linux-kernel - [PATCH 04/24] tty: Fix ref counting for port krefs

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Message-Id: <1391997564-1805-5-git-send-email-peter@hurleysoftware.com>
Date:	Sun,  9 Feb 2014 20:59:04 -0500
From:	Peter Hurley <peter@...leysoftware.com>
To:	Marcel Holtmann <marcel@...tmann.org>
Cc:	Gustavo Padovan <gustavo@...ovan.org>,
	Johan Hedberg <johan.hedberg@...il.com>,
	Gianluca Anzolin <gianluca@...tospazio.it>,
	Alexander Holler <holler@...oftware.de>,
	Andrey Vihrov <andrey.vihrov@...il.com>,
	Sander Eikelenboom <linux@...elenboom.it>,
	linux-bluetooth@...r.kernel.org, linux-kernel@...r.kernel.org,
	Peter Hurley <peter@...leysoftware.com>,
	Jiri Slaby <jslaby@...e.cz>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Subject: [PATCH 04/24] tty: Fix ref counting for port krefs

The tty core supports two models for handling tty_port lifetimes;
the tty_port can use the kref supplied by tty_port (which will
automatically destruct the tty_port when the ref count drops to
zero) or it can destruct the tty_port manually.

For tty drivers that choose to use the port kref to manage the
tty_port lifetime, it is not possible to safely acquire a port
reference conditionally. If the last reference is released after
evaluating the condition but before acquiring the reference, a
bogus reference will be held while the tty_port destruction
commences.

Rather, only acquire a port reference if the ref count is non-zero
and allow the caller to distinguish if a reference has successfully
been acquired.

Cc: Jiri Slaby <jslaby@...e.cz>
Cc: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Signed-off-by: Peter Hurley <peter@...leysoftware.com>
---
 include/linux/tty.h | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/include/linux/tty.h b/include/linux/tty.h
index 90b4fdc..4781d7b 100644
--- a/include/linux/tty.h
+++ b/include/linux/tty.h
@@ -518,9 +518,9 @@ extern void tty_port_put(struct tty_port *port);

 static inline struct tty_port *tty_port_get(struct tty_port *port)
 {
-	if (port)
-		kref_get(&port->kref);
-	return port;
+	if (port && kref_get_unless_zero(&port->kref))
+		return port;
+	return NULL;
 }

 /* If the cts flow control is enabled, return true. */
-- 
1.8.1.2

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