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SubjectRe: possible locking bug in tty_open
On Sun, 2 May 2010 22:47:33 +0200
Arnd Bergmann <arnd@arndb.de> wrote:

> While playing some more with my TTY BKL patches, I stumbled over
> what looks like a bug in tty_open, introduced in e8c6210
> "tty: push the BKL down into the handlers a bit":
>
> After the "retry_open:" label, we first get the tty_mutex
> and then the BKL. However a the end of tty_open, we jump
> back to retry_open with the BKL still held. If we run into
> this case, the tty_open function will be left with the BKL
> still held.
>
> It may be impossible to actually trigger this bug, because
> the path is only taken if a tty driver open function returns
> -ERESTARTSYS without setting signal_pending().
>
> Arnd

I think all we need is probably this

tty: Fix unbalanced BKL handling in error path

Arnd noted:

After the "retry_open:" label, we first get the tty_mutex
and then the BKL. However a the end of tty_open, we jump
back to retry_open with the BKL still held. If we run into
this case, the tty_open function will be left with the BKL
still held.

Signed-off-by: Alan Cox <alan@linux.intel.com>

diff --git a/drivers/char/tty_io.c b/drivers/char/tty_io.c
index 6da962c..fe810a7 100644
--- a/drivers/char/tty_io.c
+++ b/drivers/char/tty_io.c
@@ -1875,6 +1875,7 @@ got_driver:
*/
if (filp->f_op == &hung_up_tty_fops)
filp->f_op = &tty_fops;
+ unlock_kernel();
goto retry_open;
}
unlock_kernel();

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