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tty: reorder ldisc locking
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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>
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Arnd Bergmann authored and Greg Kroah-Hartman committed Aug 10, 2010
1 parent be1bc28 commit 60af22d
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Showing 2 changed files with 27 additions and 6 deletions.
9 changes: 7 additions & 2 deletions drivers/char/selection.c
Original file line number Diff line number Diff line change
Expand Up @@ -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);
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24 changes: 20 additions & 4 deletions drivers/char/tty_ldisc.c
Original file line number Diff line number Diff line change
Expand Up @@ -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);

/*
Expand All @@ -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);

/*
Expand Down Expand Up @@ -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 */
Expand Down Expand Up @@ -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
Expand Down Expand Up @@ -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);
/*
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