Re: [GIT PATCH] TTY patches for 2.6.33-git

[Thomas Gleixner <tglx@linutronix.de>]

On Sat, 12 Dec 2009, Alan Cox wrote:

> > The do_tty_hangup()->tty_fasync() path takes the locks in the
> > file_list_lock()->lock_kernel() direction whereas most other code takes
> > them in the other direction, which cannot be good.  But I'm not sure
> 
> Thats a bug - the BKL does want to be taken first (and will
> sleep/yield/drop)
> 
> > Have a trace.  I'm actually wondering if perhaps there's a missing
> > unlock_kernel() somewhere else, and the tty code is just the victim of
> > that.
> 
> It's introduced by the BKL shuffle. Try the following
> 
> 
> commit 1f61f07a985c7e8cfc20ad8fcced2f3d226bd0dc
> Author: Alan Cox <alan@linux.intel.com>
> Date:   Sat Dec 12 10:32:36 2009 +0000
> 
>     tty: Fix the AB-BA locking bug introduced in the BKL split
>     
>     The fasync path takes the BKL (it probably doesn't need to in fact) but
>     this causes lock inversions and deadlocks so we can't do that. Leave the
>     BKL over that bit for the moment.
>     
>     Identified by AKPM.
>     
>     Signed-off-by: Alan Cox <alan@linux.intel.com>

Acked-and-Tested-by: Thomas Gleixner <tglx@linutronix.de>

 
> diff --git a/drivers/char/tty_io.c b/drivers/char/tty_io.c
> index 684f0e0..f15df40 100644
> --- a/drivers/char/tty_io.c
> +++ b/drivers/char/tty_io.c
> @@ -516,7 +516,6 @@ static void do_tty_hangup(struct work_struct *work)
>  	/* inuse_filps is protected by the single kernel lock */
>  	lock_kernel();
>  	check_tty_count(tty, "do_tty_hangup");
> -	unlock_kernel();
>  
>  	file_list_lock();
>  	/* This breaks for file handles being sent over AF_UNIX sockets ? */
> @@ -531,7 +530,6 @@ static void do_tty_hangup(struct work_struct *work)
>  	}
>  	file_list_unlock();
>  
> -	lock_kernel();
>  	tty_ldisc_hangup(tty);
>  
>  	read_lock(&tasklist_lock);
>