tty: remove TTY_MAGIC

According to Greg, in the context of magic numbers as defined in
magic-number.rst, "the tty layer should not need this and I'll gladly
take patches"

Acked-by: Jiri Slaby <[e-mail]>
Signed-off-by: Ahelenia Ziemiańska <[e-mail]>
Ref: https://lore.kernel.org/linux-doc/[e-mail]/
Link: https://lore.kernel.org/r/476d02[e-mail]
Signed-off-by: Greg Kroah-Hartman <[e-mail]>

Documentation/process/magic-number.rst

···
 STRIP_MAGIC           0x5303           strip                    ``drivers/net/strip.c``
 SIXPACK_MAGIC         0x5304           sixpack                  ``drivers/net/hamradio/6pack.h``
 AX25_MAGIC            0x5316           ax_disp                  ``drivers/net/mkiss.h``
-TTY_MAGIC             0x5401           tty_struct               ``include/linux/tty.h``
 MGSL_MAGIC            0x5401           mgsl_info                ``drivers/char/synclink.c``
 TTY_DRIVER_MAGIC      0x5402           tty_driver               ``include/linux/tty_driver.h``
 MGSLPC_MAGIC          0x5402           mgslpc_info              ``drivers/char/pcmcia/synclink_cs.c``

Documentation/translations/it_IT/process/magic-number.rst

···
 STRIP_MAGIC           0x5303           strip                    ``drivers/net/strip.c``
 SIXPACK_MAGIC         0x5304           sixpack                  ``drivers/net/hamradio/6pack.h``
 AX25_MAGIC            0x5316           ax_disp                  ``drivers/net/mkiss.h``
-TTY_MAGIC             0x5401           tty_struct               ``include/linux/tty.h``
 MGSL_MAGIC            0x5401           mgsl_info                ``drivers/char/synclink.c``
 TTY_DRIVER_MAGIC      0x5402           tty_driver               ``include/linux/tty_driver.h``
 MGSLPC_MAGIC          0x5402           mgslpc_info              ``drivers/char/pcmcia/synclink_cs.c``

Documentation/translations/zh_CN/process/magic-number.rst

···
 STRIP_MAGIC           0x5303           strip                    ``drivers/net/strip.c``
 SIXPACK_MAGIC         0x5304           sixpack                  ``drivers/net/hamradio/6pack.h``
 AX25_MAGIC            0x5316           ax_disp                  ``drivers/net/mkiss.h``
-TTY_MAGIC             0x5401           tty_struct               ``include/linux/tty.h``
 MGSL_MAGIC            0x5401           mgsl_info                ``drivers/char/synclink.c``
 TTY_DRIVER_MAGIC      0x5402           tty_driver               ``include/linux/tty_driver.h``
 MGSLPC_MAGIC          0x5402           mgslpc_info              ``drivers/char/pcmcia/synclink_cs.c``

Documentation/translations/zh_TW/process/magic-number.rst

···
 STRIP_MAGIC           0x5303           strip                    ``drivers/net/strip.c``
 SIXPACK_MAGIC         0x5304           sixpack                  ``drivers/net/hamradio/6pack.h``
 AX25_MAGIC            0x5316           ax_disp                  ``drivers/net/mkiss.h``
-TTY_MAGIC             0x5401           tty_struct               ``include/linux/tty.h``
 MGSL_MAGIC            0x5401           mgsl_info                ``drivers/char/synclink.c``
 TTY_DRIVER_MAGIC      0x5402           tty_driver               ``include/linux/tty_driver.h``
 MGSLPC_MAGIC          0x5402           mgslpc_info              ``drivers/char/pcmcia/synclink_cs.c``

drivers/tty/tty_io.c

···
 	tty_ldisc_deinit(tty);
 	put_device(tty->dev);
 	kvfree(tty->write_buf);
-	tty->magic = 0xDEADDEAD;
 	kfree(tty);
 }
 
···
 #ifdef TTY_PARANOIA_CHECK
 	if (!tty) {
 		pr_warn("(%d:%d): %s: NULL tty\n",
-			imajor(inode), iminor(inode), routine);
-		return 1;
-	}
-	if (tty->magic != TTY_MAGIC) {
-		pr_warn("(%d:%d): %s: bad magic number\n",
 			imajor(inode), iminor(inode), routine);
 		return 1;
 	}
···
 	if (tty->ops->cleanup)
 		tty->ops->cleanup(tty);
 
-	tty->magic = 0;
 	tty_driver_kref_put(driver);
 	module_put(owner);
 
···
 		return NULL;
 
 	kref_init(&tty->kref);
-	tty->magic = TTY_MAGIC;
 	if (tty_ldisc_init(tty)) {
 		kfree(tty);
 		return NULL;

drivers/tty/tty_mutex.c

···
 
 void tty_lock(struct tty_struct *tty)
 {
-	if (WARN(tty->magic != TTY_MAGIC, "L Bad %p\n", tty))
-		return;
 	tty_kref_get(tty);
 	mutex_lock(&tty->legacy_mutex);
 }
···
 {
 	int ret;
 
-	if (WARN(tty->magic != TTY_MAGIC, "L Bad %p\n", tty))
-		return -EIO;
 	tty_kref_get(tty);
 	ret = mutex_lock_interruptible(&tty->legacy_mutex);
 	if (ret)
···
 
 void tty_unlock(struct tty_struct *tty)
 {
-	if (WARN(tty->magic != TTY_MAGIC, "U Bad %p\n", tty))
-		return;
 	mutex_unlock(&tty->legacy_mutex);
 	tty_kref_put(tty);
 }

include/linux/tty.h

···
 /**
  * struct tty_struct - state associated with a tty while open
  *
- * @magic: magic value set early in @alloc_tty_struct to %TTY_MAGIC, for
- *	   debugging purposes
  * @kref: reference counting by tty_kref_get() and tty_kref_put(), reaching zero
  *	  frees the structure
  * @dev: class device or %NULL (e.g. ptys, serdev)
···
  * &struct tty_port.
  */
 struct tty_struct {
-	int	magic;
 	struct kref kref;
 	struct device *dev;
 	struct tty_driver *driver;
···
 	struct file *file;
 	struct list_head list;
 };
-
-/* tty magic number */
-#define TTY_MAGIC		0x5401
 
 /**
  * DOC: TTY Struct Flags