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SubjectRe: [PATCH] tty: tty_mutex: fix lockdep warning in tty_lock_pair
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On Thu, 2012-05-17 at 11:28 -0700, Greg Kroah-Hartman wrote:
> > +static void __lockfunc tty_lock_nest_lock(struct tty_struct *tty,
> > + struct tty_struct *tty2)
>
> Duplicating tty_lock() just for this one issue seems wrong and prone to
> error, don't you think?
>
> > +{
> > + if (tty->magic != TTY_MAGIC) {
> > + printk(KERN_ERR "L Bad %p\n", tty);
> > + WARN_ON(1);
> > + return;
> > + }
> > + tty_kref_get(tty);
> > + mutex_lock_nest_lock(&tty->legacy_mutex, &tty2->legacy_mutex);

Yeah, its completely broken, even the lockdep annotation is the wrong
one.

Something like the (completely untested) below patch is the 'right' way.

---
drivers/tty/tty_mutex.c | 35 ++++++++++++++++++++---------------
1 file changed, 20 insertions(+), 15 deletions(-)
diff --git a/drivers/tty/tty_mutex.c b/drivers/tty/tty_mutex.c
index 69adc80..587330b 100644
--- a/drivers/tty/tty_mutex.c
+++ b/drivers/tty/tty_mutex.c
@@ -10,7 +10,7 @@
* Getting the big tty mutex.
*/

-void __lockfunc tty_lock(struct tty_struct *tty)
+static void __lockfunc tty_lock_nested(struct tty_struct *tty, int subclass)
{
if (tty->magic != TTY_MAGIC) {
printk(KERN_ERR "L Bad %p\n", tty);
@@ -18,7 +18,12 @@ void __lockfunc tty_lock(struct tty_struct *tty)
return;
}
tty_kref_get(tty);
- mutex_lock(&tty->legacy_mutex);
+ mutex_lock_nested(&tty->legacy_mutex, subclass);
+}
+
+void __lockfunc tty_lock(struct tty_struct *tty)
+{
+ tty_lock_nested(tty, 0);
}
EXPORT_SYMBOL(tty_lock);

@@ -38,25 +43,25 @@ EXPORT_SYMBOL(tty_unlock);
* Getting the big tty mutex for a pair of ttys with lock ordering
* On a non pty/tty pair tty2 can be NULL which is just fine.
*/
-void __lockfunc tty_lock_pair(struct tty_struct *tty,
- struct tty_struct *tty2)
+void __lockfunc tty_lock_pair(struct tty_struct *tty1, struct tty_struct *tty2)
{
- if (tty < tty2) {
- tty_lock(tty);
- tty_lock(tty2);
- } else {
- if (tty2 && tty2 != tty)
- tty_lock(tty2);
- tty_lock(tty);
+ if (!tty2 || tty1 == tty2) {
+ tty_lock(tty1);
+ return;
}
+
+ if (tty2 < tty1)
+ swap(tty1, tty2);
+
+ tty_lock(tty1);
+ tty_lock_nested(tty2, SINGLE_DEPTH_NESTING);
}
EXPORT_SYMBOL(tty_lock_pair);

-void __lockfunc tty_unlock_pair(struct tty_struct *tty,
- struct tty_struct *tty2)
+void __lockfunc tty_unlock_pair(struct tty_struct *tty1, struct tty_struct *tty2)
{
- tty_unlock(tty);
- if (tty2 && tty2 != tty)
+ tty_unlock(tty1);
+ if (tty2 && tty2 != tty1)
tty_unlock(tty2);
}
EXPORT_SYMBOL(tty_unlock_pair);


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