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Date:	Thu, 6 Feb 2014 11:39:33 +0000
From:	Will Deacon <will.deacon@....com>
To:	"Srivatsa S. Bhat" <srivatsa.bhat@...ux.vnet.ibm.com>
Cc:	"paulus@...ba.org" <paulus@...ba.org>,
	"oleg@...hat.com" <oleg@...hat.com>,
	"rusty@...tcorp.com.au" <rusty@...tcorp.com.au>,
	"peterz@...radead.org" <peterz@...radead.org>,
	"tglx@...utronix.de" <tglx@...utronix.de>,
	"akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
	"mingo@...nel.org" <mingo@...nel.org>,
	"paulmck@...ux.vnet.ibm.com" <paulmck@...ux.vnet.ibm.com>,
	"tj@...nel.org" <tj@...nel.org>,
	"walken@...gle.com" <walken@...gle.com>,
	"ego@...ux.vnet.ibm.com" <ego@...ux.vnet.ibm.com>,
	"linux@....linux.org.uk" <linux@....linux.org.uk>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH 08/51] arm, hw-breakpoint: Fix CPU hotplug callback
 registration

On Thu, Feb 06, 2014 at 11:25:46AM +0000, Srivatsa S. Bhat wrote:
> Hi Will,

Hello,

> On 02/06/2014 04:27 PM, Will Deacon wrote:
> > On Wed, Feb 05, 2014 at 10:06:04PM +0000, Srivatsa S. Bhat wrote:
> >> Subsystems that want to register CPU hotplug callbacks, as well as perform
> >> initialization for the CPUs that are already online, often do it as shown
> >> below:
> >>
> >> 	get_online_cpus();
> >>
> >> 	for_each_online_cpu(cpu)
> >> 		init_cpu(cpu);
> >>
> >> 	register_cpu_notifier(&foobar_cpu_notifier);
> >>
> >> 	put_online_cpus();
> >>
> >> This is wrong, since it is prone to ABBA deadlocks involving the
> >> cpu_add_remove_lock and the cpu_hotplug.lock (when running concurrently
> >> with CPU hotplug operations).
> > 
> > Hmm, the code in question (for this patch) runs from an arch_initcall. How
> > can you generate CPU hotplug operations at that stage?
> > 
> 
> You are right - in today's design of the init sequence, CPU hotplug
> operations can't be triggered at that time during boot.

Phew, so we don't have a bug as the code stands today.

> However, there have been proposals to boot CPUs in parallel along with the
> rest of the kernel initialization [1] (and that would need full synchronization
> with CPU hotplug even at the initcall stage). Of course this needs a lot of
> auditing and modifications to the CPU hotplug notifiers of various subsystems
> to make them robust enough to handle the parallel boot; so its not going to
> happen very soon. But I felt that it would be a good idea to ensure that we
> get the locking/synchronization right, even if the registrations happen very
> early during boot today.. you know, just to be on the safer side and also to
> make the job easier for whoever that is, who tries to implement parallel
> CPU booting again in the future ;-)
> 
> [1]. http://thread.gmane.org/gmane.linux.kernel/1246209

Makes sense, and this seems like a good start.

> > so it's best if you take this all via your tree.
> >
> 
> Hmm.. I'm not a maintainer myself, so I'm hoping that either Oleg or Rusty
> or any of the other CPU hotplug maintainers (Thomas/Peter/Ingo) would be
> willing to take these patches through their tree.

Well, you can have my ack for this patch:

  Acked-by: Will Deacon <will.deacon@....com>

Cheers,

Will
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