[PATCH v4 27/32] tty: Replace ldisc locking with ldisc_sem

From: Peter Hurley
Date: Wed Feb 20 2013 - 15:18:33 EST

Line discipline locking was performed with a combination of
a mutex, a status bit, a count, and a waitqueue -- basically,
a rw semaphore.

Replace the existing combination with an ld_semaphore.

Fixes:
1) the 'reference acquire after ldisc locked' bug
2) the over-complicated halt mechanism
3) lock order wrt. tty_lock()
4) dropping locks while changing ldisc
5) previously unidentified deadlock while locking ldisc from
both linked ttys concurrently
6) previously unidentified recursive deadlocks

Adds much-needed lockdep diagnostics.

Signed-off-by: Peter Hurley <peter@xxxxxxxxxxxxxxxxxx>
---
drivers/tty/tty_buffer.c | 2 +-
drivers/tty/tty_io.c | 7 +-
drivers/tty/tty_ldisc.c | 324 ++++++----------------------------------------
include/linux/tty.h | 4 +-
include/linux/tty_ldisc.h | 3 +-
5 files changed, 48 insertions(+), 292 deletions(-)

diff --git a/drivers/tty/tty_buffer.c b/drivers/tty/tty_buffer.c
index bb11993..0dd35ce 100644
--- a/drivers/tty/tty_buffer.c
+++ b/drivers/tty/tty_buffer.c
@@ -429,7 +429,7 @@ static void flush_to_ldisc(struct work_struct *work)
return;

disc = tty_ldisc_ref(tty);
- if (disc == NULL) /* !TTY_LDISC */
+ if (disc == NULL)
return;

spin_lock_irqsave(&buf->lock, flags);
diff --git a/drivers/tty/tty_io.c b/drivers/tty/tty_io.c
index 2261c21..376080d 100644
--- a/drivers/tty/tty_io.c
+++ b/drivers/tty/tty_io.c
@@ -1328,8 +1328,7 @@ static int tty_reopen(struct tty_struct *tty)
struct tty_driver *driver = tty->driver;

if (test_bit(TTY_CLOSING, &tty->flags) ||
- test_bit(TTY_HUPPING, &tty->flags) ||
- test_bit(TTY_LDISC_CHANGING, &tty->flags))
+ test_bit(TTY_HUPPING, &tty->flags))
return -EIO;

if (driver->type == TTY_DRIVER_TYPE_PTY &&
@@ -1345,7 +1344,7 @@ static int tty_reopen(struct tty_struct *tty)
}
tty->count++;

- WARN_ON(!test_bit(TTY_LDISC, &tty->flags));
+ WARN_ON(!tty->ldisc);

return 0;
}
@@ -2955,7 +2954,7 @@ void initialize_tty_struct(struct tty_struct *tty,
tty->pgrp = NULL;
mutex_init(&tty->legacy_mutex);
mutex_init(&tty->termios_mutex);
- mutex_init(&tty->ldisc_mutex);
+ init_ldsem(&tty->ldisc_sem);
init_waitqueue_head(&tty->write_wait);
init_waitqueue_head(&tty->read_wait);
INIT_WORK(&tty->hangup_work, do_tty_hangup);
diff --git a/drivers/tty/tty_ldisc.c b/drivers/tty/tty_ldisc.c
index ae0287f..a150f95 100644
--- a/drivers/tty/tty_ldisc.c
+++ b/drivers/tty/tty_ldisc.c
@@ -45,7 +45,6 @@ enum {
*/

static DEFINE_RAW_SPINLOCK(tty_ldisc_lock);
-static DECLARE_WAIT_QUEUE_HEAD(tty_ldisc_wait);
/* Line disc dispatch table */
static struct tty_ldisc_ops *tty_ldiscs[NR_LDISCS];

@@ -153,7 +152,7 @@ static void put_ldops(struct tty_ldisc_ops *ldops)
* takes tty_ldisc_lock to guard against ldisc races
*/

