| From | Peter Hurley <> | Subject | [PATCH v3 17/23] tty: Document lock requirements to halt ldisc | Date | Tue, 5 Feb 2013 15:20:32 -0500 |
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Although the tty_lock() - aka legacy_mutex - protects concurrent close/re-open of the tty->ldisc, the lock cannot be held while waiting for the line discipline to halt. The ldisc_mutex acts as a surrogate lock to prevent the ldisc close while waiting for halt. Unfortunately, to reacquire the tty_lock() requires dropping the ldisc_mutex to maintain lock order. This accounts for the complicated sequence of:
/* holding tty_lock() for atomic tty operation */
mutex_lock(); tty_unlock(); tty_ldisc_halt(); mutex_unlock();
tty_lock(); mutex_lock();
/* check for tty state change which may have occurred while tty_lock() was dropped */
/* close old ldisc & open new ldisc */
tty_ldisc_enable();
mutex_unlock(); tty_unlock();
tty_ldisc_release() is exempt from locking the halt, as the only other possible concurrent user, tty_ldisc_hangup(), checks tty state to prevent this concurrent use.
Signed-off-by: Peter Hurley <peter@hurleysoftware.com> --- drivers/tty/tty_ldisc.c | 11 +++++++---- 1 file changed, 7 insertions(+), 4 deletions(-)
diff --git a/drivers/tty/tty_ldisc.c b/drivers/tty/tty_ldisc.c index c2837b2..9599b05 100644 --- a/drivers/tty/tty_ldisc.c +++ b/drivers/tty/tty_ldisc.c @@ -505,6 +505,9 @@ static void tty_ldisc_restore(struct tty_struct *tty, struct tty_ldisc *old) * * Wait for the line discipline to become idle. The discipline must * have been halted for this to guarantee it remains idle. + * + * Callers other than tty_ldisc_release() must hold ->ldisc_mutex and + * cannot be holding tty_lock() (aka legacy_mutex). */ static int tty_ldisc_wait_idle(struct tty_struct *tty, long timeout) { @@ -533,6 +536,9 @@ static int tty_ldisc_wait_idle(struct tty_struct *tty, long timeout) * released, which in turn, guarantees that no future buffer work * can be rescheduled. * + * Callers other than tty_ldisc_release() must hold ->ldisc_mutex and + * cannot be holding tty_lock() (aka legacy_mutex). + * * You need to do a 'flush_scheduled_work()' (outside the ldisc_mutex) * in order to make sure any currently executing ldisc work is also * flushed. @@ -713,10 +719,7 @@ int tty_set_ldisc(struct tty_struct *tty, int ldisc) retval = tty_ldisc_halt(tty, o_tty, &work, &o_work, 5 * HZ); - /* - * Wait for hangup to complete, if pending. - * We must drop the mutex here in case a hangup is also in process. - */ + /* Wait for hangup to complete, if pending. */ mutex_unlock(&tty->ldisc_mutex); -- 1.8.1.2
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