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tty: Don't block on IO when ldisc change is pending

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There might be situations where tty_ldisc_lock() has blocked, but there
is already IO on tty and it prevents line discipline changes.
It might theoretically turn into dead-lock.

Basically, provide more priority to pending tty_ldisc_lock() than to
servicing reads/writes over tty.

User-visible issue was reported by Mikulas where on pa-risc with
Debian 5 reboot took either 80 seconds, 3 minutes or 3:25 after proper
locking in tty_reopen().

Cc: Jiri Slaby <jslaby@suse.com>
Reported-by: Mikulas Patocka <mpatocka@redhat.com>
Signed-off-by: Dmitry Safonov <dima@arista.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
This commit is contained in:
Dmitry Safonov
2018-11-01 00:24:48 +00:00
committed by Greg Kroah-Hartman
parent 83d817f410
commit c96cf923a9
5 changed files with 21 additions and 7 deletions
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8
drivers/tty/n_tty.c
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@@ -1702,7 +1702,7 @@ n_tty_receive_buf_common(struct tty_struct *tty, const unsigned char *cp,
down_read(&tty->termios_rwsem);
while (1) {
do {
/*
* When PARMRK is set, each input char may take up to 3 chars
* in the read buf; reduce the buffer space avail by 3x
@@ -1744,7 +1744,7 @@ n_tty_receive_buf_common(struct tty_struct *tty, const unsigned char *cp,
fp += n;
count -= n;
rcvd += n;
}
} while (!test_bit(TTY_LDISC_CHANGING, &tty->flags));
tty->receive_room = room;
@@ -2211,7 +2211,7 @@ static ssize_t n_tty_read(struct tty_struct *tty, struct file *file,
break;
if (!timeout)
break;
if (file->f_flags & O_NONBLOCK) {
if (tty_io_nonblock(tty, file)) {
retval = -EAGAIN;
break;
}
@@ -2365,7 +2365,7 @@ static ssize_t n_tty_write(struct tty_struct *tty, struct file *file,
}
if (!nr)
break;
if (file->f_flags & O_NONBLOCK) {
if (tty_io_nonblock(tty, file)) {
retval = -EAGAIN;
break;
}