linux-kernel - [PATCH] pty: Fix lock inversion

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Message-ID: <20120524145052.4698.17929.stgit@bob.linux.org.uk>
Date:	Thu, 24 May 2012 15:51:15 +0100
From:	Alan Cox <alan@...rguk.ukuu.org.uk>
To:	linux-kernel@...r.kernel.org, davej@...hat.com
Subject: [PATCH] pty: Fix lock inversion

From: Alan Cox <alan@...ux.intel.com>

The ptmx_open path takes the tty and devpts locks in the wrong order
because tty_init_dev locks and returns a locked tty. As far as I can tell
this is actually safe anyway because the tty being returned is new so
nobody can get a reference to lock it at this point.

However we don't even need the devpts lock at this point, it's only held as
a byproduct of the way the locks were pushe down.

Signed-off-by: Alan Cox <alan@...ux.intel.com>
---

 drivers/tty/pty.c |    2 --
 1 files changed, 0 insertions(+), 2 deletions(-)


diff --git a/drivers/tty/pty.c b/drivers/tty/pty.c
index 59af394..65c7c62 100644
--- a/drivers/tty/pty.c
+++ b/drivers/tty/pty.c
@@ -633,7 +633,6 @@ static int ptmx_open(struct inode *inode, struct file *filp)
 	mutex_unlock(&devpts_mutex);
 
 	mutex_lock(&tty_mutex);
-	mutex_lock(&devpts_mutex);
 	tty = tty_init_dev(ptm_driver, index);
 
 	if (IS_ERR(tty)) {
@@ -643,7 +642,6 @@ static int ptmx_open(struct inode *inode, struct file *filp)
 
 	/* The tty returned here is locked so we can safely
 	   drop the mutex */
-	mutex_unlock(&devpts_mutex);
 	mutex_unlock(&tty_mutex);
 
 	set_bit(TTY_PTY_LOCK, &tty->flags); /* LOCK THE SLAVE */

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