linux-kernel - Re: [PATCH] rcu_read_lock/unlock protect find_task_by

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Date:	Mon, 8 Nov 2010 07:04:43 +0900
From:	Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>
To:	paulmck@...ux.vnet.ibm.com
Cc:	sergey.senozhatsky@...il.com, viro@...iv.linux.org.uk,
	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
	akpm@...ux-foundation.org, mingo@...e.hu
Subject: Re: [PATCH] rcu_read_lock/unlock protect find_task_by_vpid call

Hello.

Paul E. McKenney wrote:
> > Users missing rcu_read_lock() when calling find_task_by_vpid():
> > 
> >   check_clock() in kernel/posix-cpu-timers.c
> 
> 	This one has read_lock(&tasklist_lock).
> 
Excuse me. Holding tasklist_lock lock does not help.
We must call rcu_read_lock() explicitly.
That's why 9728e5d6 "kernel/pid.c: update comment on find_task_by_pid_ns" was made.

I think there are users who needlessly call read_lock(&tasklist_lock)
when they can use rcu_read_lock() instead.
But I don't know when to use read_lock(&tasklist_lock).

If read_lock(&tasklist_lock) is needed only when we want to access
the "struct task_struct" after rcu_read_unlock(), maybe it is cleaner to
use a helper like

struct task_struct *find_task_and_get(pid_t pid)
{
	struct task_struct *task;
	read_lock(&tasklist_lock);
	rcu_read_lock();
	task = find_task_by_vpid(pid);
	rcu_read_unlock();
	if (task)
		get_task_struct(task);
	read_unlock(&tasklist_lock);
	return task;
}

and hide tasklist_lock.

Regards.
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