linux-kernel - Re: [PATCH] Update comment on find_task_by_pid

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Date:	Mon, 8 Feb 2010 14:21:01 +0100
From:	Oleg Nesterov <oleg@...hat.com>
To:	Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>
Cc:	tglx@...utronix.de, linux-kernel@...r.kernel.org,
	paulmck@...ux.vnet.ibm.com, linux-security-module@...r.kernel.org
Subject: Re: [PATCH] Update comment on find_task_by_pid_ns

On 02/08, Tetsuo Handa wrote:
>
> [PATCH] Update comment on find_task_by_pid_ns
>
> Caller of find_task_by_vpid() and find_task_by_pid_ns() needs to call
> rcu_read_lock() rather than read_lock(&tasklist_lock) because find_pid_ns()
> uses RCU primitives but spinlock does not prevent RCU callback if preemptive
> RCU ( CONFIG_TREE_PREEMPT_RCU ) is enabled.

I agree with the patch, but the changelog looks a bit confusing to me.
Perhaps this is just me, though.

tasklist does protect the task and its pid, it can't go away. The problem
is that find_pid_ns() itself is unsafe without rcu lock, it can race with
copy_process()->free_pid(any_pid).

IOW, if we change copy_process()

	--- kernel/fork.c
	+++ kernel/fork.c
	@@ -1304,8 +1304,11 @@ static struct task_struct *copy_process(
		return p;
	 
	 bad_fork_free_pid:
	-	if (pid != &init_struct_pid)
	+	if (pid != &init_struct_pid) {
	+		read_lock(&tasklist_lock);
			free_pid(pid);
	+		read_unlock(&tasklist_lock);
	+	}
	 bad_fork_cleanup_io:
		if (p->io_context)
			exit_io_context(p);

then find_task_by_pid_ns/etc could be used under tasklist safely even
with PREEMPT_RCU.

In any case, I think the patch is nice.

Oleg.

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