linux-kernel - [PATCH 2/5] TTY: serial, move locking in uart

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Date:	Thu, 25 Aug 2011 15:12:05 +0200
From:	Jiri Slaby <jslaby@...e.cz>
To:	gregkh@...e.de
Cc:	Arnd Bergmann <arnd@...db.de>, alan@...ux.intel.com,
	linux-serial@...r.kernel.org, jirislaby@...il.com,
	linux-kernel@...r.kernel.org, Jiri Slaby <jslaby@...e.cz>
Subject: [PATCH 2/5] TTY: serial, move locking in uart_close

So now, when we handle CLOSING flag, there is no point to hold
port->mutex over the start of uart_close.

Yes, there are still several things to reason about:
* port->count etc is and always was protected by a spinlock
* ->stop_rx is protected by a spinlock. Otherwise it would
  race with interrupts.
* uart_wait_until_sent -- that one is already called without
  port->mutex from set_termios and tty_set_ldisc. Should anything
  be protected there, it would be tx_empty. And by a spinlock.
  8250 does this internally...

This step is needed to fix system stalls. To not create an AB-BA lock
dependency (see next patches).

Signed-off-by: Jiri Slaby <jslaby@...e.cz>
---
 drivers/tty/serial/serial_core.c |    2 +-
 1 files changed, 1 insertions(+), 1 deletions(-)

diff --git a/drivers/tty/serial/serial_core.c b/drivers/tty/serial/serial_core.c
index cd7a0bb..e1bff1b 100644
--- a/drivers/tty/serial/serial_core.c
+++ b/drivers/tty/serial/serial_core.c
@@ -1258,7 +1258,6 @@ static void uart_close(struct tty_struct *tty, struct file *filp)

 	pr_debug("uart_close(%d) called\n", uport->line);

-	mutex_lock(&port->mutex);
 	spin_lock_irqsave(&port->lock, flags);

 	if (tty_hung_up_p(filp)) {
@@ -1317,6 +1316,7 @@ static void uart_close(struct tty_struct *tty, struct file *filp)
 		uart_wait_until_sent(tty, uport->timeout);
 	}

+	mutex_lock(&port->mutex);
 	uart_shutdown(tty, state);
 	uart_flush_buffer(tty);

-- 
1.7.6

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