Re: [PATCH v9 06/13] task_isolation: add debug boot flag

[Chris Metcalf]

On 1/4/2016 5:52 PM, Steven Rostedt wrote:
> On Mon, 4 Jan 2016 14:34:44 -0500
> Chris Metcalf<cmetcalf@xxxxxxxxxx>  wrote:
>
>
> > >+#ifdef CONFIG_TASK_ISOLATION
> > >+void task_isolation_debug(int cpu)
> > >+{
> > >+	struct task_struct *p;
> > >+
> > >+	if (!task_isolation_possible(cpu))
> > >+		return;
> > >+
> > >+	rcu_read_lock();
> What's the rcu_read_lock() for? I don't see what is being protected by
> rcu here?

I'm not completely clear either, but this is the same idiom as is used throughout
kernel/sched/core.c when mapping from a pid or a cpu to a task_struct, since
obviously you could end up racing with the task_struct being removed after the
task dies.  My best understanding is that the rcu_read_lock() holds up the final
free of the structure so that we have time here to get another reference to it.

See for example sched_setaffinity() for a similar use of the idiom.

--
Chris Metcalf, EZChip Semiconductor
http://www.ezchip.com

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