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    SubjectRe: [PATCH 1/1] TTY: fix PTY hangup vs close race
    On Sun, Feb 19, 2012 at 11:41 PM, Jiri Slaby <jirislaby@gmail.com> wrote:
    > On 02/19/2012 10:19 PM, Jiri Slaby wrote:
    >> Commit d3bda5298 (TTY: get rid of BTM around devpts_*) moved
    >> devpts_pty_kill out of BTM, but the BTM was not protecting only
    >> devpts_pty_kill, but also tty->link. Hence move the function back at
    >> this late stage until this gets resolved properly some time later.
    >>
    >> I was confused by tty_vhangup(tty->link) outside BTM. But inside of
    >> tty_vhangup, there is a check for tty == NULL. But we cannot add such
    >> a check here. We have to have the tty and free the devpts node...
    >>
    >> Signed-off-by: Jiri Slaby <jslaby@suse.cz>
    >> Reported-by: Sasha Levin <levinsasha928@gmail.com>
    >> ---
    >>
    >> Gee, I messed up Greg's address again...
    >>
    >>  drivers/tty/pty.c |    3 ++-
    >>  1 files changed, 2 insertions(+), 1 deletions(-)
    >>
    >> diff --git a/drivers/tty/pty.c b/drivers/tty/pty.c
    >> index fa1bd2e..95037aa 100644
    >> --- a/drivers/tty/pty.c
    >> +++ b/drivers/tty/pty.c
    >> @@ -54,8 +54,9 @@ static void pty_close(struct tty_struct *tty, struct file *filp)
    >>       wake_up_interruptible(&tty->link->write_wait);
    >>       if (tty->driver->subtype == PTY_TYPE_MASTER) {
    >>               set_bit(TTY_OTHER_CLOSED, &tty->flags);
    >> -             tty_unlock();
    >> +             /* BTM protects tty->link here */
    >>               devpts_pty_kill(tty->link);
    >> +             tty_unlock();
    >
    > I'm afraid this won't help. As this is based on an assumption that
    > tty->link is NULL [*] and that is not just true.
    >
    > Greg, please revert commit d3bda5298 completely.
    >
    > [*] Your dump reveals that the code fetches tty->driver_data (mov
    > 0x428(%rdi),%rbx) and traps at a fetch of inode->i_sbm because inode is
    > NULL (mov    0x28(%rbx),%rax).

    It actually looks even more complex than that. I reverted the patch
    above, but still got the error. A quick bisection pointed me to
    a50f724a432997321cabb6c9e665c28e34850f78.

    Looks like reverting both actually solves the problem. Reverting just
    one of them doesn't.
    --
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