linux-kernel - Re: Deadlock between cpu_hotplug_begin and cpu_add_remove

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Message-ID: <20140123170242.GA8051@redhat.com>
Date:	Thu, 23 Jan 2014 18:02:42 +0100
From:	Oleg Nesterov <oleg@...hat.com>
To:	"Srivatsa S. Bhat" <srivatsa.bhat@...ux.vnet.ibm.com>
Cc:	Paul Mackerras <paulus@...ba.org>, linux-kernel@...r.kernel.org,
	Peter Zijlstra <peterz@...radead.org>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Ingo Molnar <mingo@...nel.org>, Tejun Heo <tj@...nel.org>,
	Michel Lespinasse <walken@...gle.com>, ego@...ux.vnet.ibm.com,
	"rusty@...tcorp.com.au" <rusty@...tcorp.com.au>,
	Thomas Gleixner <tglx@...utronix.de>,
	"akpm@...ux-foundation.org" <akpm@...ux-foundation.org>
Subject: Re: Deadlock between cpu_hotplug_begin and cpu_add_remove_lock

On 01/23, Srivatsa S. Bhat wrote:
>
> On 01/23/2014 12:48 AM, Oleg Nesterov wrote:
> > On 01/22, Srivatsa S. Bhat wrote:
> >>
> >> Wait a min, that _will_ actually work for all cases because I have provided
> >> an option to invoke _any_ arbitrary function as the "setup" routine.
> >
> > And probably the generic solution makes sense. I am not sure I actually
> > understand the semantics of register_allcpu_notifier(), but the problem
> > it tries to solve looks clear/valid.
> >
>
> Thank you. But I was wondering whether its usage is a bit unintuitive/
> convoluted. So I was contemplating between going with that solution or the
> below one, where the call-sites are expected to do:
>
> cpu_maps_update_begin();
> for_each_online_cpu(cpu) {
> 	...
> }
> __register_cpu_notifier(); //use the __reg() variant, which doesn't take locks
> cpu_maps_update_done();
>
> Of course, that requires exporting the functions cpu_maps_update_begin/done(),
> but this latter form of callback registration might look more natural.

Yes, I thought about this too ;)

> But for some of the other call-sites, we might have to use one
> of the solutions mentioned above.

Yes, yes, sure, I agree.

I suggested this change only for discussion, for the case we need
an "urgent" fix without changes outside of drivers/md/. The generic
solution is better.

Oleg.

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