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Subject[PATCH v2 09/11] tty: Strengthen no-subsequent-use guarantee of tty_ldisc_halt()
Date
In preparation for destructing and freeing the tty, the line discipline
must first be brought to an inactive state before it can be destructed.
This line discipline shutdown must:
- disallow new users of the ldisc
- wait for existing ldisc users to finish
- only then, cancel/flush their pending/running work

Factor tty_ldisc_wait_idle() from tty_set_ldisc() and tty_ldisc_kill()
to ensure this shutdown order.

Failure to provide this guarantee can result in scheduled work
running after the tty has already been freed, as indicated in the
following log message:

[ 88.331234] WARNING: at /home/peter/src/kernels/next/drivers/tty/tty_buffer.c:435 flush_to_ldisc+0x194/0x1d0()
[ 88.334505] Hardware name: Bochs
[ 88.335618] tty is bad=-1
[ 88.335703] Modules linked in: netconsole configfs bnep rfcomm bluetooth ......
[ 88.345272] Pid: 39, comm: kworker/1:1 Tainted: G W 3.7.0-next-20121129+ttydebug-xeon #20121129+ttydebug
[ 88.347736] Call Trace:
[ 88.349024] [<ffffffff81058aff>] warn_slowpath_common+0x7f/0xc0
[ 88.350383] [<ffffffff81058bf6>] warn_slowpath_fmt+0x46/0x50
[ 88.351745] [<ffffffff81432bd4>] flush_to_ldisc+0x194/0x1d0
[ 88.353047] [<ffffffff816f7fe1>] ? _raw_spin_unlock_irq+0x21/0x50
[ 88.354190] [<ffffffff8108a809>] ? finish_task_switch+0x49/0xe0
[ 88.355436] [<ffffffff81077ad1>] process_one_work+0x121/0x490
[ 88.357674] [<ffffffff81432a40>] ? __tty_buffer_flush+0x90/0x90
[ 88.358954] [<ffffffff81078c84>] worker_thread+0x164/0x3e0
[ 88.360247] [<ffffffff81078b20>] ? manage_workers+0x120/0x120
[ 88.361282] [<ffffffff8107e230>] kthread+0xc0/0xd0
[ 88.362284] [<ffffffff816f0000>] ? cmos_do_probe+0x2eb/0x3bf
[ 88.363391] [<ffffffff8107e170>] ? flush_kthread_worker+0xb0/0xb0
[ 88.364797] [<ffffffff816fff6c>] ret_from_fork+0x7c/0xb0
[ 88.366087] [<ffffffff8107e170>] ? flush_kthread_worker+0xb0/0xb0
[ 88.367266] ---[ end trace 453a7c9f38fbfec0 ]---

Signed-off-by: Peter Hurley <peter@hurleysoftware.com>
---
drivers/tty/tty_ldisc.c | 35 +++++++++++++++++++++--------------
1 file changed, 21 insertions(+), 14 deletions(-)

diff --git a/drivers/tty/tty_ldisc.c b/drivers/tty/tty_ldisc.c
index c3dec37..9f4c7b0 100644
--- a/drivers/tty/tty_ldisc.c
+++ b/drivers/tty/tty_ldisc.c
@@ -528,24 +528,38 @@ static int tty_ldisc_wait_idle(struct tty_struct *tty, long timeout)
/**
* tty_ldisc_halt - shut down the line discipline
* @tty: tty device
+ * @pending: returns true if work was scheduled when cancelled
+ * (can be set to NULL)
+ * @timeout: # of jiffies to wait for ldisc refs to be released
*
* Shut down the line discipline and work queue for this tty device.
* The TTY_LDISC flag being cleared ensures no further references can
* be obtained while the delayed work queue halt ensures that no more
* data is fed to the ldisc.
*
+ * Furthermore, guarantee that existing ldisc references have been
+ * released, which in turn, guarantees that no future buffer work
+ * can be rescheduled.
+ *
* 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.
*/

-static int tty_ldisc_halt(struct tty_struct *tty)
+static int tty_ldisc_halt(struct tty_struct *tty, int *pending, long timeout)
{
- int scheduled;
+ int scheduled, retval;
+
clear_bit(TTY_LDISC, &tty->flags);
+ retval = tty_ldisc_wait_idle(tty, timeout);
+ if (retval)
+ return retval;
+
scheduled = cancel_work_sync(&tty->port->buf.work);
set_bit(TTY_LDISC_HALTED, &tty->flags);
- return scheduled;
+ if (pending)
+ *pending = scheduled;
+ return 0;
}

/**
@@ -687,9 +701,9 @@ int tty_set_ldisc(struct tty_struct *tty, int ldisc)
* parallel to the change and re-referencing the tty.
*/

- work = tty_ldisc_halt(tty);
+ retval = tty_ldisc_halt(tty, &work, 5 * HZ);
if (o_tty)
- o_work = tty_ldisc_halt(o_tty);
+ tty_ldisc_halt(o_tty, &o_work, 0);

/*
* Wait for ->hangup_work and ->buf.work handlers to terminate.
@@ -700,8 +714,6 @@ int tty_set_ldisc(struct tty_struct *tty, int ldisc)

tty_ldisc_flush_works(tty);

- retval = tty_ldisc_wait_idle(tty, 5 * HZ);
-
tty_lock(tty);
mutex_lock(&tty->ldisc_mutex);

@@ -920,11 +932,6 @@ int tty_ldisc_setup(struct tty_struct *tty, struct tty_struct *o_tty)

static void tty_ldisc_kill(struct tty_struct *tty)
{
- /* There cannot be users from userspace now. But there still might be
- * drivers holding a reference via tty_ldisc_ref. Do not steal them the
- * ldisc until they are done. */
- tty_ldisc_wait_idle(tty, MAX_SCHEDULE_TIMEOUT);
-
mutex_lock(&tty->ldisc_mutex);
/*
* Now kill off the ldisc
@@ -958,10 +965,10 @@ void tty_ldisc_release(struct tty_struct *tty, struct tty_struct *o_tty)
*/

tty_lock_pair(tty, o_tty);
- tty_ldisc_halt(tty);
+ tty_ldisc_halt(tty, NULL, MAX_SCHEDULE_TIMEOUT);
tty_ldisc_flush_works(tty);
if (o_tty) {
- tty_ldisc_halt(o_tty);
+ tty_ldisc_halt(o_tty, NULL, MAX_SCHEDULE_TIMEOUT);
tty_ldisc_flush_works(o_tty);
}

--
1.8.0.1


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