tty: cumulate and document tty_struct::flow* members

Group the flow flags under a single struct called flow. The new struct
contains 'stopped' and 'tco_stopped' bools which used to be bits in a
bitfield. The struct also contains the lock protecting them to
potentially share the same cache line.

Note that commit c545b66c6922b (tty: Serialize tcflow() with other tty
flow control changes) added a padding to the original bitfield. It was
for the bitfield to occupy a whole 64b word to avoid interferring stores
on Alpha (cannot we evaporate this arch with weird implications to C
code yet?). But it doesn't work as expected as the padding
(tty_struct::unused) is aligned to a 8B boundary too and occupies some
bytes from the next word.

So make it reliable by:
1) setting __aligned of the struct -- that aligns the start, and
2) making 'unsigned long unused[0]' as the last member of the struct --
   pads the end.

This is also the perfect time to start the documentation of tty_struct
where all this lives. So we start by documenting what these bools
actually serve for. And why we do all the alignment dances. Only the few
up-to-date information from the Theodore's comment made it into this new
Kerneldoc comment.

Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Cc: "David S. Miller" <davem@davemloft.net>
Cc: Jakub Kicinski <kuba@kernel.org>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Ulf Hansson <ulf.hansson@linaro.org>
Cc: Heiko Carstens <hca@linux.ibm.com>
Cc: Vasily Gorbik <gor@linux.ibm.com>
Cc: Christian Borntraeger <borntraeger@de.ibm.com>
Cc: Shawn Guo <shawnguo@kernel.org>
Cc: Sascha Hauer <s.hauer@pengutronix.de>
Cc: Vineet Gupta <vgupta@synopsys.com>
Cc: "Maciej W. Rozycki" <macro@orcam.me.uk>
Link: https://lore.kernel.org/r/20210505091928.22010-13-jslaby@suse.cz
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

1 files changed, 12 insertions, 12 deletions

diff --git a/drivers/tty/tty_io.c b/drivers/tty/tty_io.c
index 5b5e99604989..9734be2eb00e 100644
--- a/drivers/tty/tty_io.c
+++ b/drivers/tty/tty_io.c
@@ -778,14 +778,14 @@ EXPORT_SYMBOL(tty_hung_up_p);
  *	but not always.
  *
  *	Locking:
- *		flow_lock
+ *		flow.lock
  */
 
 void __stop_tty(struct tty_struct *tty)
 {
-	if (tty->stopped)
+	if (tty->flow.stopped)
 		return;
-	tty->stopped = 1;
+	tty->flow.stopped = true;
 	if (tty->ops->stop)
 		tty->ops->stop(tty);
 }
@@ -794,9 +794,9 @@ void stop_tty(struct tty_struct *tty)
 {
 	unsigned long flags;
 
-	spin_lock_irqsave(&tty->flow_lock, flags);
+	spin_lock_irqsave(&tty->flow.lock, flags);
 	__stop_tty(tty);
-	spin_unlock_irqrestore(&tty->flow_lock, flags);
+	spin_unlock_irqrestore(&tty->flow.lock, flags);
 }
 EXPORT_SYMBOL(stop_tty);
 
@@ -809,14 +809,14 @@ EXPORT_SYMBOL(stop_tty);
  *	start method is invoked and the line discipline woken.
  *
  *	Locking:
- *		flow_lock
+ *		flow.lock
  */
 
 void __start_tty(struct tty_struct *tty)
 {
-	if (!tty->stopped || tty->flow_stopped)
+	if (!tty->flow.stopped || tty->flow.tco_stopped)
 		return;
-	tty->stopped = 0;
+	tty->flow.stopped = false;
 	if (tty->ops->start)
 		tty->ops->start(tty);
 	tty_wakeup(tty);
@@ -826,9 +826,9 @@ void start_tty(struct tty_struct *tty)
 {
 	unsigned long flags;
 
-	spin_lock_irqsave(&tty->flow_lock, flags);
+	spin_lock_irqsave(&tty->flow.lock, flags);
 	__start_tty(tty);
-	spin_unlock_irqrestore(&tty->flow_lock, flags);
+	spin_unlock_irqrestore(&tty->flow.lock, flags);
 }
 EXPORT_SYMBOL(start_tty);
 
@@ -1172,7 +1172,7 @@ ssize_t redirected_tty_write(struct kiocb *iocb, struct iov_iter *iter)
 
 int tty_send_xchar(struct tty_struct *tty, char ch)
 {
-	int	was_stopped = tty->stopped;
+	bool was_stopped = tty->flow.stopped;
 
 	if (tty->ops->send_xchar) {
 		down_read(&tty->termios_rwsem);
@@ -3141,7 +3141,7 @@ struct tty_struct *alloc_tty_struct(struct tty_driver *driver, int idx)
 	INIT_WORK(&tty->hangup_work, do_tty_hangup);
 	mutex_init(&tty->atomic_write_lock);
 	spin_lock_init(&tty->ctrl_lock);
-	spin_lock_init(&tty->flow_lock);
+	spin_lock_init(&tty->flow.lock);
 	spin_lock_init(&tty->files_lock);
 	INIT_LIST_HEAD(&tty->tty_files);
 	INIT_WORK(&tty->SAK_work, do_SAK_work);