linux-kernel - Re: [PATCH] move tty_kref_put() outside of __cleanup

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Date:	Fri, 19 Mar 2010 11:08:37 +0000
From:	Alan Cox <alan@...rguk.ukuu.org.uk>
To:	Oleg Nesterov <oleg@...hat.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Alan Cox <alan@...ux.intel.com>,
	Roland McGrath <roland@...hat.com>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] move tty_kref_put() outside of __cleanup_signal()

> This task is new, nobody can see/use it before we drop the locks. NULL or not,
> its signal->tty is just uninitialized yet.

Fair enough.

> 
> > > --- 34-rc1/kernel/exit.c~7_TTY_PUT	2010-03-17 20:05:38.000000000 +0100
> > > +++ 34-rc1/kernel/exit.c	2010-03-18 22:46:41.000000000 +0100
> > > @@ -150,6 +150,7 @@ static void __exit_signal(struct task_st
> > >  		 * see account_group_exec_runtime().
> > >  		 */
> > >  		task_rq_unlock_wait(tsk);
> > > +		tty_kref_put(sig->tty);
> >
> > and a sig->tty = NULL assignment to trap races might not go amiss here
> > perhaps ?
> 
> Indeed ;)
> 
> The subsequent patches will do this, we need more changes anyway. Currently
> this doesn't matter because we are going to kfree() this memory unconditionally.
> But when we pin ->signal to task_struct, we should clear ->signal->tty before
> we drop ->siglock, then tty_kref_put().

Ok - yes the moment you start refcounting ->signal that changes (or do you
expect to free ->tty when you destruct the signals ?)
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