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

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Message-ID: <568B0348.2040601@ezchip.com>
Date:	Mon, 4 Jan 2016 18:42:00 -0500
From:	Chris Metcalf <cmetcalf@...hip.com>
To:	Steven Rostedt <rostedt@...dmis.org>
CC:	Gilad Ben Yossef <giladb@...hip.com>,
	Ingo Molnar <mingo@...nel.org>,
	Peter Zijlstra <peterz@...radead.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 v9 06/13] task_isolation: add debug boot flag

On 1/4/2016 5:52 PM, Steven Rostedt wrote:
> On Mon, 4 Jan 2016 14:34:44 -0500
> Chris Metcalf<cmetcalf@...hip.com>  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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