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

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Message-ID: <20170512202316.49df3548@gandalf.local.home>
Date:   Fri, 12 May 2017 20:23:16 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Thomas Gleixner <tglx@...utronix.de>
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 Sat, 13 May 2017 00:15:03 +0200 (CEST)
Thomas Gleixner <tglx@...utronix.de> wrote:

> 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 ....
> 

After I sent the updated changes to Paul, I realized this. But didn't
update it as I turned my attention to seeing if we can still get it
done without it.

But I'm not convinced that we can yet. I'll reply to Peter's email
again later.

-- Steve