Re: [PATCH v12 07/13] task_isolation: add debug boot flag

[Peter Zijlstra]

On Wed, May 18, 2016 at 12:35:19PM -0400, Chris Metcalf wrote:
> On 5/18/2016 9:56 AM, Peter Zijlstra wrote:
> >On Tue, Apr 05, 2016 at 01:38:36PM -0400, Chris Metcalf wrote:
> >>+#ifdef CONFIG_TASK_ISOLATION
> >>+void task_isolation_debug(int cpu)
> >>+{
> >>+	struct task_struct *p;
> >>+
> >>+	if (!task_isolation_possible(cpu))
> >>+		return;
> >>+
> >>+	rcu_read_lock();
> >>+	p = cpu_curr(cpu);
> >>+	get_task_struct(p);
> >>+	rcu_read_unlock();
> >>+	task_isolation_debug_task(cpu, p);
> >>+	put_task_struct(p);
> >
> >This is still broken...
> 
> I don't know how or why, though. :-)  Can you give me a better idiom?
> This looks to my eye just like how it's done for something like
> sched_setaffinity() by one task on another task, and I would have
> assumed the risks there of the other task evaporating part way
> through would be the same as the risks here.

Because rcu_read_lock() does not stop the task pointed to by
cpu_curr(cpu) from disappearing on you entirely.

See also the discussion around:

lkml.kernel.org/r/20160518170218.GY3192@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx