On Fri, Jan 11, 2019 at 01:33:09PM -0500, Nicolas Pitre wrote: > I use Linux with the help of a braille display and the brltty daemon. It > turns out that the latest mainline kernel I can work with comes from > commit 231f8fd0cc. Anything past that and I lose the ability to read the > console barely a few seconds after the system has booted as brltty is > thrown a wrench and the braille display becomes completely inoperable. > > Things get somewhat better with commit c96cf923a9 as brltty is not > longer incapacitated, but some programs would randomly crash. Even the > very first login attempt won't work as I soon as I hit enter after my > user name the password prompt is skipped over, just like if the enter > key had been hit twice. Then lynx (the text web browser) would crash as > soon as I switch the virtual console with LeftAlt+FN. Mind you, this > isn't easy to perform bisection in those conditions. > > And the worst commit i.e. 83d817f410 is marked for stable! :-( > > Some interaction with brltty must be at play here otherwise such > breakage would never have survived up to the mainline kernel. > > As far as latest mainline is concerned, I managed to reproduce at least > one of the unwelcome behavior change (hoping that's all there is to this > issue) with a very simple test case so you won't have to learn braille > to debug this: > > # from any vt, make sure tty40 is allocated and empty > openvt -c 40 -f -- true > > # open it and wait on read() > cat /dev/tty40 > > # from a second vt, simply open tty40 again > true < /dev/tty40 > > # come back to the first vt and watch cat bailing out with EAGAIN. > > Please fix.
Please try the patch below, it was just queued up to my tree and should resolve the issue. If not, please let us know. thanks, greg k-h >From d3736d82e8169768218ee0ef68718875918091a0 Mon Sep 17 00:00:00 2001 From: Dmitry Safonov <d...@arista.com> Date: Wed, 9 Jan 2019 01:17:40 +0000 Subject: tty: Don't hold ldisc lock in tty_reopen() if ldisc present Try to get reference for ldisc during tty_reopen(). If ldisc present, we don't need to do tty_ldisc_reinit() and lock the write side for line discipline semaphore. Effectively, it optimizes fast-path for tty_reopen(), but more importantly it won't interrupt ongoing IO on the tty as no ldisc change is needed. Fixes user-visible issue when tty_reopen() interrupted login process for user with a long password, observed and reported by Lukas. Fixes: c96cf923a98d ("tty: Don't block on IO when ldisc change is pending") Fixes: 83d817f41070 ("tty: Hold tty_ldisc_lock() during tty_reopen()") Cc: Jiri Slaby <jsl...@suse.com> Reported-by: Lukas F. Hartmann <lu...@mntmn.com> Tested-by: Lukas F. Hartmann <lu...@mntmn.com> Cc: stable <sta...@vger.kernel.org> Signed-off-by: Dmitry Safonov <d...@arista.com> Signed-off-by: Greg Kroah-Hartman <gre...@linuxfoundation.org> --- drivers/tty/tty_io.c | 20 +++++++++++++------- 1 file changed, 13 insertions(+), 7 deletions(-) diff --git a/drivers/tty/tty_io.c b/drivers/tty/tty_io.c index bfe9ad85b362..23c6fd238422 100644 --- a/drivers/tty/tty_io.c +++ b/drivers/tty/tty_io.c @@ -1256,7 +1256,8 @@ static void tty_driver_remove_tty(struct tty_driver *driver, struct tty_struct * static int tty_reopen(struct tty_struct *tty) { struct tty_driver *driver = tty->driver; - int retval; + struct tty_ldisc *ld; + int retval = 0; if (driver->type == TTY_DRIVER_TYPE_PTY && driver->subtype == PTY_TYPE_MASTER) @@ -1268,13 +1269,18 @@ static int tty_reopen(struct tty_struct *tty) if (test_bit(TTY_EXCLUSIVE, &tty->flags) && !capable(CAP_SYS_ADMIN)) return -EBUSY; - retval = tty_ldisc_lock(tty, 5 * HZ); - if (retval) - return retval; + ld = tty_ldisc_ref_wait(tty); + if (ld) { + tty_ldisc_deref(ld); + } else { + retval = tty_ldisc_lock(tty, 5 * HZ); + if (retval) + return retval; - if (!tty->ldisc) - retval = tty_ldisc_reinit(tty, tty->termios.c_line); - tty_ldisc_unlock(tty); + if (!tty->ldisc) + retval = tty_ldisc_reinit(tty, tty->termios.c_line); + tty_ldisc_unlock(tty); + } if (retval == 0) tty->count++; -- 2.20.1