Subject: Re: [PATCH v10 07/12] task_isolation: add debug boot flag
To: Peter Zijlstra <peterz@infradead.org>
References: <1456949376-4910-1-git-send-email-cmetcalf@ezchip.com>
 <1456949376-4910-8-git-send-email-cmetcalf@ezchip.com>
 <20160302203748.GH6356@twins.programming.kicks-ass.net>
CC: Gilad Ben Yossef <giladb@ezchip.com>, Steven Rostedt <rostedt@goodmis.org>,
        Ingo Molnar <mingo@kernel.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Rik van Riel <riel@redhat.com>, Tejun Heo <tj@kernel.org>,
        Frederic Weisbecker <fweisbec@gmail.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>,
        Christoph Lameter <cl@linux.com>,
        Viresh Kumar <viresh.kumar@linaro.org>,
        Catalin Marinas <catalin.marinas@arm.com>,
        Will Deacon <will.deacon@arm.com>,
        Andy Lutomirski <luto@amacapital.net>, <linux-doc@vger.kernel.org>,
        <linux-kernel@vger.kernel.org>, Oleg Nesterov <oleg@redhat.com>
From: Chris Metcalf <cmetcalf@mellanox.com>
Message-ID: <56D7537B.5040804@mellanox.com>
Date: Wed, 2 Mar 2016 15:56:27 -0500
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On 3/2/2016 3:37 PM, Peter Zijlstra wrote:
> On Wed, Mar 02, 2016 at 03:09:31PM -0500, Chris Metcalf wrote:
>> +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);
> As I think Oleg keeps reminding me, this is not actually a safe thing to
> do.

So what's the right solution?  The fast path in task_isolation_debug_task basically
just uses the new "task_isolation_flags", and "pid" and "comm".  I would think those
would all have to be safe because of the get_task_struct().

The piece that might be problematic is the eventual call to send_sig_info() using the
task_struct pointer (called via task_isolation_debug_task -> task_isolation_interrupt).
Clearly this is safe at some level, since that's more or less what sys_kill() does and the
process could similarly evaporate half way through sending the signal.

Suggestions?  Thanks!

-- 
Chris Metcalf, Mellanox Technologies
http://www.mellanox.com