Re: [PATCH 1/1] driver/tty: Fix a warning in check_tty_count

[Peter Hurley <peter@hurleysoftware.com>]

Hi Zhen-Hua,

On 07/14/2014 01:54 AM, Li, Zhen-Hua wrote:
> When there are to many open/close on a tty device in the same time,
> there may be a warning like:
>    Warning: dev (ttyS0) tty->count(4) != #fd's(3) in tty_release_dev
>
> That's because tty->count and files in tty->tty_files are not synchronized
> in time.
> So I add a lock to avoid this.

Every place you added the count_lock spinlock is already protected
by tty_lock(tty), so the count is out of sync for some other reason.

What kernel version are you having this problem on and what are the
circumstances?

Regards,
Peter Hurley


> Signed-off-by: Li, Zhen-Hua <zhen-hual@hp.com>
> ---
>   drivers/tty/tty_io.c | 13 +++++++++----
>   include/linux/tty.h  |  1 +
>   2 files changed, 10 insertions(+), 4 deletions(-)
>
> diff --git a/drivers/tty/tty_io.c b/drivers/tty/tty_io.c
> index 3411071..9283fc2 100644
> --- a/drivers/tty/tty_io.c
> +++ b/drivers/tty/tty_io.c
> @@ -297,6 +297,7 @@ static int check_tty_count(struct tty_struct *tty, const char *routine)
>   	struct list_head *p;
>   	int count = 0;
>
> +	spin_lock(&tty->count_lock);
>   	spin_lock(&tty_files_lock);
>   	list_for_each(p, &tty->tty_files) {
>   		count++;
> @@ -310,8 +311,10 @@ static int check_tty_count(struct tty_struct *tty, const char *routine)
>   		printk(KERN_WARNING "Warning: dev (%s) tty->count(%d) "
>   				    "!= #fd's(%d) in %s\n",
>   		       tty->name, tty->count, count, routine);
> +		spin_unlock(&tty->count_lock);
>   		return count;
>   	}
> +	spin_unlock(&tty->count_lock);
>   #endif
>   	return 0;
>   }
> @@ -1337,7 +1340,6 @@ int tty_standard_install(struct tty_driver *driver, struct tty_struct *tty)
>   		return ret;
>
>   	tty_driver_kref_get(driver);
> -	tty->count++;
>   	driver->ttys[tty->index] = tty;
>   	return 0;
>   }
> @@ -1408,7 +1410,6 @@ static int tty_reopen(struct tty_struct *tty)
>
>   		tty->link->count++;
>   	}
> -	tty->count++;
>
>   	WARN_ON(!tty->ldisc);
>
> @@ -1796,6 +1797,7 @@ int tty_release(struct inode *inode, struct file *filp)
>   	 * We must *not* drop the tty_mutex until we ensure that a further
>   	 * entry into tty_open can not pick up this tty.
>   	 */
> +	spin_lock(&tty->count_lock);
>   	if (pty_master) {
>   		if (--o_tty->count < 0) {
>   			printk(KERN_WARNING "%s: bad pty slave count (%d) for %s\n",
> @@ -1819,7 +1821,7 @@ int tty_release(struct inode *inode, struct file *filp)
>   	 *    something that needs to be handled for hangups.
>   	 */
>   	tty_del_file(filp);
> -
> +	spin_unlock(&tty->count_lock);
>   	/*
>   	 * Perform some housekeeping before deciding whether to return.
>   	 *
> @@ -2046,8 +2048,10 @@ retry_open:
>   		retval = PTR_ERR(tty);
>   		goto err_file;
>   	}
> -
> +	spin_lock(&tty->count_lock);
> +	tty->count++;
>   	tty_add_file(tty, filp);
> +	spin_unlock(&tty->count_lock);
>
>   	check_tty_count(tty, __func__);
>   	if (tty->driver->type == TTY_DRIVER_TYPE_PTY &&
> @@ -3031,6 +3035,7 @@ void initialize_tty_struct(struct tty_struct *tty,
>   	INIT_WORK(&tty->hangup_work, do_tty_hangup);
>   	mutex_init(&tty->atomic_write_lock);
>   	spin_lock_init(&tty->ctrl_lock);
> +	spin_lock_init(&tty->count_lock);
>   	INIT_LIST_HEAD(&tty->tty_files);
>   	INIT_WORK(&tty->SAK_work, do_SAK_work);
>
> diff --git a/include/linux/tty.h b/include/linux/tty.h
> index 1c3316a..2959300 100644
> --- a/include/linux/tty.h
> +++ b/include/linux/tty.h
> @@ -284,6 +284,7 @@ struct tty_struct {
>   	/* If the tty has a pending do_SAK, queue it here - akpm */
>   	struct work_struct SAK_work;
>   	struct tty_port *port;
> +	spinlock_t count_lock;
>   };
>
>   /* Each of a tty's open files has private_data pointing to tty_file_private */
>