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tty: Fix pty master poll() after slave closes v2

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Commit f95499c303 ("n_tty: Don't wait for buffer work in read() loop")
introduces a race window where a pty master can be signalled that the pty
slave was closed before all the data that the slave wrote is delivered.
Commit f8747d4a46 ("tty: Fix pty master read() after slave closes") fixed the
problem in case of n_tty_read, but the problem still exists for n_tty_poll.
This can be seen by running 'for ((i=0; i<100;i++));do ./test.py ;done'
where test.py is:

import os, select, pty

(pid, pty_fd) = pty.fork()

if pid == 0:
   os.write(1, 'This string should be received by parent')
else:
   poller = select.epoll()
   poller.register( pty_fd, select.EPOLLIN )
   ready = poller.poll( 1 * 1000 )
   for fd, events in ready:
      if not events & select.EPOLLIN:
         print 'missed POLLIN event'
      else:
         print os.read(fd, 100)
   poller.close()

The string from the slave is missed several times.
This patch takes the same approach as the fix for read and special cases
this condition for poll.
Tested on 3.16.

Signed-off-by: Francesco Ruggeri <fruggeri@arista.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
This commit is contained in:
Francesco Ruggeri 2014-10-10 13:09:53 -07:00 committed by Greg Kroah-Hartman
parent 7e12e675c1
commit c4dc304677
1 changed files with 9 additions and 4 deletions
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13
drivers/tty/n_tty.c
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@ -2413,12 +2413,17 @@ static unsigned int n_tty_poll(struct tty_struct *tty, struct file *file,
poll_wait(file, &tty->read_wait, wait);
poll_wait(file, &tty->write_wait, wait);
if (input_available_p(tty, 1))
mask |= POLLIN | POLLRDNORM;
if (tty->packet && tty->link->ctrl_status)
mask |= POLLPRI | POLLIN | POLLRDNORM;
if (test_bit(TTY_OTHER_CLOSED, &tty->flags))
mask |= POLLHUP;
if (input_available_p(tty, 1))
mask |= POLLIN | POLLRDNORM;
else if (mask & POLLHUP) {
tty_flush_to_ldisc(tty);
if (input_available_p(tty, 1))
mask |= POLLIN | POLLRDNORM;
}
if (tty->packet && tty->link->ctrl_status)
mask |= POLLPRI | POLLIN | POLLRDNORM;
if (tty_hung_up_p(file))
mask |= POLLHUP;
if (!(mask & (POLLHUP | POLLIN | POLLRDNORM))) {