Re: [PATCH] do_task_stat: don't take tty_mutex

[Peter Zijlstra <a.p.zijlstra@chello.nl>]

On Sun, 2006-10-22 at 19:57 +0400, Oleg Nesterov wrote:
> Depends on
> 	tty-signal-tty-locking.patch
> 
> ->signal->tty is protected by ->siglock, no need to take the global tty_mutex.
> 
> Signed-off-by: Oleg Nesterov <oleg@tv-sign.ru>

->siglock protects the value of ->signal->tty, not memory it points to;
however since destroying the tty also means clearing all ->signal->tty
references, which means taking all ->siglocks, just holding the
->siglock around this piece of code looks sufficient indeed.

Acked-by: Peter Zijlstra <a.p.zijlstra@chello.nl>

> --- rc2-mm2/fs/proc/array.c~	2006-10-22 19:28:17.000000000 +0400
> +++ rc2-mm2/fs/proc/array.c	2006-10-22 19:45:52.000000000 +0400
> @@ -346,20 +346,13 @@ static int do_task_stat(struct task_stru
>  	sigemptyset(&sigcatch);
>  	cutime = cstime = utime = stime = cputime_zero;
>  
> -	mutex_lock(&tty_mutex);
>  	rcu_read_lock();
>  	if (lock_task_sighand(task, &flags)) {
>  		struct signal_struct *sig = task->signal;
> -		struct tty_struct *tty = sig->tty;
>  
> -		if (tty) {
> -			/*
> -			 * sig->tty is not stable, but tty_mutex
> -			 * protects us from release_dev(tty)
> -			 */
> -			barrier();
> -			tty_pgrp = tty->pgrp;
> -			tty_nr = new_encode_dev(tty_devnum(tty));
> +		if (sig->tty) {
> +			tty_pgrp = sig->tty->pgrp;
> +			tty_nr = new_encode_dev(tty_devnum(sig->tty));
>  		}
>  
>  		num_threads = atomic_read(&sig->count);
> @@ -395,7 +388,6 @@ static int do_task_stat(struct task_stru
>  		unlock_task_sighand(task, &flags);
>  	}
>  	rcu_read_unlock();
> -	mutex_unlock(&tty_mutex);
>  
>  	if (!whole || num_threads<2)
>  		wchan = get_wchan(task);
>