-static struct tty_ldisc *tty_ldisc_get(int disc)
+static struct tty_ldisc *tty_ldisc_get(struct tty_struct *tty, int disc)
{
struct tty_ldisc *ld;
struct tty_ldisc_ops *ldops;
@@ -180,8 +179,7 @@ static struct tty_ldisc *tty_ldisc_get(int disc)
}

ld->ops = ldops;
- atomic_set(&ld->users, 1);
- init_waitqueue_head(&ld->wq_idle);
+ ld->tty = tty;

return ld;
}
@@ -193,20 +191,11 @@ static struct tty_ldisc *tty_ldisc_get(int disc)
*/
static inline void tty_ldisc_put(struct tty_ldisc *ld)
{
- unsigned long flags;
-
if (WARN_ON_ONCE(!ld))
return;

- raw_spin_lock_irqsave(&tty_ldisc_lock, flags);
-
- /* unreleased reader reference(s) will cause this WARN */
- WARN_ON(!atomic_dec_and_test(&ld->users));
-
- ld->ops->refcount--;
- module_put(ld->ops->owner);
+ put_ldops(ld->ops);
kfree(ld);
- raw_spin_unlock_irqrestore(&tty_ldisc_lock, flags);
}

static void *tty_ldiscs_seq_start(struct seq_file *m, loff_t *pos)
@@ -258,34 +247,6 @@ const struct file_operations tty_ldiscs_proc_fops = {
};

/**
- * tty_ldisc_try - internal helper
- * @tty: the tty
- *
- * Make a single attempt to grab and bump the refcount on
- * the tty ldisc. Return 0 on failure or 1 on success. This is
- * used to implement both the waiting and non waiting versions
- * of tty_ldisc_ref
- *
- * Locking: takes tty_ldisc_lock
- */
-
-static struct tty_ldisc *tty_ldisc_try(struct tty_struct *tty)
-{
- unsigned long flags;
- struct tty_ldisc *ld;
-
- /* FIXME: this allows reference acquire after TTY_LDISC is cleared */
- raw_spin_lock_irqsave(&tty_ldisc_lock, flags);
- ld = NULL;
- if (test_bit(TTY_LDISC, &tty->flags) && tty->ldisc) {
- ld = tty->ldisc;
- atomic_inc(&ld->users);
- }
- raw_spin_unlock_irqrestore(&tty_ldisc_lock, flags);
- return ld;
-}
-
-/**
* tty_ldisc_ref_wait - wait for the tty ldisc
* @tty: tty device
*
@@ -298,16 +259,15 @@ static struct tty_ldisc *tty_ldisc_try(struct tty_struct *tty)
* against a discipline change, such as an existing ldisc reference
* (which we check for)
*
- * Locking: call functions take tty_ldisc_lock
+ * Note: only callable from a file_operations routine (which
+ * guarantees tty->ldisc !- NULL when the lock is acquired).
*/

struct tty_ldisc *tty_ldisc_ref_wait(struct tty_struct *tty)
{
- struct tty_ldisc *ld;
-
- /* wait_event is a macro */
- wait_event(tty_ldisc_wait, (ld = tty_ldisc_try(tty)) != NULL);
- return ld;
+ ldsem_down_read(&tty->ldisc_sem, MAX_SCHEDULE_TIMEOUT);
+ WARN_ON(!tty->ldisc);
+ return tty->ldisc;
}
EXPORT_SYMBOL_GPL(tty_ldisc_ref_wait);

@@ -318,13 +278,16 @@ EXPORT_SYMBOL_GPL(tty_ldisc_ref_wait);
* Dereference the line discipline for the terminal and take a
* reference to it. If the line discipline is in flux then
* return NULL. Can be called from IRQ and timer functions.
- *
- * Locking: called functions take tty_ldisc_lock
*/

struct tty_ldisc *tty_ldisc_ref(struct tty_struct *tty)
{
- return tty_ldisc_try(tty);
+ if (ldsem_down_read_trylock(&tty->ldisc_sem)) {
+ if (!tty->ldisc)
+ ldsem_up_read(&tty->ldisc_sem);
+ return tty->ldisc;
+ }
+ return NULL;
}
EXPORT_SYMBOL_GPL(tty_ldisc_ref);

