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

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Message-ID: <20160518170647.GL3193@twins.programming.kicks-ass.net>
Date:	Wed, 18 May 2016 19:06:47 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Chris Metcalf <cmetcalf@...lanox.com>
Cc:	Gilad Ben Yossef <giladb@...hip.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Ingo Molnar <mingo@...nel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Rik van Riel <riel@...hat.com>, Tejun Heo <tj@...nel.org>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Christoph Lameter <cl@...ux.com>,
	Viresh Kumar <viresh.kumar@...aro.org>,
	Catalin Marinas <catalin.marinas@....com>,
	Will Deacon <will.deacon@....com>,
	Andy Lutomirski <luto@...capital.net>,
	linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v12 07/13] task_isolation: add debug boot flag

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@...ns.programming.kicks-ass.net