tty: Add timed, writer-prioritized rw semaphore

The semantics of a rw semaphore are almost ideally suited
for tty line discipline lifetime management;  multiple active
threads obtain "references" (read locks) while performing i/o
to prevent the loss or change of the current line discipline
(write lock).

Unfortunately, the existing rw_semaphore is ill-suited in other
ways;
1) TIOCSETD ioctl (change line discipline) expects to return an
   error if the line discipline cannot be exclusively locked within
   5 secs. Lock wait timeouts are not supported by rwsem.
2) A tty hangup is expected to halt and scrap pending i/o, so
   exclusive locking must be prioritized.
   Writer priority is not supported by rwsem.

Add ld_semaphore which implements these requirements in a
semantically similar way to rw_semaphore.

Writer priority is handled by separate wait lists for readers and
writers. Pending write waits are priortized before existing read
waits and prevent further read locks.

Wait timeouts are trivially added, but obviously change the lock
semantics as lock attempts can fail (but only due to timeout).

This implementation incorporates the write-lock stealing work of
Michel Lespinasse <walken@google.com>.

Cc: Michel Lespinasse <walken@google.com>
Signed-off-by: Peter Hurley <peter@hurleysoftware.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/Makefile

 obj-$(CONFIG_TTY)		+= tty_io.o n_tty.o tty_ioctl.o tty_ldisc.o \
-				   tty_buffer.o tty_port.o tty_mutex.o
+				   tty_buffer.o tty_port.o tty_mutex.o tty_ldsem.o
 obj-$(CONFIG_LEGACY_PTYS)	+= pty.o
 obj-$(CONFIG_UNIX98_PTYS)	+= pty.o
 obj-$(CONFIG_AUDIT)		+= tty_audit.o

include/linux/tty_ldisc.h

@@ -110,6 +110,52 @@
 #include <linux/wait.h>
 #include <linux/wait.h>
 
+
+/*
+ * the semaphore definition
+ */
+struct ld_semaphore {
+	long			count;
+	raw_spinlock_t		wait_lock;
+	unsigned int		wait_readers;
+	struct list_head	read_wait;
+	struct list_head	write_wait;
+#ifdef CONFIG_DEBUG_LOCK_ALLOC
+	struct lockdep_map	dep_map;
+#endif
+};
+
+extern void __init_ldsem(struct ld_semaphore *sem, const char *name,
+			 struct lock_class_key *key);
+
+#define init_ldsem(sem)						\
+do {								\
+	static struct lock_class_key __key;			\
+								\
+	__init_ldsem((sem), #sem, &__key);			\
+} while (0)
+
+
+extern int ldsem_down_read(struct ld_semaphore *sem, long timeout);
+extern int ldsem_down_read_trylock(struct ld_semaphore *sem);
+extern int ldsem_down_write(struct ld_semaphore *sem, long timeout);
+extern int ldsem_down_write_trylock(struct ld_semaphore *sem);
+extern void ldsem_up_read(struct ld_semaphore *sem);
+extern void ldsem_up_write(struct ld_semaphore *sem);
+
+#ifdef CONFIG_DEBUG_LOCK_ALLOC
+extern int ldsem_down_read_nested(struct ld_semaphore *sem, int subclass,
+				  long timeout);
+extern int ldsem_down_write_nested(struct ld_semaphore *sem, int subclass,
+				   long timeout);
+#else
+# define ldsem_down_read_nested(sem, subclass, timeout)		\
+		ldsem_down_read(sem, timeout)
+# define ldsem_down_write_nested(sem, subclass, timeout)	\
+		ldsem_down_write(sem, timeout)
+#endif
+
+
 struct tty_ldisc_ops {
 	int	magic;
 	char	*name;