@@ -334,27 +297,11 @@ EXPORT_SYMBOL_GPL(tty_ldisc_ref);
*
* Undoes the effect of tty_ldisc_ref or tty_ldisc_ref_wait. May
* be called in IRQ context.
- *
- * Locking: takes tty_ldisc_lock
*/

void tty_ldisc_deref(struct tty_ldisc *ld)
{
- unsigned long flags;
-
- if (WARN_ON_ONCE(!ld))
- return;
-
- raw_spin_lock_irqsave(&tty_ldisc_lock, flags);
- /*
- * WARNs if one-too-many reader references were released
- * - the last reference must be released with tty_ldisc_put
- */
- WARN_ON(atomic_dec_and_test(&ld->users));
- raw_spin_unlock_irqrestore(&tty_ldisc_lock, flags);
-
- if (waitqueue_active(&ld->wq_idle))
- wake_up(&ld->wq_idle);
+ ldsem_up_read(&ld->tty->ldisc_sem);
}
EXPORT_SYMBOL_GPL(tty_ldisc_deref);

@@ -439,26 +386,6 @@ static void __lockfunc tty_ldisc_enable_pair(struct tty_struct *tty,


/**
- * tty_ldisc_enable - allow ldisc use
- * @tty: terminal to activate ldisc on
- *
- * Set the TTY_LDISC flag when the line discipline can be called
- * again. Do necessary wakeups for existing sleepers. Clear the LDISC
- * changing flag to indicate any ldisc change is now over.
- *
- * Note: nobody should set the TTY_LDISC bit except via this function.
- * Clearing directly is allowed.
- */
-
-static void tty_ldisc_enable(struct tty_struct *tty)
-{
- clear_bit(TTY_LDISC_HALTED, &tty->flags);
- set_bit(TTY_LDISC, &tty->flags);
- clear_bit(TTY_LDISC_CHANGING, &tty->flags);
- wake_up(&tty_ldisc_wait);
-}
-
-/**
* tty_ldisc_flush - flush line discipline queue
* @tty: tty
*
@@ -555,14 +482,14 @@ static void tty_ldisc_restore(struct tty_struct *tty, struct tty_ldisc *old)
int r;

/* There is an outstanding reference here so this is safe */
- old = tty_ldisc_get(old->ops->num);
+ old = tty_ldisc_get(tty, old->ops->num);
WARN_ON(IS_ERR(old));
tty->ldisc = old;
tty_set_termios_ldisc(tty, old->ops->num);
if (tty_ldisc_open(tty, old) < 0) {
tty_ldisc_put(old);
/* This driver is always present */
- new_ldisc = tty_ldisc_get(N_TTY);
+ new_ldisc = tty_ldisc_get(tty, N_TTY);
if (IS_ERR(new_ldisc))
panic("n_tty: get");
tty->ldisc = new_ldisc;
@@ -576,101 +503,6 @@ static void tty_ldisc_restore(struct tty_struct *tty, struct tty_ldisc *old)
}

