tty: implement BTM as mutex instead of BKL

The tty locking now follows the rules for mutexes, so
we can replace the BKL usage with a new subsystem
wide mutex.

Using a regular mutex here will change the behaviour
when blocked on the BTM from spinning to sleeping,
but that should not be visible to the user.

Using the mutex also means that all the BTM is now
covered by lockdep.

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>

1 files changed, 5 insertions, 13 deletions

diff --git a/include/linux/tty.h b/include/linux/tty.h
index 0fbafb0b69bf..1437da3ddc62 100644
--- a/include/linux/tty.h
+++ b/include/linux/tty.h
@@ -578,20 +578,12 @@ extern int vt_ioctl(struct tty_struct *tty, struct file *file,
 extern long vt_compat_ioctl(struct tty_struct *tty, struct file * file,
                      unsigned int cmd, unsigned long arg);
 
+/* tty_mutex.c */
 /* functions for preparation of BKL removal */
-static inline void tty_lock(void) __acquires(kernel_lock)
-{
-#ifdef CONFIG_LOCK_KERNEL
-        /* kernel_locked is 1 for !CONFIG_LOCK_KERNEL */
-        WARN_ON(kernel_locked());
-#endif
-        lock_kernel();
-}
-static inline void tty_unlock(void) __releases(kernel_lock)
-{
-        unlock_kernel();
-}
-#define tty_locked()            (kernel_locked())
+extern void __lockfunc tty_lock(void) __acquires(tty_lock);
+extern void __lockfunc tty_unlock(void) __releases(tty_lock);
+extern struct task_struct *__big_tty_mutex_owner;
+#define tty_locked()            (current == __big_tty_mutex_owner)
 
 /*
  * wait_event_interruptible_tty -- wait for a condition with the tty lock held