Re: [PATCH -next v2 10/26] tty: Don't take tty_mutex for tty count changes

[Greg Kroah-Hartman]

On Wed, Nov 05, 2014 at 12:12:53PM -0500, Peter Hurley wrote:
> Holding tty_mutex is no longer required to serialize changes to
> the tty_count or to prevent concurrent opens of closing ttys;
> tty_lock() is sufficient.
> 
> Reviewed-by: Alan Cox <alan@xxxxxxxxxxxxxxx>
> Signed-off-by: Peter Hurley <peter@xxxxxxxxxxxxxxxxxx>
> ---
>  drivers/tty/tty_io.c | 6 ------
>  1 file changed, 6 deletions(-)
> 
> diff --git a/drivers/tty/tty_io.c b/drivers/tty/tty_io.c
> index ea8c6cae8d12..e59de81c39a9 100644
> --- a/drivers/tty/tty_io.c
> +++ b/drivers/tty/tty_io.c
> @@ -1804,10 +1804,6 @@ int tty_release(struct inode *inode, struct file *filp)
>  	 * each iteration we avoid any problems.
>  	 */
>  	while (1) {
> -		/* Guard against races with tty->count changes elsewhere and
> -		   opens on /dev/tty */
> -
> -		mutex_lock(&tty_mutex);
>  		tty_lock_pair(tty, o_tty);
>  		tty_closing = tty->count <= 1;
>  		o_tty_closing = o_tty &&
> @@ -1840,7 +1836,6 @@ int tty_release(struct inode *inode, struct file *filp)
>  		printk(KERN_WARNING "%s: %s: read/write wait queue active!\n",
>  				__func__, tty_name(tty, buf));
>  		tty_unlock_pair(tty, o_tty);
> -		mutex_unlock(&tty_mutex);
>  		schedule();
>  	}
>  

The code in my tree in this section of tty_release() looks a bit
different, so I had to hand-apply this patch.  I've included the version
I used below, please verify I didn't mess it up.

thanks,

greg k-h



diff --git a/drivers/tty/tty_io.c b/drivers/tty/tty_io.c
index ea8c6cae8d12..e59de81c39a9 100644
--- a/drivers/tty/tty_io.c
+++ b/drivers/tty/tty_io.c
@@ -1804,10 +1804,6 @@ int tty_release(struct inode *inode, struct file *filp)
 	 * each iteration we avoid any problems.
 	 */
 	while (1) {
-		/* Guard against races with tty->count changes elsewhere and
-		   opens on /dev/tty */
-
-		mutex_lock(&tty_mutex);
 		tty_lock_pair(tty, o_tty);
 		tty_closing = tty->count <= 1;
 		o_tty_closing = o_tty &&
@@ -1840,7 +1836,6 @@ int tty_release(struct inode *inode, struct file *filp)
 		printk(KERN_WARNING "%s: %s: read/write wait queue active!\n",
 				__func__, tty_name(tty, buf));
 		tty_unlock_pair(tty, o_tty);
-		mutex_unlock(&tty_mutex);
 		schedule();
 	}
 
@@ -1891,7 +1886,6 @@ int tty_release(struct inode *inode, struct file *filp)
 		read_unlock(&tasklist_lock);
 	}
 
-	mutex_unlock(&tty_mutex);
 	tty_unlock_pair(tty, o_tty);
 	/* At this point, the tty->count == 0 should ensure a dead tty
 	   cannot be re-opened by a racing opener */
--
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