/**
- * tty_ldisc_wait_idle - wait for the ldisc to become idle
- * @tty: tty to wait for
- * @timeout: for how long to wait at most
- *
- * Wait for the line discipline to become idle. The discipline must
- * have been halted for this to guarantee it remains idle.
- */
-static int tty_ldisc_wait_idle(struct tty_struct *tty, long timeout)
-{
- long ret;
- ret = wait_event_timeout(tty->ldisc->wq_idle,
- atomic_read(&tty->ldisc->users) == 1, timeout);
- return ret > 0 ? 0 : -EBUSY;
-}
-
-/**
- * tty_ldisc_halt - shut down the line discipline
- * @tty: tty device
- * @o_tty: paired pty device (can be NULL)
- * @timeout: # of jiffies to wait for ldisc refs to be released
- *
- * Shut down the line discipline and work queue for this tty device and
- * its paired pty (if exists). Clearing the TTY_LDISC flag ensures
- * no further references can be obtained, while waiting for existing
- * references to be released ensures no more data is fed to the ldisc.
- *
- * 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, struct tty_struct *o_tty,
- long timeout)
-{
- int retval;
-
- clear_bit(TTY_LDISC, &tty->flags);
- if (o_tty)
- clear_bit(TTY_LDISC, &o_tty->flags);
-
- retval = tty_ldisc_wait_idle(tty, timeout);
- if (!retval && o_tty)
- retval = tty_ldisc_wait_idle(o_tty, timeout);
- if (retval)
- return retval;
-
- set_bit(TTY_LDISC_HALTED, &tty->flags);
- if (o_tty)
- set_bit(TTY_LDISC_HALTED, &o_tty->flags);
-
- return 0;
-}
-
-/**
- * tty_ldisc_hangup_halt - halt the line discipline for hangup
- * @tty: tty being hung up
- *
- * Shut down the line discipline and work queue for the tty device
- * being hungup. Clear the TTY_LDISC flag to ensure no further
- * references can be obtained and wait for remaining references to be
- * released to ensure no more data is fed to this ldisc.
- * Caller must hold legacy and ->ldisc_mutex.
- *
- * NB: tty_set_ldisc() is prevented from changing the ldisc concurrently
- * with this function by checking the TTY_HUPPING flag.
- */
-static bool tty_ldisc_hangup_halt(struct tty_struct *tty)
-{
- char cur_n[TASK_COMM_LEN], tty_n[64];
- long timeout = 3 * HZ;
-
- clear_bit(TTY_LDISC, &tty->flags);
-
- if (tty->ldisc) { /* Not yet closed */
- tty_unlock(tty);
-
- while (tty_ldisc_wait_idle(tty, timeout) == -EBUSY) {
- timeout = MAX_SCHEDULE_TIMEOUT;
- printk_ratelimited(KERN_WARNING
- "%s: waiting (%s) for %s took too long, but we keep waiting...\n",
- __func__, get_task_comm(cur_n, current),
- tty_name(tty, tty_n));
- }
-
- set_bit(TTY_LDISC_HALTED, &tty->flags);
-
- /* must reacquire both locks and preserve lock order */
- mutex_unlock(&tty->ldisc_mutex);
- tty_lock(tty);
- mutex_lock(&tty->ldisc_mutex);
- }
- return !!tty->ldisc;
-}
-
-/**
* tty_set_ldisc - set line discipline
* @tty: the terminal to set
* @ldisc: the line discipline
@@ -679,103 +511,45 @@ static bool tty_ldisc_hangup_halt(struct tty_struct *tty)
* context. The ldisc change logic has to protect itself against any
* overlapping ldisc change (including on the other end of pty pairs),
* the close of one side of a tty/pty pair, and eventually hangup.
- *
- * Locking: takes tty_ldisc_lock, termios_mutex
*/

int tty_set_ldisc(struct tty_struct *tty, int ldisc)
{
int retval;
struct tty_ldisc *o_ldisc, *new_ldisc;
- struct tty_struct *o_tty;
+ struct tty_struct *o_tty = tty->link;

- new_ldisc = tty_ldisc_get(ldisc);
+ new_ldisc = tty_ldisc_get(tty, ldisc);
if (IS_ERR(new_ldisc))
return PTR_ERR(new_ldisc);

- tty_lock(tty);
- /*
- * We need to look at the tty locking here for pty/tty pairs
- * when both sides try to change in parallel.
- */
-
- o_tty = tty->link; /* o_tty is the pty side or NULL */
-
+ retval = tty_ldisc_lock_pair_timeout(tty, o_tty, 5 * HZ);
+ if (retval)
+ return retval;

/*
* Check the no-op case
*/

if (tty->ldisc->ops->num == ldisc) {
- tty_unlock(tty);
+ tty_ldisc_enable_pair(tty, o_tty);
tty_ldisc_put(new_ldisc);
return 0;
}

- mutex_lock(&tty->ldisc_mutex);
-
- /*
- * We could be midstream of another ldisc change which has
- * dropped the lock during processing. If so we need to wait.
- */
-
- while (test_bit(TTY_LDISC_CHANGING, &tty->flags)) {
- mutex_unlock(&tty->ldisc_mutex);
- tty_unlock(tty);
- wait_event(tty_ldisc_wait,
- test_bit(TTY_LDISC_CHANGING, &tty->flags) == 0);
- tty_lock(tty);
- mutex_lock(&tty->ldisc_mutex);
- }
-
- set_bit(TTY_LDISC_CHANGING, &tty->flags);
-
- /*
- * No more input please, we are switching. The new ldisc
- * will update this value in the ldisc open function
- */
-
+ /* FIXME: why 'shutoff' input if the ldisc is locked? */
tty->receive_room = 0;

