From: Dmitry Safonov dima@arista.com
[ Upstream commit c96cf923a98d1b094df9f0cf97a83e118817e31b ]
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 Signed-off-by: Sasha Levin sashal@kernel.org --- drivers/tty/n_hdlc.c | 4 ++-- drivers/tty/n_r3964.c | 2 +- drivers/tty/n_tty.c | 8 ++++---- drivers/tty/tty_ldisc.c | 7 +++++++ include/linux/tty.h | 7 +++++++ 5 files changed, 21 insertions(+), 7 deletions(-)
diff --git a/drivers/tty/n_hdlc.c b/drivers/tty/n_hdlc.c index 8d6253903f24f..0c12dec110bc4 100644 --- a/drivers/tty/n_hdlc.c +++ b/drivers/tty/n_hdlc.c @@ -614,7 +614,7 @@ static ssize_t n_hdlc_tty_read(struct tty_struct *tty, struct file *file, } /* no data */ - if (file->f_flags & O_NONBLOCK) { + if (tty_io_nonblock(tty, file)) { ret = -EAGAIN; break; } @@ -681,7 +681,7 @@ static ssize_t n_hdlc_tty_write(struct tty_struct *tty, struct file *file, if (tbuf) break;
- if (file->f_flags & O_NONBLOCK) { + if (tty_io_nonblock(tty, file)) { error = -EAGAIN; break; } diff --git a/drivers/tty/n_r3964.c b/drivers/tty/n_r3964.c index 345111467b850..ee0e07b4a13d0 100644 --- a/drivers/tty/n_r3964.c +++ b/drivers/tty/n_r3964.c @@ -1080,7 +1080,7 @@ static ssize_t r3964_read(struct tty_struct *tty, struct file *file, pMsg = remove_msg(pInfo, pClient); if (pMsg == NULL) { /* no messages available. */ - if (file->f_flags & O_NONBLOCK) { + if (tty_io_nonblock(tty, file)) { ret = -EAGAIN; goto unlock; } diff --git a/drivers/tty/n_tty.c b/drivers/tty/n_tty.c index 904fc9c37fdea..8214b0326b3a1 100644 --- a/drivers/tty/n_tty.c +++ b/drivers/tty/n_tty.c @@ -1704,7 +1704,7 @@ 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 @@ -1746,7 +1746,7 @@ 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;
@@ -2213,7 +2213,7 @@ 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; } @@ -2367,7 +2367,7 @@ 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; } diff --git a/drivers/tty/tty_ldisc.c b/drivers/tty/tty_ldisc.c index 3eb3f2a03bbb9..706faca834f27 100644 --- a/drivers/tty/tty_ldisc.c +++ b/drivers/tty/tty_ldisc.c @@ -348,6 +348,11 @@ int tty_ldisc_lock(struct tty_struct *tty, unsigned long timeout) { int ret;
+ /* Kindly asking blocked readers to release the read side */ + set_bit(TTY_LDISC_CHANGING, &tty->flags); + wake_up_interruptible_all(&tty->read_wait); + wake_up_interruptible_all(&tty->write_wait); + ret = __tty_ldisc_lock(tty, timeout); if (!ret) return -EBUSY; @@ -358,6 +363,8 @@ int tty_ldisc_lock(struct tty_struct *tty, unsigned long timeout) void tty_ldisc_unlock(struct tty_struct *tty) { clear_bit(TTY_LDISC_HALTED, &tty->flags); + /* Can be cleared here - ldisc_unlock will wake up writers firstly */ + clear_bit(TTY_LDISC_CHANGING, &tty->flags); __tty_ldisc_unlock(tty); }
diff --git a/include/linux/tty.h b/include/linux/tty.h index fe1b8623a3a1f..fe483976b1193 100644 --- a/include/linux/tty.h +++ b/include/linux/tty.h @@ -356,6 +356,7 @@ struct tty_file_private { #define TTY_NO_WRITE_SPLIT 17 /* Preserve write boundaries to driver */ #define TTY_HUPPED 18 /* Post driver->hangup() */ #define TTY_HUPPING 19 /* Hangup in progress */ +#define TTY_LDISC_CHANGING 20 /* Change pending - non-block IO */ #define TTY_LDISC_HALTED 22 /* Line discipline is halted */
/* Values for tty->flow_change */ @@ -373,6 +374,12 @@ static inline void tty_set_flow_change(struct tty_struct *tty, int val) smp_mb(); }
+static inline bool tty_io_nonblock(struct tty_struct *tty, struct file *file) +{ + return file->f_flags & O_NONBLOCK || + test_bit(TTY_LDISC_CHANGING, &tty->flags); +} + static inline bool tty_io_error(struct tty_struct *tty) { return test_bit(TTY_IO_ERROR, &tty->flags);