This patch series addresses the causes of flush_to_ldisc accessing the tty after freeing. The most common cause stems from the n_tty_close() path spuriously scheduling buffer work, when the ldisc has already been halted. This is fixed in 'tty: Don't reschedule buffer work while closing' The other causes have a central theme: incorrect order-of-operations when halting a line discipline. In general, to prevent buffer work from being scheduled requires: 1. Disallowing further ldisc references 2. Waiting for all existing ldisc references to be released 3. Cancelling existing buffer work If the wait takes place _after_ cancellation, then new work can be scheduled by existing holder(s) of ldisc reference(s). That's bad. Halting the line discipline is performed when, - hanging up the tty (tty_ldisc_hangup()) - TIOCSETD ioctl (tty_set_ldisc()) - finally closing the tty (pair) (tty_ldisc_release()) Concurrent halts are governed by the following requirements: 1. tty_ldisc_release is not concurrent with the other two and so does not need lock or state protection during the ldiscs halt. 2. Accesses through tty->ldisc must be protected by the ldisc_mutex. The wait operation checks the user count (ldisc references) in tty->ldisc->users. 3. tty_set_ldisc() reschedules buffer work that was pending when the ldiscs were halted. That must be an atomic operation in conjunction with re-enabling the ldisc -- which necessitates locking concurrent halts (tty_ldisc_release is exempt here) 4. The legacy mutex cannot be held while waiting for ldisc reference(s) release -or- for cancelling buffer work. 5. Because of #4, the legacy mutex must be dropped prior to or during the halt. Which means reacquiring after the halt. But to preserve lock order the ldisc_mutex must be dropped and reacquired after reacquiring the legacy mutex. 6. Because of #5, the ldisc state may have changed while the ldisc mutex was dropped. Note: this series does not include the lock correction initially reported on by Sebastian Andrzej Siewior <bigeasy@xxxxxxxxxxxxx> here https://lkml.org/lkml/2012/11/21/267 . I commented on the latest version here https://lkml.org/lkml/2012/12/3/362 This series also does not include Jiri's debug patch here https://lkml.org/lkml/2012/11/2/278 for identifying which itty cleanup path is used. I think it may be helpful to include this in -next. Peter Hurley (9): tty: WARN if buffer work racing with tty free tty: Add diagnostic for halted line discipline tty: Don't reschedule buffer work while closing tty: Refactor wait for ldisc refs out of tty_ldisc_hangup() tty: Remove unnecessary re-test of ldisc ref count tty: Fix ldisc halt sequence on hangup tty: Strengthen no-subsequent-use guarantee of tty_ldisc_halt() tty: Remove unnecessary buffer work flush tty: Halt both ldiscs concurrently drivers/tty/n_tty.c | 8 ++- drivers/tty/tty_buffer.c | 3 + drivers/tty/tty_io.c | 2 + drivers/tty/tty_ldisc.c | 171 +++++++++++++++++++++++++++++------------------ include/linux/tty.h | 1 + 5 files changed, 119 insertions(+), 66 deletions(-) -- 1.8.0 -- To unsubscribe from this list: send the line "unsubscribe linux-serial" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
