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DateSat, 12 Dec 2009 23:34:07 +0100 (CET)
FromThomas Gleixner <>
SubjectRe: [GIT PATCH] TTY patches for 2.6.33-git


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);
>


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