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

Peter Zijlstra
2006-10-23T11:16:55Z

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);
>