linux-kernel - Re: [RFC][PATCH 2/5] cpu-hotplug: Allow get_online

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Date:   Sat, 13 May 2017 00:15:03 +0200 (CEST)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Steven Rostedt <rostedt@...dmis.org>
cc:     linux-kernel@...r.kernel.org,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...nel.org>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
        Masami Hiramatsu <mhiramat@...nel.org>
Subject: Re: [RFC][PATCH 2/5] cpu-hotplug: Allow get_online_cpus() to nest

On Fri, 12 May 2017, Steven Rostedt wrote:
>  void get_online_cpus(void)
>  {
> +#ifdef CONFIG_LOCKDEP
> +	if (current->goc_depth++)
> +		return;

This must be unconditional and not depend on lockdep. The percpu rwsem is
going to deadlock silently otherwise when a writer is waiting ....

Thanks,

	tglx