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Subject[PATCH 40/67] tty: reorder ldisc locking
Date
From: Arnd Bergmann <arnd@arndb.de>

We need to release the BTM in paste_selection() when
sleeping in tty_ldisc_ref_wait to avoid deadlocks
with tty_ldisc_enable.

In tty_set_ldisc, we now always grab the BTM before
taking the ldisc_mutex in order to avoid AB-BA
deadlocks between the two.

tty_ldisc_halt potentially blocks on a workqueue
function that takes the BTM, so we must release
the BTM before calling it.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Cc: Alan Cox <alan@lxorguk.ukuu.org.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>
---
drivers/char/selection.c | 9 +++++++--
drivers/char/tty_ldisc.c | 24 ++++++++++++++++++++----
2 files changed, 27 insertions(+), 6 deletions(-)
diff --git a/drivers/char/selection.c b/drivers/char/selection.c
index 85211a3..75889cd 100644
--- a/drivers/char/selection.c
+++ b/drivers/char/selection.c
@@ -319,8 +319,13 @@ int paste_selection(struct tty_struct *tty)
poke_blanked_console();
release_console_sem();

- ld = tty_ldisc_ref_wait(tty);
-
+ ld = tty_ldisc_ref(tty);
+ if (!ld) {
+ tty_unlock();
+ ld = tty_ldisc_ref_wait(tty);
+ tty_lock();
+ }
+
add_wait_queue(&vc->paste_wait, &wait);
while (sel_buffer && sel_buffer_lth > pasted) {
set_current_state(TASK_INTERRUPTIBLE);
diff --git a/drivers/char/tty_ldisc.c b/drivers/char/tty_ldisc.c
index 97681ff..0f49479 100644
--- a/drivers/char/tty_ldisc.c
+++ b/drivers/char/tty_ldisc.c
@@ -582,6 +582,7 @@ int tty_set_ldisc(struct tty_struct *tty, int ldisc)

tty_wait_until_sent(tty, 0);

+ tty_lock();
mutex_lock(&tty->ldisc_mutex);

/*
@@ -591,13 +592,13 @@ int tty_set_ldisc(struct tty_struct *tty, int ldisc)

while (test_bit(TTY_LDISC_CHANGING, &tty->flags)) {
mutex_unlock(&tty->ldisc_mutex);
+ tty_unlock();
wait_event(tty_ldisc_wait,
test_bit(TTY_LDISC_CHANGING, &tty->flags) == 0);
+ tty_lock();
mutex_lock(&tty->ldisc_mutex);
}

- tty_lock();
-
set_bit(TTY_LDISC_CHANGING, &tty->flags);

/*
@@ -634,8 +635,8 @@ int tty_set_ldisc(struct tty_struct *tty, int ldisc)

flush_scheduled_work();

- mutex_lock(&tty->ldisc_mutex);
tty_lock();
+ mutex_lock(&tty->ldisc_mutex);
if (test_bit(TTY_HUPPED, &tty->flags)) {
/* We were raced by the hangup method. It will have stomped
the ldisc data and closed the ldisc down */
@@ -782,7 +783,20 @@ void tty_ldisc_hangup(struct tty_struct *tty)
* Avoid racing set_ldisc or tty_ldisc_release
*/
mutex_lock(&tty->ldisc_mutex);
- tty_ldisc_halt(tty);
+
+ /*
+ * this is like tty_ldisc_halt, but we need to give up
+ * the BTM before calling cancel_delayed_work_sync,
+ * which may need to wait for another function taking the BTM
+ */
+ clear_bit(TTY_LDISC, &tty->flags);
+ tty_unlock();
+ cancel_delayed_work_sync(&tty->buf.work);
+ mutex_unlock(&tty->ldisc_mutex);
+
+ tty_lock();
+ mutex_lock(&tty->ldisc_mutex);
+
/* At this point we have a closed ldisc and we want to
reopen it. We could defer this to the next open but
it means auditing a lot of other paths so this is
@@ -853,8 +867,10 @@ void tty_ldisc_release(struct tty_struct *tty, struct tty_struct *o_tty)
* race with the set_ldisc code path.
*/

+ tty_unlock();
tty_ldisc_halt(tty);
flush_scheduled_work();
+ tty_lock();

mutex_lock(&tty->ldisc_mutex);
/*
--
1.7.2


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