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Subject[PATCH v4 20/32] tty: Separate release semantics of ldisc reference
Date
tty_ldisc_ref()/tty_ldisc_unref() have usage semantics
equivalent to down_read_trylock()/up_read(). Only
callers of tty_ldisc_put() are performing the additional
operations necessary for proper ldisc teardown, and then only
after ensuring no outstanding 'read lock' remains.

Thus, tty_ldisc_unref() should never be the last reference;
WARN if it is. Conversely, tty_ldisc_put() should never be
destructing if the use count != 1.

Signed-off-by: Peter Hurley <peter@hurleysoftware.com>
---
drivers/tty/tty_ldisc.c | 69 +++++++++++++++++++++++++------------------------
1 file changed, 35 insertions(+), 34 deletions(-)

diff --git a/drivers/tty/tty_ldisc.c b/drivers/tty/tty_ldisc.c
index 328ff5b..9362a10 100644
--- a/drivers/tty/tty_ldisc.c
+++ b/drivers/tty/tty_ldisc.c
@@ -49,37 +49,6 @@ static inline struct tty_ldisc *get_ldisc(struct tty_ldisc *ld)
return ld;
}

-static void put_ldisc(struct tty_ldisc *ld)
-{
- unsigned long flags;
-
- if (WARN_ON_ONCE(!ld))
- return;
-
- /*
- * If this is the last user, free the ldisc, and
- * release the ldisc ops.
- *
- * We really want an "atomic_dec_and_raw_lock_irqsave()",
- * but we don't have it, so this does it by hand.
- */
- raw_spin_lock_irqsave(&tty_ldisc_lock, flags);
- if (atomic_dec_and_test(&ld->users)) {
- struct tty_ldisc_ops *ldo = ld->ops;
-
- ldo->refcount--;
- module_put(ldo->owner);
- raw_spin_unlock_irqrestore(&tty_ldisc_lock, flags);
-
- kfree(ld);
- return;
- }
- raw_spin_unlock_irqrestore(&tty_ldisc_lock, flags);
-
- if (waitqueue_active(&ld->wq_idle))
- wake_up(&ld->wq_idle);
-}
-
/**
* tty_register_ldisc - install a line discipline
* @disc: ldisc number
@@ -363,13 +332,45 @@ EXPORT_SYMBOL_GPL(tty_ldisc_ref);

void tty_ldisc_deref(struct tty_ldisc *ld)
{
- put_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);
}
EXPORT_SYMBOL_GPL(tty_ldisc_deref);

+/**
+ * tty_ldisc_put - release the ldisc
+ *
+ * Complement of tty_ldisc_get().
+ */
static inline void tty_ldisc_put(struct tty_ldisc *ld)
{
- put_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);
+ kfree(ld);
+ raw_spin_unlock_irqrestore(&tty_ldisc_lock, flags);
}

/**
@@ -1001,7 +1002,7 @@ void tty_ldisc_init(struct tty_struct *tty)
*/
void tty_ldisc_deinit(struct tty_struct *tty)
{
- put_ldisc(tty->ldisc);
+ tty_ldisc_put(tty->ldisc);
tty_ldisc_assign(tty, NULL);
}

--
1.8.1.2


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