o_ldisc = tty->ldisc;
-
- tty_unlock(tty);
- /*
- * Make sure we don't change while someone holds a
- * reference to the line discipline. The TTY_LDISC bit
- * prevents anyone taking a reference once it is clear.
- * We need the lock to avoid racing reference takers.
- *
- * We must clear the TTY_LDISC bit here to avoid a livelock
- * with a userspace app continually trying to use the tty in
- * parallel to the change and re-referencing the tty.
- */
-
- retval = tty_ldisc_halt(tty, o_tty, 5 * HZ);
-
- /*
- * Wait for hangup to complete, if pending.
- * We must drop the mutex here in case a hangup is also in process.
- */
-
- mutex_unlock(&tty->ldisc_mutex);
-
- flush_work(&tty->hangup_work);
-
tty_lock(tty);
- mutex_lock(&tty->ldisc_mutex);

- /* handle wait idle failure locked */
- if (retval) {
- tty_ldisc_put(new_ldisc);
- goto enable;
- }
+ /* FIXME: for testing only */
+ WARN_ON(test_bit(TTY_HUPPED, &tty->flags));

if (test_bit(TTY_HUPPING, &tty->flags)) {
/* We were raced by the hangup method. It will have stomped
the ldisc data and closed the ldisc down */
- clear_bit(TTY_LDISC_CHANGING, &tty->flags);
- mutex_unlock(&tty->ldisc_mutex);
+ tty_ldisc_enable_pair(tty, o_tty);
tty_ldisc_put(new_ldisc);
tty_unlock(tty);
return -EIO;
@@ -804,14 +578,10 @@ int tty_set_ldisc(struct tty_struct *tty, int ldisc)

tty_ldisc_put(o_ldisc);

-enable:
/*
* Allow ldisc referencing to occur again
*/
-
- tty_ldisc_enable(tty);
- if (o_tty)
- tty_ldisc_enable(o_tty);
+ tty_ldisc_enable_pair(tty, o_tty);

/* Restart the work queue in case no characters kick it off. Safe if
already running */
@@ -819,7 +589,6 @@ enable:
if (o_tty)
schedule_work(&o_tty->port->buf.work);

- mutex_unlock(&tty->ldisc_mutex);
tty_unlock(tty);
return retval;
}
@@ -852,7 +621,7 @@ static void tty_reset_termios(struct tty_struct *tty)

static int tty_ldisc_reinit(struct tty_struct *tty, int ldisc)
{
- struct tty_ldisc *ld = tty_ldisc_get(ldisc);
+ struct tty_ldisc *ld = tty_ldisc_get(tty, ldisc);

if (IS_ERR(ld))
return -1;
@@ -891,14 +660,8 @@ void tty_ldisc_hangup(struct tty_struct *tty)

tty_ldisc_debug(tty, "closing ldisc: %p\n", tty->ldisc);

- /*
- * FIXME! What are the locking issues here? This may me overdoing
- * things... This question is especially important now that we've
- * removed the irqlock.
- */
ld = tty_ldisc_ref(tty);
if (ld != NULL) {
- /* We may have no line discipline at this point */
if (ld->ops->flush_buffer)
ld->ops->flush_buffer(tty);
tty_driver_flush_buffer(tty);
@@ -909,21 +672,22 @@ void tty_ldisc_hangup(struct tty_struct *tty)
ld->ops->hangup(tty);
tty_ldisc_deref(ld);
}
- /*
- * FIXME: Once we trust the LDISC code better we can wait here for
- * ldisc completion and fix the driver call race
- */
+
wake_up_interruptible_poll(&tty->write_wait, POLLOUT);
wake_up_interruptible_poll(&tty->read_wait, POLLIN);
+
+ tty_unlock(tty);
+
/*
* Shutdown the current line discipline, and reset it to
* N_TTY if need be.
*
* Avoid racing set_ldisc or tty_ldisc_release
*/
- mutex_lock(&tty->ldisc_mutex);
+ tty_ldisc_lock_pair(tty, tty->link);
+ tty_lock(tty);

