >>>> flushed until the last user, so it explains why the "echo dummy" >>>> returns directly, despite the crtscts flags. >>>> And in tty_mode_ioctl(), there are the lines: >>>> case TCSETSW: >>>> return set_termios(real_tty, p, TERMIOS_WAIT | >>>> TERMIOS_OLD); >>>> That explain why the stty blocks. >>>> >>>> But this behavior seems really strange. >>>> ... Or it's meant to be like that ? >>> >>> Yeah, meant to be like that. >>> >>> When mgetty writes the login prompt but h/w flow control is enabled >>> and nothing's connected, the output is buffered. >>> >>> Since stty uses tcsetattr(TCSADRAIN), the attempt to turn off h/w flow >>> control blocks, waiting for output to empty. >> Damn ! I forgot about the TCSADRAIN/FLUSH/NOW flags >> >>> In this situation, stopping mgetty will allow the other process >>> to unblock and advance. >>> >>> Hmmm, I could add a -f,--force flag to stty so it uses tcsetattr(TCSANOW)... >> Yes, I think such a flag will make sense. > In fact, a similar flag exist in the very last version of coreutils (8.25) > http://git.savannah.gnu.org/cgit/coreutils.git/commit/?id=cb7d8b90a213c0186a8c8ba66da959e1f5930e78 > :) >
Thanks for saving me the time doing it myself. Regards, Peter Hurley
