Date: Thu, 9 Mar 2017 07:24:59 -0800
From: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
To: Thomas Gleixner <tglx@linutronix.de>
Cc: linux-kernel@vger.kernel.org, mingo@redhat.com, peterz@infradead.org,
        fweisbec@gmail.com
Subject: Re: RCU used on incoming CPU before rcu_cpu_starting() called
Reply-To: paulmck@linux.vnet.ibm.com
References: <20170308221656.GA11949@linux.vnet.ibm.com>
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On Thu, Mar 09, 2017 at 02:08:23PM +0100, Thomas Gleixner wrote:
> On Wed, 8 Mar 2017, Paul E. McKenney wrote:
> > [   30.694013]  lockdep_rcu_suspicious+0xe7/0x120
> > [   30.694013]  get_work_pool+0x82/0x90
> > [   30.694013]  __queue_work+0x70/0x5f0
> > [   30.694013]  queue_work_on+0x33/0x70
> > [   30.694013]  clear_sched_clock_stable+0x33/0x40
> > [   30.694013]  early_init_intel+0xe7/0x2f0
> > [   30.694013]  init_intel+0x11/0x350
> > [   30.694013]  identify_cpu+0x344/0x5a0
> > [   30.694013]  identify_secondary_cpu+0x18/0x80
> > [   30.694013]  smp_store_cpu_info+0x39/0x40
> > [   30.694013]  start_secondary+0x4e/0x100
> > [   30.694013]  start_cpu+0x14/0x14
> > 
> > Here is the relevant code from x86's smp_callin():
> > 
> > 	/*
> > 	 * Save our processor parameters. Note: this information
> > 	 * is needed for clock calibration.
> > 	 */
> > 	smp_store_cpu_info(cpuid);
> >
> > The problem is that smp_store_cpu_info() indirectly invokes
> > schedule_work(), which wants to use RCU.  But RCU isn't informed
> > of the incoming CPU until the call to notify_cpu_starting(), which
> > causes lockdep to complain bitterly about the use of RCU by the
> > premature call to schedule_work().
> 
> Right. And that want's to be fixed, not hacked around by silencing RCU.

Fair enough!

I have updated my commit to indicate yours and Frederic's discomfort with
it and marked it as not intended for upstream.  If we get an alternative
fix shortly, I will drop my commit -- but failing that at some point I
will of course start pushing this patch again.

							Thanx, Paul

> Peter????
> 
> Thanks,
> 
> 	tglx
>