- if (tty_ldisc_hangup_halt(tty)) {
+ if (tty->ldisc) {

/* At this point we have a halted ldisc; we want to close it and
reopen a new ldisc. We could defer the reopen to the next
@@ -942,9 +706,8 @@ void tty_ldisc_hangup(struct tty_struct *tty)
BUG_ON(tty_ldisc_reinit(tty, N_TTY));
WARN_ON(tty_ldisc_open(tty, tty->ldisc));
}
- tty_ldisc_enable(tty);
}
- mutex_unlock(&tty->ldisc_mutex);
+ tty_ldisc_enable_pair(tty, tty->link);
if (reset)
tty_reset_termios(tty);

@@ -976,15 +739,12 @@ int tty_ldisc_setup(struct tty_struct *tty, struct tty_struct *o_tty)
tty_ldisc_close(tty, ld);
return retval;
}
- tty_ldisc_enable(o_tty);
}
- tty_ldisc_enable(tty);
return 0;
}

static void tty_ldisc_kill(struct tty_struct *tty)
{
- mutex_lock(&tty->ldisc_mutex);
/*
* Now kill off the ldisc
*/
@@ -995,7 +755,6 @@ static void tty_ldisc_kill(struct tty_struct *tty)

/* Ensure the next open requests the N_TTY ldisc */
tty_set_termios_ldisc(tty, N_TTY);
- mutex_unlock(&tty->ldisc_mutex);
}

/**
@@ -1017,15 +776,16 @@ void tty_ldisc_release(struct tty_struct *tty, struct tty_struct *o_tty)

tty_ldisc_debug(tty, "closing ldisc: %p\n", tty->ldisc);

- tty_ldisc_halt(tty, o_tty, MAX_SCHEDULE_TIMEOUT);
-
+ tty_ldisc_lock_pair(tty, o_tty);
tty_lock_pair(tty, o_tty);
- /* This will need doing differently if we need to lock */
+
tty_ldisc_kill(tty);
if (o_tty)
tty_ldisc_kill(o_tty);

tty_unlock_pair(tty, o_tty);
+ tty_ldisc_unlock_pair(tty, o_tty);
+
/* And the memory resources remaining (buffers, termios) will be
disposed of when the kref hits zero */

@@ -1042,7 +802,7 @@ void tty_ldisc_release(struct tty_struct *tty, struct tty_struct *o_tty)

void tty_ldisc_init(struct tty_struct *tty)
{
- struct tty_ldisc *ld = tty_ldisc_get(N_TTY);
+ struct tty_ldisc *ld = tty_ldisc_get(tty, N_TTY);
if (IS_ERR(ld))
panic("n_tty: init_tty");
tty->ldisc = ld;
diff --git a/include/linux/tty.h b/include/linux/tty.h
index bfa6fca..2c109a3 100644
--- a/include/linux/tty.h
+++ b/include/linux/tty.h
@@ -238,7 +238,7 @@ struct tty_struct {
int index;

/* Protects ldisc changes: Lock tty not pty */
- struct mutex ldisc_mutex;
+ struct ld_semaphore ldisc_sem;
struct tty_ldisc *ldisc;

struct mutex atomic_write_lock;
@@ -306,8 +306,6 @@ struct tty_file_private {
#define TTY_DO_WRITE_WAKEUP 5 /* Call write_wakeup after queuing new */
#define TTY_PUSH 6 /* n_tty private */
#define TTY_CLOSING 7 /* ->close() in progress */
-#define TTY_LDISC 9 /* Line discipline attached */
-#define TTY_LDISC_CHANGING 10 /* Line discipline changing */
#define TTY_LDISC_OPEN 11 /* Line discipline is open */
#define TTY_HW_COOK_OUT 14 /* Hardware can do output cooking */
#define TTY_HW_COOK_IN 15 /* Hardware can do input cooking */
diff --git a/include/linux/tty_ldisc.h b/include/linux/tty_ldisc.h
index bbefe71..a8c58c2 100644
--- a/include/linux/tty_ldisc.h
+++ b/include/linux/tty_ldisc.h
@@ -198,8 +198,7 @@ struct tty_ldisc_ops {

struct tty_ldisc {
struct tty_ldisc_ops *ops;
- atomic_t users;
- wait_queue_head_t wq_idle;
+ struct tty_struct *tty;
};

#define TTY_LDISC_MAGIC 0x5403
--
1.8.1.2

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