linux-kernel - rfc, leader_pid

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Message-ID: <20080318153811.GA3488@tv-sign.ru>
Date:	Tue, 18 Mar 2008 18:38:11 +0300
From:	Oleg Nesterov <oleg@...sign.ru>
To:	"Eric W. Biederman" <ebiederm@...ssion.com>,
	Pavel Emelyanov <xemul@...nvz.org>
Cc:	linux-kernel@...r.kernel.org
Subject: rfc, leader_pid_type()

Eric, Pavel.

Without tasklist lock held, task_tgid/task_pgrp/task_session can return the
bogus NULL. Note that the last 2 can return NULL even if task == current.

What do you think if we add yet another helper?

	struct pid *leader_pid_type(struct task_struct *task, enum pid_type type)
	{
		struct pid *ret;
	retry:
		ret = task->group_leader->pids[type].pid;
		if (likely(ret != NULL) || !pid_alive(task))
			return ret;
		/*
		 * We hit the old leader in the middle of de_thread(),
		 * or setsid/setpgrp is in progress.
		 */
		cpu_relax();
		goto retry;
	}

Yes, we already have a lot helpers... The one potential user is
check_kill_permission(), but it can live without it.

What do you think, do you see other possible users? Say, do_task_stat() may
report sid = 0, but this is minor of course...

Oleg.	

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