[PATCH 29/31] tty: implement BTM as mutex instead of BKL

[Arnd Bergmann <arnd@arndb.de>]

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

This patch for now makes the new behaviour an optional
experimental feature that can be enabled for testing
purposes.

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>
---
 drivers/char/Makefile    |    1 +
 drivers/char/tty_mutex.c |   47 ++++++++++++++++++++++++++++++++++++++++++++++
 include/linux/tty.h      |    8 +++++++
 lib/Kconfig.debug        |   10 +++++++++
 4 files changed, 66 insertions(+), 0 deletions(-)
 create mode 100644 drivers/char/tty_mutex.c

diff --git a/drivers/char/Makefile b/drivers/char/Makefile
index 88d6eac..658bfbe 100644
--- a/drivers/char/Makefile
+++ b/drivers/char/Makefile
@@ -9,6 +9,7 @@ FONTMAPFILE = cp437.uni
 
 obj-y	 += mem.o random.o tty_io.o n_tty.o tty_ioctl.o tty_ldisc.o tty_buffer.o tty_port.o
 
+obj-$(CONFIG_TTY_MUTEX)		+= tty_mutex.o
 obj-$(CONFIG_LEGACY_PTYS)	+= pty.o
 obj-$(CONFIG_UNIX98_PTYS)	+= pty.o
 obj-y				+= misc.o
diff --git a/drivers/char/tty_mutex.c b/drivers/char/tty_mutex.c
new file mode 100644
index 0000000..1336975
--- /dev/null
+++ b/drivers/char/tty_mutex.c
@@ -0,0 +1,47 @@
+/*
+ * drivers/char/tty_lock.c
+ */
+#include <linux/tty.h>
+#include <linux/module.h>
+#include <linux/kallsyms.h>
+#include <linux/semaphore.h>
+#include <linux/sched.h>
+
+/*
+ * The 'big tty mutex'
+ *
+ * This mutex is taken and released by tty_lock() and tty_unlock(),
+ * replacing the older big kernel lock.
+ * It can no longer be taken recursively, and does not get
+ * released implicitly while sleeping.
+ *
+ * Don't use in new code.
+ */
+static DEFINE_MUTEX(big_tty_mutex);
+struct task_struct *__big_tty_mutex_owner;
+EXPORT_SYMBOL_GPL(__big_tty_mutex_owner);
+
+/*
+ * Getting the big tty mutex.
+ */
+void __lockfunc tty_lock(void)
+{
+	struct task_struct *task = current;
+
+	WARN_ON(__big_tty_mutex_owner == task);
+
+	mutex_lock(&big_tty_mutex);
+	__big_tty_mutex_owner = task;
+}
+EXPORT_SYMBOL(tty_lock);
+
+void __lockfunc tty_unlock(void)
+{
+	struct task_struct *task = current;
+
+	WARN_ON(__big_tty_mutex_owner != task);
+	__big_tty_mutex_owner = NULL;
+
+	mutex_unlock(&big_tty_mutex);
+}
+EXPORT_SYMBOL(tty_unlock);
diff --git a/include/linux/tty.h b/include/linux/tty.h
index 21a6de2..3e7e04a 100644
--- a/include/linux/tty.h
+++ b/include/linux/tty.h
@@ -574,7 +574,14 @@ 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 */
+#ifdef CONFIG_TTY_MUTEX
+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)
+#else
 static inline void tty_lock(void) __acquires(kernel_lock)
 {
 #ifdef CONFIG_LOCK_KERNEL
@@ -588,6 +595,7 @@ static inline void tty_unlock(void) __releases(kernel_lock)
 	unlock_kernel();
 }
 #define tty_locked()		(kernel_locked())
+#endif
 
 /*
  * wait_event_interruptible_tty -- wait for a condition with the tty lock held
diff --git a/lib/Kconfig.debug b/lib/Kconfig.debug
index e722e9d..fd1a2f1 100644
--- a/lib/Kconfig.debug
+++ b/lib/Kconfig.debug
@@ -428,6 +428,16 @@ config RT_MUTEX_TESTER
 	help
 	  This option enables a rt-mutex tester.
 
+config TTY_MUTEX
+	bool "Use a mutex instead of BKL for TTY locking"
+	depends on EXPERIMENTAL && SMP
+	help
+	  The TTY subsystem traditionally depends on the big kernel lock
+	  for serialization. Saying Y here replaces the BKL with the Big
+	  TTY Mutex (BTM).
+	  Building a kernel without the BKL is only possible with TTY_MUTEX
+	  enabled.
+
 config DEBUG_SPINLOCK
 	bool "Spinlock and rw-lock debugging: basic checks"
 	depends on DEBUG_KERNEL
-- 
1.7.0.4