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

Greg Kroah-Hartman
2014-11-06T02:33:13Z

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@linux.intel.com>
> Signed-off-by: Peter Hurley <peter@hurleysoftware.com>
> ---
> 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 */